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FOCUS om CONCEPTS

o What is latent heat?

in What four mechanisms cause air to rise?e How is atmospheric stability determined?

fir How is precipitation measured?

qr" ater vapor is an odorless, colorless gas that mixes freely with the other gases of the atmo-. sphere. Unlike oxygen and nitrogen-—the two most abundant components of the atmo-

sphere—water can change from one state of matter to another (solid, liquid, or gas) at thetemperatures and pressures experienced on Earth. Because of this unique property, water freely leaves

T the oceans as a gas and returns again as a liquid.l As you observe day-to-day weather changes, you might ask: Why is it generally more humid in the

summer than in the winter? Why do clouds form on some occasions but not on others? I/Vhy do someclouds look thin and harmless whereas others form gray and ominous towers? Answers to these ques-

I tions involve the role ofwater vapor in the atmosphere, the central theme of this chapter.

To assist you in learning the important concepts in this chapter, focus on the following questions:What are the processes that cause water to change from one state of matter to another?

<‘*;:- What is humidity? What is the most common method used to express humidity?What processes cause relative humidity to change?

rs-2 What are the necessary conditions for condensation?* Which two criteria are used for cloud classification?

What is fog? How do the various types of fog form?as What two processes produce precipitation in a cloud?tin What are the different types of precipitation and how does each form?

Water’s Changes of StateEarth’s Dynamic Atmosphere

mm

r Moisture and Cloud Formation

Water is the only substance that exists in the atmosphere as asolid, liquid, and gas It is made ofhydrogen and oxy-gen atoms that are bonded together to form water molecules(H20). In all three states ofmatter (even ice) these molecules arein constant motion—the higher the temperature, the more vig-orous the movement. The chief difference among liquid water,ice, and water vapor is the arrangement of the water molecules.

Ice, Liquid Water, and Water VaporIce is composed of water molecules that are held together bymutual molecular attractions. Here the molecules form a tight,orderly network, as shown in As a consequence, thewater molecules in ice are not free to move relative to each otherbut rather vibrate about fixed sites. When ice is heated, the mol-ecules oscillate more rapidly. When the rate of molecular move-ment increases sufficiently, the bonds between some of the watermolecules are broken, resulting in melting.

In the liquid state, water molecules are still tightly packed butare moving fast enough that they are able to slide past oneanother. As a result, liquid water is fluid and will take the shapeof its container.

As liquid water gains heat from its environment, some of themolecules will acquire enough energy to break the remainingmolecular attractions and escape from the surface, becomingwater vapor. Water-vapor molecules are widely spaced comparedto liquid water and exhibit very energetic random motion. Whatdistinguishes a gas from a liquid is its compressibility (andexpandability). For example, you can easily put more and more airinto a tire and increase its volume only slightly. However, don’ttry to put 10 gallons of gasoline into a five-gallon can.

To summarize, when water changes state, it does not turn intoa different substance; only the distances and interactions amongthe water molecules change.

Latent HeatWhenever water changes state, heat is exchanged between waterand its surroundings. I/Vhen water evaporates, heat is absorbed(Figure 17.2). Meteorologists often measure heat energy in calo-ries. One calorie is the amount of heat required to raise the tem-perature of l gram of water 1° C (1.8° F). Thus, when 10 calories

Water’s Changes of State 491

i?‘iIt§tll1i'.€ ’i‘!.ii Caught in a downpour. (Photo by ImageState/Alarny)

of heat are absorbed by 1 gram of water, the molecules vibratefaster and a 10° C (18° F) temperature rise occurs.

Under certain conditions, heat may be added to a substanceWithout an accompanying rise in temperature. For example, whena glass of ice water is warmed, the temperature of the ice-watermixture remains a constant 0° C (32° F) Lmtil all the ice has melted.If adding heat does not raise the temperature, where does this

energy go? In this case, the added energy went to break the mol-ecular attractions between the water molecules in the ice cubes.

Because the heat used to melt ice does not produce a tem-perature change, it is referred to as latent heat. (Latent means“hidden,” like the latent fingerprints hidden at a crime scene.)This energy can be thought of as being stored in liquid water, andit is not released to its surroundings as heat until the liquid returnsto the solid state.

It requires 80 calories to melt one gram of ice, an amountreferred to as latent heat ofmelting. Freezing, the reverse process,releases these 80 calories per gram to the environment as latentheat offusion.

Evaporation and Condensation We sawthat heat is absorbedwhen ice is converted to liquid water. Heat is also absorbed dur-ing evaporation, the process of converting a liquid to a gas(vapor). The energy absorbed by water molecules during evapo-ration is used to give them the motion needed to escape the sur-face of the liquid and become a gas. This energy is referred to asthe latent heat ofvaporization. During the process ofevaporation,it is the higher-temperature (faster-moving) molecules that escapethe surface. As a result, the average molecular motion (tempera-ture) of the remaining water is reduced—hence the commonexpression, “evaporation is a cooling process.” You have undoubt-edly experienced this cooling effect on stepping dripping wet froma swimming pool or bathtub. In this situation the energy used toevaporate water comes from your skin—hence, you feel cool.

FIGURE T12 Changes of state always involve an exchange of heat. The numbers shown here are the approximate number ofcalories either absorbed or released when 1 gram of water changes from one state of matter to another.

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492 CHAPTER 17 Moisture, Clouds, and Precipitation

Condensation, the reverse process, occurs when water vaporchanges to the liquid state. During condensation, water-vapormolecules release energy (latent heat of condensation) in anamount equivalent to what was absorbed during evaporation.When condensation occurs in the atmosphere, it results in theformation of such phenomena as fog and clouds.

As you will see, latent heat plays an important role in manyatmospheric processes. In particular, when water vapor con-denses to form cloud droplets, latent heat of condensation isreleased, warming the surrounding air and giving it buoyancy.Vllhen the moisture content of air is high, this process can spurthe growth of towering storm clouds.

Sublimation and Deposition You are probably least familiarwith the last two processes illustrated in Figure 17.2—sublimationand deposition. Sublimation is the conversion of a solid directlyto a gas without passing through the liquid state. Examples youmay have observed include the gradual shrinking of unused icecubes in the freezer and the rapid conversion of dry ice (frozencarbon dioxide) to wispy clouds that quickly disappear.

Deposition refers to the reverse process, the conversion of avapor directly to a solid. This change occurs, for example, whenwater vapor is deposited as ice on solid objects such as grass orwindows iris). These deposits are called white frost orhoarfirost and are frequently referred to simply asfrost. A house-hold example of the process of deposition is the “frost” that accu-mulates in a freezer. As shown in Figure 17.2, deposition releasesan amount of energy equal to the total amount released by con-densation and freezing.

CONCEPT cnrzcx 1 7.1Q Summarize the processes bywhich water changes from one

state ofmatter to another. Indicate whether energy isabsorbed or released.

Q What is latent heat?Q What is a common example of sublimation?Q How does frost form?

".i‘}’.i-i Frost on a window pane is an example of deposition.(Photo by Stockxpert/Jupiter Images Unlimited)

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Students Sometimes Ask...What is freezer burn?

"Freezer burn" is a commonexpression used to describefoods that have been left in afrost-free refrigerator forextended periods of time.Frost-free refrigerators circu-late comparatively dry airthrough the freezer compart-

(change from a solid to a gas)so it can be removed by the cir-culating air. Unfortunately, thisprocess also removes moisturefrom those frozen foods thatare not in airtight containers.Consequently, over a period ofa few months these foodsbegin to dry out rather thanactually being burned.

ments. This causes ice on thefreezer walls to sublimate

Humidity: Water Vaporin the Air

Earth's Dynamic Atmosphere

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Water vapor constitutes only a small fraction of the atmosphere,varying from as little as one-tenth of 1 percent up to about 4 percentby volume. But the importance ofwater in the air is far greater thanthese small percentages would indicate. Indeed, scientists agreethat water vapor is the most important gas in the atmosphere whenit comes to understanding atmospheric processes.

Humidity is the general term for the amount ofwater vaporin air. Meteorologists employ several methods to express thewater-vapor content of the air; we examine three: mixing ratio,relative humidity, and dew-point temperature.

SaturationBefore we consider these humidity measures further, it is impor-tant to understand the concept ofsaturation. Imagine a closed jarcontaining water overlain by dry air, both at the same tempera-ture. As the water begins to evaporate from the water surface, asmall increase in pressure can be detected in the air above. Thisincrease is the result of the motion of the water-vapor moleculesthat were added to the air through evaporation. In the openatmosphere, this pressure is termed vapor pressure and isdefined as that part of the total atmospheric pressure that can beattributed to the water-vapor content.

In the closed container, as more and more molecules escapefrom the water surface, the steadily increasing vapor pressure inthe air above forces more and more of these molecules to returnto the liquid. Eventually the number of vapor molecules return-ing to the surface will balance the number leaving. At that point,the air is said to be saturated. If we add heat to the container,thereby increasing the temperature of the water and air, morewater will evaporate before a balance is reached. Consequently,

i Humidity: Water Vapor in the Air 493

at higher temperatures, more moisture is required for saturation.The amount ofwater vapor required for saturation at various tem-peratures is shown in Table 17.1.

Mixing RatioNot all air is saturated, of course. Thus, we need ways to expresshow humid a parcel of air is. One method specifies the amount ofwater vapor contained in a unit of air. The mixing ratio is themass of water vapor in a unit of air compared to the remainingmass of dry air.

mass of water vapor (grams)mmng ram) mass of dry air (kilograms)

Table 17 .1 shows the mixing ratios of saturated air at varioustemperatures. For example, at 25° C (77° F) a saturated parcel ofair (one kilogram) would contain 20 grams ofwater vapor.

Because the mixing ratio is expressed in units of mass (usu-ally in grams per kilogram), it is not affected by changes in pres-sure or temperature. However, the mixing ratio is time-consuming to measure by direct sampling. Thus, other methodsare employed to express the moisture content of the air. Theseinclude relative humidity and dew-point temperature.

Relative HumidityThe most familiar and, unfortunately, the most misunderstoodterm used to describe the moisture content of air is relativehumidity. Relative humidity is a ratio ofthe air ’s actual water-vapor content compared with the amount of water vaporrequiredfor saturation at that temperature (and pressure). Thus,relative humidity indicates how near the air is to saturation,rather than the actual quantity of water vapor in the air (seeBox 17.1).

To illustrate, we see from Table 17.1 that at 25° C (77° F), air issaturated when it contains 20 grams ofwater vapor per kilogram

TABLE 17.1 Amount of Water Vapor Needed to Saturatea Kilogram of Air at Various Temperatures

Temperature °C (°F) Water-Vapor Content at Saturation (grams)

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Students Sometimes Ask...Why do my lips get chapped in the winter?

tive humidity to plunge. Unlessyour home is equipped with ahumidifier, you are likely toexperience chapped lips anddry skin at that time of year.

During the winter months, out-side air is comparatively cooland dry. When this air isdrawn into the home, it isheated, which causes the rela-

of air. Thus, if the air contains 10 grams per kilogram on a 25° Cday, the relative humidity is expressed as 10/20, or 50 percent. Ifair with a temperature of 25° C had a water-vapor content of 20grams per kilogram, the relative humidity would be expressed as20/20, or 100 percent. When the relative humidity reaches 100percent, the air is said to be saturated.

Because relative humidity is based on the air’s water-vaporcontent, as well as the amount of moisture required for satura-tion, it can be changed in either of two ways. First, relative humid-ity can be changed by the addition or removal of water vapor.Second, because the amount of moisture required for saturationis a function of air temperature, relative humidity varies with tem-perature. (Recall that the amount of water vapor required forsaturation is temperature dependent, such that at highertemperatures it takes more water vapor to saturate air than atlower temperatures.)

Adding or Subtracting Moisture Notice in : thatwhen water vapor is added to a parcel of air, its relative humidityincreases until saturation occurs (100 percent relative humidity).What if even more moisture is added to this parcel of saturatedair? Does the relative humidity exceed 100 percent? Normally,this situation does not occur. Instead, the excess water vapor con-denses to form liquid water.

In nature, moisture is added to the air mainly via evaporationfrom the oceans. However, plants, soil, and smaller bodies ofwaterdo make substantial contributions.

Changes with Temperature The second condition thataffects relative humidity is air temperature. Examine < = 2 :1;carefully. Note in Part A that when air at 20° C contains 7 gramsofwater vapor per kilogram, it has a relative humidity of 50 per-cent. This can be verified by referring to Table 17.1. Here we cansee that at 20° C, air is saturated when it contains 14 grams ofwater vapor per kilogram of air. Because the air in Figure 17.5Acontains 7 grams of water vapor, its relative humidity is 7/ 14, or50 percent.

When the flask is cooled from 20° to 10° C, as shown inFigure 17.5B, the relative humidity increases from 50 to 100 per-cent. We can conclude from this that when the water-vapor con-tent remains constant, a decrease in temperature results in anincrease in relative humidity.

494 CHAPTER 17 Moisture, Clouds, and Precipitation _ _

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What happens when the air is cooled below the temperatureat which saturation occurs? Part C in Figure 17.5 illustrates thissituation. Notice from Table 17.1 that when the flask is cooled to0° C, the air is saturated at 3.5 grams ofwater vapor per kilogramof air. Because this flask originally contained 7 grams of water

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vapor, 3.5 grams of water vapor will condense to form liquiddroplets that collect on the walls of the container. The relativehumidity of the air inside remains at I00 percent. This raises animportant concept. When air aloft is cooled below its saturationlevel, some of the water vapor condenses to form clouds. As

. Relative humidity varies whenEll temperatures change. When the water-vapor

content (mixing ratio) remains constant,the relative humidity can be changed by

) increasing or decreasing the air temperature.In this example, when the temperature of theair in the flask was lowered from 20° to 10° C,

2 the relative humidity increased from 50 to 100percent. Further cooling (from 10° to 0° C)causes one-half of the water vapor to

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Humidity: Water Vapor in the Air 495

clouds are made of liquid droplets (or ice crystals), they are nolonger part of the water-vapor content of the air. (Clouds are notwater vapor; they are composed of liquid water droplets or icecrystals too tiny to fall to Earth.)

We can summarize the effects of temperature on relativehumidity as follows. When the water-vapor content of air remainsat a constant level, a decrease in air temperature results in anincrease in relative humidity and an increase in temperaturecauses a decrease in relative humidity. In =5? the varia-tions in temperature and relative humidity during a typical daydemonstrate the relationship just described.

Dew-Point TemperatureAnother important measure of humidity is the dew-point tem-perature. The dew-point temperature or simply the dew point isthe temperature to which a parcel of air would need to be cooledto reach saturation. For example, in Figure 17.5, the unsaturatedair in the flask had to be cooled to 10° C before saturationoccurred. Therefore, 10° C is the dew-point temperature for thisair. In nature, cooling below the dew point causes water vapor tocondense, typically as dew, fog, or clouds. The term dew pointstems from the fact that during nighttime hours, objects near the

"1i.’.?-‘.-.ii& Typical daily variations in temperature and relativehumidity during a spring day at Washington, D.C. When temperatureincreases, relative humidity drops (see mid-afternoon) and vice versa.

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ground often cool below the dew-point temperature and becomecoated with dew :~:.‘r:-

Unlike relative humidity, which is a measure ofhow near theair is to being saturated, dew-point temperature is a meastue of itsactual moisture content. Because the dew-point temperature isdirectly related to the amount of water vapor in the air, andbecause it is easy to determine, it is one of the most useful mea-sures of humidity.

The amount of water vapor needed for saturation is temper-ature dependent and that for every 10° C (18° F) increase in tem-perature, the amount of water vapor needed for saturationdoubles (see Table 17.1). Therefore, relatively cold saturated air(0° C or 32° F) contains about half the water vapor ofsaturated airhaving a temperature of 10° C (50° F) and roughly one-fourth thatof hot saturated airwith a temperature of 20° C (68° F). Becausethe dew point is the temperature at which saturation occurs, wecan conclude that high dew-point temperatures equate to moistair, and low dew-point temperatures indicate dry air. More pre-cisely, based on what we have learned about vapor pressure andsaturation, we can state that for every 10° C (18° F) increase in thedew-point temperature, the air contains about twice as muchwater vapor. Therefore, we know that air over Fort Myers, Florida,with a dew point of 25° C (77° F), contains about twice the watervapor of air situated over St. Louis, Missouri, with a dew point of

496 CHAPTER 1'7 Moisture, Clouds, and Precipitation

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15° C (59° F), and four times that of Tucson, Arizona, with a dewpoint of 5° C (41° F).

Because the dew-point temperature is a good measure oftheamount ofwater vapor in the air, it is the measure of atmosphericmoisture that appears on a variety of weather maps. Notice onthe map in that most of the places located near thewarm Gulf of Mexico have dew-point temperatures that exceed70° F (21° C). When the dew point exceeds 65° F (18° C), it is con-sidered humid by most people, and air with a dew point of 75° F(24° C) or higher is oppressive. Also notice in Figure 17.8 thatalthough the Southeast is dominated by hurn.id conditions (dewpoints above 65° F), most of the remainder of the countryis expe-riencing drier air. I

Measuring HumidityRelative humidity is commonly measured using a hygrometer(hygro = moisture, metron = measuring instrument). 0ne typeof hygrometer, called a psychrometer, consists of two identicalthermometers mounted side by side One ther-mometer, the dry bulb, gives the current air temperature. Theother, called the wet-bulb thermometer, has a thin muslin wicktied around the end (see end of thermometers in the photo).

To use the psychrometer, the cloth sleeve is saturated withwater and a continuous current of air is passed over the wick(Figure I7.9). This is done either by swinging the instrument freelyin the air or by fanning air past it. As a consequence, water evap-orates from the wick, and the heat absorbed by the evaporatingwater makes the temperature of the wet bulb drop. The loss ofheat that was required to evaporate water from the wet bulb low-ers the thermometer reading.

The amount ofcooling that takes place is directly proportionalto the dryness of the air. The drier the air, the more moisture evap-

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orates. The more heat the evaporating water absorbs, the greaterthe cooling. Therefore, the larger the difference that is observedbetween the thermometer readings, the lower the relative humid-ity; the smaller the difference, the higher the relative humidity. Ifthe air is saturated, no evaporation will occur, and the two ther-mometers will have identical readings.

To determine the precise relative humidity from the ther-mometer readings, a standard table is used (refer to Appendix C,Table C.l). With the same information but using a different table(Table C.2), the dew-point temperature can also be calculated.

Students Sometimes Ask. ..Why is the air in buildings so dry in the winter?

The answer lies in the relation-ship between temperature andrelative humidity. Recall that ifthe water-vapor content of airremains at a constant level, anincrease in temperature lowersthe relative humidity, and adrop in temperature raises therelative humidity. During thewinter months, outside air iscomparatively cool and has alow mixing ratio. When this airis drawn into the home, it isheated to room temperature.

This process causes the rela-tive humidity to plunge, oftento uncomfortably low levels ofl0 percent or lower. Livingwith dry air can mean staticelectrical shocks, dry skin,sinus headaches, or even nose-bleeds. Consequently, thehomeowner may install ahumidifier, which adds waterto the air and increases the rel-ative humidity to a more com-fortable level.

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DryAir at 1 O0 PercentRelative Humidity?

A common misconception relating to meteo-rology is the notion that air with a highrelative humidity must have a greaterwater-vapor content than air with a lowerrelative humidity. Frequently, this is not thecase (Figure 17.A). To illustrate, consider atypical January day at two different loca-tions—International Falls, Minnesota, and ;Phoenix, Arizona. On this hypothetical daythe temperature in International Falls is acold -1 0° C (11-1° F) and the relative humidityis 100 percent. By referring to Table 17.1,we can see that saturated —l0° C (14° F) airhas a water-vapor content (mixing ratio) of2 grams per kilogram (g/kg). By contrast,the desert air at Phoenix is a warm25° C (77° F), and the relative humidity is20 percent. A look at Table 17.1 reveals thatthe 25° C (77° F) air has a saturation mixingratio of 20 g/kg. Therefore, with a relativehumidity of 20 percent, the air at Phoenixhas a water-vapor content of 4 g/kg(20 grams x 20 percent). Consequently, the“ dry" air at Phoenix actually contains twicethe water vapor as the "moist" air at Interna-tional Falls. -

This comparison illustrates why placesthat are very cold are also very dry. The lowwater-vapor content of frigid air (even whensaturated) helps explain why many Arcticareas receive only meager amounts of pre-cipitation and are referred to as "polardeserts." Furthermore, the dry skin andchapped lips people frequently experienceduring the winter months in the UnitedStates can be attributed to dry air. Thewater vapor content of the cold air is low,even when compared to some hot, aridregions.

Another instrument used for measuring relative hurnidity, thehair hygrometer, can be read directlywithout the use of tables. Thehair hygrometer operates on the principle that hair or certain syn-thetic fibers change their length in proportion to changes in therelative humidity, lengthening as relative humidity increases andshrinking as the relative humidity drops. The tension of a bundleof hairs is linked mechanically to an indicator that is calibratedbetween 0 and 100 percent. Thus, we need only glance at the dialto determine the relative humidity. Unfortunately, the hair

FIGURE 17.A Moisture content of hot air versus frigid air. Hot desert air with alow relative humidity generally has a higher water-vapor content than frigid airWith 8. high relative humidity. (Top photo by Matt Duvall, bottom photo by E. J. Tarbuck)

hygrometer is less accurate than the psychrometer. Furthermore,it requires frequent calibration and is slow in responding tochanges in humidity, especially at low temperatures.

A different type of hygrometer is used in remote-sensinginstrument packages such as radiosondes that transmit upper-air observations back to ground stations. The electric hygrometercontains an electrical conductor coated with a moisture-absorbing chemical. It works on the principle that the passage ofcurrent varies as the relative humidityvaries.

498 CHAPTER 17 Moisture, Clouds, and Precipitation

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water vapor in the air decreases, howwill relative humiditychange?

Q Explain the principle of the sling psychrometer and the hairhygrometer.

Q 0n a warm summer day when the relative humidity is high, itmay seem even warmer than the thermometer indicates. Whydo we feel so tmcomfortable on these muggy days?

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The Basis of CloudFormation: Adiabatic Cooling

Earth's Dynamic Atmosphere

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P Moisture and Cloud Formation

We have considered basic properties of water vapor and how itsvariability is measured. This section examines some ofthe impor-tant roles that water vapor plays in weather, especially the for-mation of clouds.

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Sling psychrometer.A. Sling psychrometers are used todetermine both relative humidity anddew point. B. The dry-bulb thermometergives the current air temperature. Theweb-bulb thermometer is covered with

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Fog and Dew versus Cloud FormationRecall that condensation occurs when water vapor changes to aliquid. Condensation may form dew, fog, or clouds. Althoughthese three forms are different, all require that air reach saturation.As indicated earlier, saturation occurs either when sufficientwatervapor is added to the air or, more commonly, when the air iscooled to its dew point.

Near Earth’s surface, heat is readily exchanged between theground and the air above. During evening hours, the surface radi-ates heat away, causing the surface and adjacent air to coolrapidly. This radiation cooling accounts for the formation of dewand some types of fog. Thus, surface cooling that occurs after sun-set accounts for some condensation. However, cloud formationoften takes place during the warmest part of the day. Clearly someother mechanism must operate aloft that cools air sufficiently togenerate clouds.

Adiabatic Temperature ChangesThe process that is responsible for most cloud formation is easilydemonstrated if you have ever pumped up a bicycle tire andnoticed that the pump barrel became quite warm. The heat youfelt was the consequence of the work you did on the air to com-press it. When energy is used to compress air, the motion of the gasmolecules increases and therefore the temperature of the an rises.Conversely, air that is allowed to escape from a bicycle tireexpands and cools. This results because the expanding air pushes

The Basis of Cloud Formation: Adiabatic Cooling 499

Students Sometimes Ask. ..What is the difference between a winter storm warningand a blizzard warning?

A winter storm warning is usu-ally issued when heavy snowexceeding 6 inches in 12 hoursor possible icing conditions arelikely. It is interesting to notethat in upper Michigan andmountainous areas wheresnowfall is abundant, winterstorm warnings are issuedonly if 8 or more inches of

snow is expected in 12 hours.By contrast, blizzard warningsare issued for periods whenconsiderable falling or blowingsnow is accompanied by windsof 35 or more miles per hour.Thus, a blizzard is a type ofwinter storm in which windsare the determining factor, notthe amount of snowfall.

(does work on) the surrounding air and must cool by an amountequivalent to the energy expended.

You have probably felt the cooling effect of the propellant gasexpanding as you applied hairspray or spray deodorant. As thecompressed gas in the aerosol can is released, it quickly exp andsand cools. This drop in temperature occurs even though heat isneither added nor subtracted. Such variations are known asadiabatic temperature changes and result when air is com-pressed or allowed to expand. In summary, when air is allowed toexpand, it cools, and when it is compressed, it warms.

Adiabatic Cooling and CondensationTo simplify the following discussion, it helps to imagine a volume ofair enclosed in a thin elastic cover. Meteorologists call this imagi-naryvolume ofair a parcel. Typically, we consider , . 2 , . ._a parcel to be a few hundred cubic meters in vol-ume, and we assume that it acts independently ofthe sturounding air. It is also asstuned that no heatis transferred into, or out of, the parcel. Althoughhighly idealized, over short time spans a parcel ofair behaves in a manner much like an actual vol-ume of air moving vertically in the atmosphere. 5000

Dry Adiabatic Rate As you travel from Earth'ssurface upward through the atmosphere, the 4000atmospheric pressure rapidly diminishes,because there are fewer and fewer gas molecules.Thus, any time a parcel of air moves upward, it »- 3000passes through regions of successively lower F’pressure, and the ascending air expands. As it —expands, it cools adiabatically. Unsaturated air 000cools at the constant rate of 10° C for every 1,000meters of ascent (5.5° F per 1,000 feet).

Conversely, descending air comes underincreasingly higher pressures, compresses, andis heated 10° C for every 1,000 meters of descent.This rate of cooling or heating applies only tounsaturated air and is known as the dry adia-

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Wet Adiabatic Rate If a parcel of air rises high enough, it willeventually cool to its dew point, where the process of condensa-tion begins. From this point on along its ascent, latent heat ofcon-densation stored in the water vapor will be liberated. Althoughthe air will continue to cool after condensation begins, thereleased latent heat works against the adiabatic process, therebyreducing the rate at which the air cools. This slower rate of cool-ing caused by the addition of latent heat is called the wet adia-batic rate of cooling. Because the amount of latent heat releaseddepends on the quantity of moisture present in the air, the wetadiabatic rate varies from 5° C per 1,000 meters for air with a highmoisture content to 9° C per 1,000 meters for dry air.

1 1 . it . illustrates the role of adiabatic cooling in the for-mation of clouds. Note that from the surface up to the condensa-tion level the air cools at the dry adiabatic rate. The wet adiabaticrate begins at the condensation level.

CONCEPT cnzcx 1 7.3Q How is the formation of dew different than the formation of a

cloud? How are they similar?Q Why does air cool when it rises through the atmosphere?Q What do meteorologists mean by the word parcel?Q Why does the adiabatic rate of cooling change when conden-

sation begins? Why is the wet adiabatic rate not a constantfigure?

Q The contents of an aerosol can are under very high pressure.When you push the nozzle on such a can, the spray feels cold.Explain.

Rising air cools at the dry adiabatic rate of 10° per 1,000 meters, until theair reaches the dew point and condensation (cloud formation) begins. As air continues torise, the latent heat released by condensation reduces the rate of cooling. The wetadiabatic rate is therefore always less than the dry adiabatic rate.

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500 2 2 CHAPTER 17 Moisture, Clouds, and Precipitation

Processes that Lift AirEx Earth's Dynamic Atmosphere

P Moisture and Cloud Formation‘,"-,m ->"'2:Z--I CI 1'71

To review, when air rises, it expands and cools adiabatically. If airis lifted sufficiently, it will eventually cool to its dew-point tem-perature, saturation will occur, and clouds will develop. But whydoes air rise on some occasions and not on others?

It turns out that, in general, air tends to resist vertical movement.Therefore, air located near the surface tends to stay near the surface,and air aloft tends to remain aloft. Exceptions to this rule, as we shallsee, include conditions in the atmosphere that give air sufficientbuoyancy to rise without the aid of outside forces. In many situa-tions, however, whenyou see clouds forming there is some mechan-ical phenomenon atwork that forces the air to rise (at least initially).

There are four mechanisms that force air to rise:

1. Orographic lifting—air is forced to rise over a mountain-ous barrier.

2. Frontal wedging—warmer, less dense air is forced overcooler, denser air.

3. Convergence-—a pileup of horizontal air flow results inupward movement.

4. Localized convective hfting—-unequal surface heating causeslocalized pockets of air to rise because of their buoyancy.

In later chapters we consider other mechanisms that causeair to rise. In particular, horizontal airflow at upper levels signif-icantly contributes to vertical lifting across the middle latitudes.

'3? Orographic lifting leads to precipitation on windward slopes. By the time air reaches theleeward side of the mountains, much of the moisture has been lost. The Great Basin desert is a rainshadow desert that covers nearly all of Nevada and portions of adjacent states. (Photo on left by DeanPennala/Shutterstock, photo on right by Dennis Tasa)

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Orographic LiftingOrographic lifting occurs when elevated terrains, such as moun-tains, act as barriers to the flow of air (i?“ir.i xi).As air ascendsa mountain slope, adiabatic cooling often generates clouds andcopious precipitation. In fact, many of the rainiest places in theworld are located on windward mountain slopes.

By the time air reaches the leeward side of a mountain, muchof its moisture has been lost. If the air descends, it warms adia-batically, making condensation and precipitation even less likely.As shown in Figure 17.1 1, the result can be a rain shadow desert.The Great Basin Desert of the western United States lies only afew hundred kilometers from the Pacific Ocean, but it is effec-tively cut off from the ocean’s moisture by the imposing SierraNevada (Figure 17.11). The Gobi Desert of Mongolia, the TaldaMakan of China, and the Patagonia Desert ofArgentina are otherexamples of deserts that exist because they are on the leewardsides of mountains (for a map showing deserts, see Figure 20.8,p. 589).

Frontal WedgingIf orographic lifting were the only mechanism that forced air aloft,the relatively flat central portion of North America would be anexpansive desert instead of the nation’s breadbasket. Fortunately,this is not the case.

In central North America, masses ofwarm and cold air collide,producing a front. Here the cooler, denser air acts as a barrierover which the warmer, less dense air rises. This process, calledfrontal Wedging, is illustrated in iiiiir. ii is.

It should be noted thatweather-producing fronts areassociated with storm systemscalled middle-latitude cyclones.Because these storms are respon-sible for producing a high per-centage of the precipitation in themiddle latitudes, we examine

- them closely in Chapter 19.

Basin - Convergence., . A ‘- We saw that the collision of con-

trasting air masses forces air torise. In a more general sense,whenever air in the lower atmo-sphere flows together, lifting

I l results. This phenomenon iscalled convergence. When airflows in from more than one di-rection, it must go somewhere.Because it cannot go down, it goesup iii). This, of course,

"tr leads to adiabatic cooling and~-- - possibly cloud formation.

Convergence can also occurwhenever an obstacle slows orrestricts horizontal air flow (wind).

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‘I.-*'..?i-itFrontal wedging. Colder, denser air acts as a barrier over which warmer, less dense air rises.

When air moves from a relatively smooth surface, such as theocean, onto an irregular landscape, its speed is reduced. Theresult is a pileup of air (convergence). This is similar to what hap-pens when people leave a well-attended sporting event and apileup results at the exits. l/Vhen air converges, the air molecules

lf.?*i<~f3ii_i‘;tii; Convergence. When surface air converges, thecolumn of air increases in height to allow for the decreased area itoccupies. This photo shows southern Florida viewed from the spaceshuttle. On warm days, airflow from the Atlantic Ocean and Gulf ofMexico onto the Florida peninsula generates many mid-afternoontl1Ll11C18ISt0II1’1S. (Photo by NASA/Media Services)

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The Florida peninsula pro-vides an excellent example of therole that convergence can play ininitiating cloud development andprecipitation. On warm days, theairflow is from the ocean to theland along both coasts of Florida.

r ,:_ . , This leads to a pileup of air alongiléad girl g the coasts and general conver-

; gence over the peninsula. Thispattern of air movement and theuplift that results is aided byintense solar heating of the land.This is why the peninsula ofFlorida experiences the greatestfrequency of mid-afternoonthunderstorms in the UnitedStates.

More importantly, convergence as a mechanism of forcefullifting is a major contributor to the weather associated withmiddle-latitude cyclones and hurricanes. The low-level hori-zontal air flow associated with these systems is inward andupward around their centers. These important weather pro-ducers are covered in more detail later, but for now rememberthat convergence near the surface results in a general upwardflow.

Localized Convective LiftingOn warm summer days, unequal heating of Earth's surface maycause pockets of air to be warmed more than the surrounding air.For instance, air above a paved parking lot will be warmed morethan the air above an adjacent wooded park. Consequently, theparcel of air above the parking lot, which is warmer (less dense)than the surrounding air, will be buoyed upwardThese rising parcels ofwarmer air are called thermals. Birds suchas hawks and eagles use these thermals to carry them to greatheights where they can gaze down on unsuspecting prey. Peoplehave learned to employ these rising parcels using hang gliders asa way to “fly.”

The phenomenon that produces rising thermals is calledlocalized convective lifting. When these warm parcels of airrise above the lifting condensation level, clouds form, whichcan produce mid-afternoon rain showers. The accompanyingrains, although occasionally heavy, are of short duration andwidely scattered.

Although localized convective lifting by itself is not a majorproducer of precipitation, the added buoyancy that results fromsurface heating contributes significantly to the lifting initiatedby the other mechanisms. It should also be remembered thateven though the other mechanisms force air to rise, convectivelifting occurs because the air is warmer (less dense) than the sur-rounding air and rises for the same reasons as does a hot-air bal-loon.

502 CHAPTER 17 Moisture, Clouds, and Precipitation

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FIGURE 1‘?.1.4l Localized convective lifting. Unequal heating of Earth’s surface causes pockets of air to be warmed more than thesurrounding air. These buoyant parcels of hot air rise, producing thermals, and if they reach the condensation level, clouds form.

comcsrr cnscrc 1 7 .4Q List the four mechanisms that cause air to rise.Q How do orographic lifting and frontal Wedging force air to

rise?Q Explain why the Great Basin area of the western United States

is so dry. What term is applied to this situation?Q What causes the Florida peninsula to experience the greatest

frequency ofmid-afternoon thunderstorms in the UnitedStates?

The Weathermaker:Atmospheric Stability

Earth's Dynamic Atmosphere

Rm "'2:Z-\_ /11 I1"!> Moisture and Cloud Formation

When air rises, it cools and eventually produces clouds. Why doclouds vary so much in size, and why does the resulting precipi-tation vary so much? The answers are closely related to thestability of the air.

Recall that a parcel ofair can be thought of as having a thin, flex-ible cover that allows it to expand but prevents it from mixing withthe surrounding air (picture a hot-air balloon). If this parcel wereforced to rise, its temperature will decrease because ofexpansion. Bycomparing the parcel’s temperature to that of the surrounding air,we can determine the stability of the bubble. Ifthe parcel’s temper-ature is lower than that of the surrounding environment, it will bedenser; and ifallowed to move freely, itwill sink to its original posi-tion. Air of this type, called stable air, resists vertical movement.

If however, our imaginary rising parcel is warmer and henceless dense than the surrounding air, itwill continue to rise until itreaches an altitude where its temperature equals that of its sur-roundings. This is exactly how a hot-air balloon works, rising aslong as it is warmer and less dense than the surrounding air(Figure 17.15). This type of air is classified as unstable air. In sum-mary, stability is a property of air that describes its tendency toremain in its original position (stable) or to rise (unstable).

Types of StabilityThe stability of air is determined by measuring the temperature ofthe atmosphere at various heights. Recall from Chapter 16 thatthis measure is termed the environmental lapse rate. The envi-ronmental lapse rate is the temperature of the atmosphere asdetermined from observations made by radiosondes and aircraft.

FIGURE 17.15 As long as air is warmer and therefore less densethan its surroundings, it will rise. Hot-air balloons rise up through theatmosphere for this reason. (Photo by Steve Vidler/SuperSt0ck)

The Weathermaker: Atmospheric Stability 503

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In a stable atmosphere, as an unsaturated parcel ofair is lifted, it expands and cools at the dry adiabatic rate of 10° C per1,000 meters. Because the temperature of the rising parcel of air islower than the surrounding environment, it will be heavier and, ifallowed to do so, will sink to its original position.

It is important not to confuse this with adiabatic temperaturechanges, which are changes in temperature caused by expansionor compression as a parcel of air rises or descends.

To illustrate, we examine a situation in which the environ-mental lapse rate is 5° C per 1,000 meters -iii),Underthis condition, when air at the surface has a temperature of25° C,the air at 1,000 meters will be 5 degrees cooler, or 20° C, the air at2,000 meters will have a temperature of 15° C, and so forth. At firstglance it appears that the air at the surface is less dense than theair at 1,000 meters, because it is 5 degrees warmer. However, if theair near the surface is unsaturated and were to rise to 1,000 meters,it would expand and cool at the dry adiabatic rate of 10° C per1,000 meters. Therefore, upon reaching 1,000 meters, its temper-ature would have dropped 10° C. Being 5 degrees cooler than itsenvironment, it would be denser and tend to sink to its originalposition. Hence, we say that the air near the surface is potentiallycooler than the air aloft and therefore will not rise on its own. Theair just described is stable and resists vertical movement.

Absolute Stability Stated quantitatively, absolute stabilityprevails when the environmental lapse rate is less than the wetadiabatic rate. depicts this situation using an envi-ronmental lapse rate of 5° C per 1,000 meters and a wet adiabaticrate of 6° C per 1,000 meters. Note that at 1,000 meters the tem-perature of the surrounding air is 15° C, while the rising parcel ofair has cooled to 10° C and is therefore the denser air. Even if thisstable air were to be forced above the condensation level, itwouldremain cooler and denser than its environment, and thus it wouldtend to return to the surface.

i Absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. A. The risingparcel of air is always cooler and heavier than the surrounding air, producing stability. B. Graphic representation of the conditionsshown in part (A).

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504 CHAPTER 17 Moisture, Clouds, and Precipitation

Students Sometimes Ask...What are the wettest places on Earth?

Many of the rainiest places in occurred at Cherrapunji, India,the world are located on wind- where an astounding 2,647 cen-ward mountain slopes. A sta- timeters (1,042 inches, over 86tion at Mount Waialeale, feet) fell. Much of this rainfallHawaii, records the highest occurred in the month of July-average annual rainfall, some a record 930 centimeters (3661,234 centimeters (486 inches). inches, over 30 feet). This is 10The greatest recorded rainfall times more rain than Chicagofor a single 12-month period receives in an average year.

The most stable conditions occur when the temperature in alayer of air actually increases with altitude. When such a reversaloccurs, a temperature inversion is said to exist. Temperature inver-sions frequently occur on clear nights as a result of radiation cool-ing of Earth’s surface. Under these conditions, an inversion iscreated because the ground and the air immediately above willcool more rapidly than the air aloft. When warm air overlies coolerair, it acts as a lid and prevents appreciable vertical mixing.Because of this, temperature inversions are responsible for trap-ping pollutants in a narrow zone near Earth’s surface.

Absolute Instability At the other extreme, air is said to exhibitabsolute instability when the environmental lapse rate is greaterthan the dry adiabatic rate. As shown in I the ascend-ing parcel of air is always warmer than its environment and willcontinue to rise because of its own buoyancy. However, absoluteinstability is generally found near Earth’s surface. On hot, sunnydays the air above some surfaces, such as shopping center park-ing lots, is heated more than the air over adjacent surfaces. Theseinvisible pockets of more intensely heated air, being less densethan the air aloft, will rise like a hot-air balloon. This phenome-non produces the small, fluffy clouds we associate with fairweather. Occasionally, when the surface air is considerablywarmer than the air aloft, clouds with considerable vertical devel-opment can form.

Conditional Instability A more common type of atmosphericinstability is called conditional instability. This occurs whenmoist air has an environmental lapse rate between the dry andwet adiabatic rates (between 5 and 10° C per 1,000 meters). Sim-ply, the atmosphere is said to be conditionally unstable when it isstable for an ttnsatitrated parcel of air, but unstable for a satnratedparcel of air. Notice in 1 ‘ I i : “ that the rising parcel of air iscooler than the surrounding air for nearly 3,000 meters. With theaddition of latent heat above the lifting condensation level, theparcel becomes warmer than the surrounding air. From this pointalong its ascent the parcel will continue to rise because of its ownbuoyancy, without an outside force. Thus, conditional instabilitydepends on whether or not the rising air is saturated. The word

1 .1. . : Illustration of absolute instability. A. Absolute instability develops when solar heating causes the lowermost layerof the atmosphere to be warmed to a higher temperature than the air aloft. The result is a steep environmental lapse rate thatrenders the atmosphere unstable. B. Graphic representation of the conditions shown in part (A).

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The Weatherrnaker: Atmospheric Stability 505

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2-1: Illustration of conditional instability, where warm air is forced to rise along a frontal boundary. A. Note that theenvironmental lapse rate of 9° C per 1,000 meters lies between the dry and wet adiabatic rates. The parcel of air is cooler than thesurrounding air up to nearly 3,000 meters, where its tendency is to sink toward the surface (stable). Above this level, however,the parcel is warmer than its environment and will rise because of its own buoyancy (unstable). Thus, when conditionally unstableair is forced to rise, the result can be towering cumulus clouds. B. Graphic representation of the conditions shown in part (A).

conditional is used because the air must be forced upward, suchas over mountainous terrain, before it becomes unstable and risesbecause of its own buoyancy.

In summary, the stability of air is determined by measuringthe temperature of the atmosphere at various heights. In simpleterms, a column of air is deemed unstable when the air near thebottom ofthis layer is significantly warmer (less dense) than the airaloft, indicating a steep environmental lapse rate. Under these con-ditions the air actually turns over, as the warm air below rises anddisplaces the colder air aloft. Conversely, the air is considered tobe stable when the temperature drops gradually with increasingaltitude. The most stable conditions occur during a temperatureinversion when the temperature actually increases with height.Under these conditions, there is very little vertical air movement.

Stability and Daily WeatherFrom the previous discussion, we can conclude that stable airresists vertical movement, whereas unstable air ascends freelybecause of its own buoyancy. But how do these facts manifestthemselves in our daily weather?

Because stable air resists upward movement, we might con-clude that clouds will not form when stable conditions prevail inthe atmosphere. Although this seems reasonable, recall thatprocesses exist thatforce air aloft. These include orographic lift-ing, frontal wedging, and convergence. Vllhen stable air is forcedaloft, the clouds that form are widespread and have little verticalthickness when compared to their horizontal dimension, and pre-cipitation, if any, is light to moderate.

By contrast, clouds associated with the lifting of unstable airare towering and often generate thunderstorms and occasionally

Students Sometimes Ask...What is the driest place on Earth?

The record for the lowest aver- per year. Over a span of 59age annual precipitation years this region in Southbelongs to Arica, Chile, which America received a total of lessreceives 0.03 inch of rainfall than 2 inches of precipitation.

506 CHAPTER 17 Moisture, Clouds, and Precipitation

even a tornado. For this reason, we can conclude that on a dreary,overcast day with light drizzle, stable air has been forced aloft. Onthe other hand, during a day when cauliflower-shaped cloudsappear to be growing as if bubbles of hot air are surging upward,we can be fairly certain that the ascending air is unstable.

In summary, stability plays an important role in determiningour daily weather. To a large degree, stability determines the typeof clouds that develop and whether precipitation will come as agentle shower or a heavy downpour.

CONCEPT cnscx 17. 50 Explain the difference between the environmental lapse rate

and adiabatic cooling.Q Describe absolute stability in your own words.Q Compare absolute instability and conditional instability.Q What types of clouds and precipitation, if any, form when sta-

ble air is forced aloft?Q What type ofweather is associated with unstable air?

Condensation and CloudFormationTo review briefly, condensation occurs when water vapor in the airchanges to a liquid. The result of this process may be dew, fog, orclouds. For any of these forms of condensation to occur, the airmust be saturated. Saturation occurs most commonly when airis cooled to its dew point, or less often when water vapor is addedto the air.

Generally, there must be a surface on which the water vaporcan condense. When dew occurs, objects at or near the groundsuch as grass and car windows serve this purpose. But when con-densation occurs in the air above the ground, tiny bits of partic-ulate matter, known as condensation nuclei, serve as surfacesfor water-vapor condensation. These nuclei are very important, forin their absence a relative humidity well in excess of 100 percentis needed to produce clouds.

Condensation nuclei such as microscopic dust, smoke, andsalt particles (from the ocean) are profuse in the lower atmo-sphere. Because of this abundance ofparticles, relative humidityrarely exceeds 101 percent. Some particles, such as ocean salt, areparticularly good nuclei because they absorb water. These parti-cles are termed hygroscopic (hygro = moisture, scopic = to seek)nuclei. When condensation takes place, the initial growth rate ofcloud droplets is rapid. It diminishes quickly because the excesswater vapor is quickly absorbed by the numerous competing par-ticles. This results in the formation of a cloud consisting ofmillionsupon millions of tiny water droplets, all so fine that they remainsuspended in air. When cloud formation occurs at below-freezingtemperatures, tiny ice crystals form. Thus, a cloud might consistof water droplets, ice crystals, or both.

The slow growth of cloud droplets by additional condensa-tion and the immense size difference between cloud droplets andraindrops suggest that condensation alone is not responsible forthe formation of drops large enough to fall as rain. We first exam-ine clouds and then return to the question of how precipitationforms.

Types of CloudsClouds are among the most conspicuous and observable aspectsof the atmosphere and its weather. Clouds are a form of conden-sation best described as visible aggregates of minute droplets ofwater or tiny crystals of ice. In addition to being prominent andsometimes spectacular features in the sky, clouds are of contin-ual interest to meteorologists, because they provide a visible indi-cation of what is going on in the atmosphere. Anyone whoobserves clouds with the hope of recognizing different types oftenfinds that there is a bewildering variety of these familiar whiteand gray masses streaming across the sky. Still, once one comesto know the basic classification scheme for clouds, most of theconfusion vanishes.

Clouds are classified on the basis of their form and height( .s 1-). Three basic forms are recognized: cirrus, cumulus,and stratus.

o Cirrus (cirrus = a curl of hair) clouds are high, white, andthin. They can occur as patches or as delicate veil-likesheets or extended wispy fibers that often have a featheryappearance.

o Cumulus (cttmmulns = a pile) clouds consist of globularindividual cloud masses. They normally exhibit a flat base andhave the appearance of rising domes or towers. Such cloudsare frequently described as having a cauliflower structure.

o Stratus (stratum = a layer) clouds are best described assheets or layers that cover much or all of the sky. Whilethere may be minor breaks, there are no distinct individualcloud units.

All other clouds reflect one of these three basic forms or arecombinations or modifications of them.

Three levels of cloud heights are recognized: high, middle,and low (Figure 17.20). High clouds normally have bases above6,000 meters (20,000 feet), middle clouds generally occupyheights from 2,000 to 6,000 meters (6,500-20,000 feet), and lowclouds form below 2,000 meters (6,500 feet). The altitudes listedfor each height category are not hard and fast. There is some sea-sonal as well as latitudinal variation. For example, at high lati-tudes or during cold winter months in the mid-latitudes, highclouds are often found at lower altitudes.

High Clouds Three cloud types make up the family of highclouds (above 6,000 meters): cirrus, cirrostratus, and cirrocumu-lus. Cirrus clouds are thin and delicate and sometimes appear ashooked filaments called “mares’ tails” *5). As thenames suggest, cirrocnmttltts clouds consist of fluffy masses(Figure 17.21B), whereas cirrostrattts clouds are flat layers(Figure 17.2lC). Because of the low temperatures and small quan-tities of water vapor present at high altitudes, all high clouds arethin and white and are made up of ice crystals. Furthermore, theseclouds are not considered precipitation makers. However, whencirrus clouds are followed by cirrocumulus clouds and increasedsky coverage, they may warn of impending stormy weather.

Middle Clouds Clouds that appear in the middle range(2,000-6,000 meters) have the prefix alto as part of their name.Altocttrnttlus clouds are composed of globular masses that differ

Condensation and Cloud Formation 507

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from cirrocumulus clouds in that they are larger and denser(Figure 17.21D). Altostratus clouds create a uniformwhite to gray-ish sheet covering the skywith the Sun or Moonvisible as a brightspot (Figure 1'7.21E). Infrequent light snow or drizzle may accom-pany these clouds.

Students Sometimes Ask...How much water is found in a cloud?

contains an average of 0.5 cubiccentimeters of liquid per cubicmeter, it would contain 45 I billion

cloud thatis.rough1y 3,000 cubic ceniiilfleters of water (3,000. meters (HD9111; 2 rnilesiin width -><- 3,000 101,000 0,5).Tha1; I

That depends a lot on the size ofthe cloud. Let's consider amodest- sized cumulonimbus

and depth and;10.00’0 meters iequetessto.1 .- our .eneu11-.i)ite-;f11lI}iQII<ii-- -

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Low Clouds There are three members in the family of lowclouds: stratus, stratocumulus, and nimbostratus. Stratus are auniform foglike layer of clouds that frequently covers much ofthesky. On occasions these clouds may produce light precipitation.When stratus clouds develop a scalloped bottom that appears aslong parallel rolls or broken globular patches, they are calledstratocumulus clouds.

Nimbostratus clouds derive their name from the Latin nimbus,which means rainy cloud, and stratus, which means to coverwitha layer (Figure 1'7.21F). As the name suggests, nimbostratus cloudsare one ofthe chiefprecipitation producers. Nimbostratus cloudsform in association with stable conditions. We might not expectclouds to grow or persist in stable air, yet cloud growth ofthis typeis common when air is forced to rise, as occurs along a mountainrange, a front, or near the center of a cyclone where convergingwinds cause air to ascend. Such forced ascent of stable air leadsto the formation of a stratified cloudlayer that is large horizontallycompared to its depth.

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FIGURE 17.21 These photos depict common forms of several different cloud types. (Photos A, B, D, E, F, and G by E. J. Tarbuck. Photo C Jung-Pang/GettyImages. Photo H by Doug Millar/Science Source/Photo Researchers, Inc.)

Clouds of Vertical Development Some clouds do not fit intoany one of the three height categories mentioned. Such cloudshave their bases in the low height range but often extend upwardinto the middle or high altitudes. Consequently, clouds in thiscategory are called clouds ofvertical development. They are allrelated to one another and are associated with unstable air.Although cumulus clouds are often connected with fair weather(Figure 17.21 G), they may grow dramatically under the propercircumstances. Once upward movement is triggered, accelera-tion is powerful, and clouds with great vertical extent form. Theend result is often a towering cloud, called a cumulonimbus, thatmay produce rain showers or a thunderstorm (Figure 17.21H).

Definite weather patterns can often be associated with par-ticular clouds or certain combinations of cloud types, so it is

508

important to become familiar with cloud descriptions and char-acteristics. Table 17.2 lists the 10 basic cloud types that are recog-nized internationally and gives some characteristics of each.

17.6 Q As you drink an ice-cold beverage on a warm humid day, the

outside of the glass or bottle becomes wet. Explain.Q What is the function ofcondensation nuclei in cloud formation?Q What is the basis for the classification of clouds?Q Why are high clouds always thin?Q Which cloud types are associated with the following charac-

teristics: thtmder, halos, precipitation, hail, mackerel sky,lightning, mares’ tails?

E. Altostratus F. Nimbostratus

Condensation and Cloud Formation 509

FiGURE'1‘?.2il (Continued)

G. Cumulus H. Cumulonimbus

TABLE 17.2 Cloud Types and Characteristics

cloudramily and Height; cloudHigh clouds—above 6,000 meters CiII11S(20,000 feet)

Cirrocumulus

Cirrostratus

Middle clouds—2,000—6,000 meters(6,500—20,000 feet)

Altocumulus

Altostratus

"' i _ i _Thin, delicate, fibrous, ice-crystal clouds. Sometimes appear as hooked filaments called“mares‘ tails." (Figure l7.21A)

Thin, white, ice-crystal clouds in the form of ripples, waves, or globular masses all in a row.May produce a "mackerel sky." Least common of the high clouds. (Figure 17.21B)Thin sheet of white, ice-crystal clouds that may give the sky a milky look. Sometimes producehalos around the Sun or Moon. (Figure 1'7.21C).White to gray clouds often composed of separate globules; “sheep-back" clouds.(Figure l7.21D)Stratified veil of clouds that are generally thin and may produce very light precipitation. Whenthin, the Sun or Moon may be visible as a bright spot, but no halos are produced.(Figure l'7.21E)

Low clouds—below 2,000 meters(6,500 feet)

Stratocumulus

StratusNimbostratus

Soft, gray clouds in globular patches or rolls. Rolls may join together to make a continuouscloud.Low uniform layer resembling fog but not resting on the ground. May produce drizzle.Amorphous layer of dark gray clouds. One of the chief precipitation-producing clouds.(Figure 17.21F) _ i

Clouds of vertical development—500—l8,000 meters (1,600—60,000 feet)

Cumulus

Cumulonimbus

Dense, billowy clouds often characterized by flat bases. May occur as isolated clouds orclosely packed. (Figure 17.21C-)Towering cloud sometimes spreading out on top to form an “ anvil head. " Associated withheavy rainfall, thunder, lightning, hail, and tornadoes. (Figure 17.21H)

510 CHAPTER 17 Moisture, Clouds, and Precipitation

FogFog is generally considered to be an atmospheric hazard. Whenit is light, visibility is reduced to 2 or 3 kilometers (1 or 2 miles).However, when it is dense, visibility may be cut to a few dozenmeters or less, making travel by any mode not only difficult butoften dangerous. Officially, visibility must be reduced to 1 kilo-meter or less before fog is reported. While this figure is arbitrary,it does provide an objective criterion for comparing fog frequen-cies at different locations.

Fog is defined as a cloud with its base at or very near the ground.There is no basic physical difference between a fog and a cloud;their appearance and structure are the same. The essential differ-ence is the method and place of formation. Whereas clouds resultwhen air rises and cools adiabatically, most fogs are the conse-quence ofradiation cooling or the movement of air over a cold sur-face. (The exception is upslope fog.) In other circumstances, fogsform when enough water vapor is added to the air to bring aboutsaturation (evaporation fogs). We examine both of these types.

Fogs Caused by CoolingThree common fogs form when air at Earth’s surface is chilledbelow its dew point. Often a fog is a combination of these types.

Advection Fog When warm, moist air moves over a cool sur-face, the result might be a blanket of fog called advection fog(Figure T122). Examples of such fogs are very common. The fog-giest location in the United States, and perhaps in the world, isCape Disappointment, Washington. The name is indeed appro-priate because this station averages 2,552 hours of fog each year(there are about 8,760 hours in a year). The fog experienced atCape Disappointment, and at other West Coast locations, is pro-duced when warm, moist air from the Pacific Ocean moves overthe cold California Current and is then carried onshore by theprevailing winds. Advection fogs are also relatively common inthe winter season when warm air from the Gulf ofMexico movesacross cold, often snow-covered surfaces of the Midwest and East.

Radiation Fog Radiation fog forms on cool, clear, calm nights,when Earth’s surface cools rapidly by radiation. As the night pro-gresses, a thin layer of air in contact with the ground is cooledbelow its dew point. As the air cools and becomes denser, it drainsinto low areas, resulting in “pockets” of fog. The largest pockets areoften river valleys, where thick accumulations may occur(Figure 17.23).

Radiation fog may also form when the skies clear after a rain-fall. In these situations the air near the surface is close to satura-tion and only a small amount of radiation cooling is needed to

I-“IC.‘%URE 17.22 Advection fog rolling into San Francisco Bay. (Photo by Ed Pritchard/Getty Images, Inc.—Stone Allstock)

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promote condensation. Radiation fog of this type often occursaround sunset and can make driving hazardous.

Upslope Fog As its name implies, upslope fog is created whenrelatively humid air moves up a gradually sloping plain or up thesteep slopes ofa mountain. As a result ofthis upward movement, airexpands and cools adiabatically. If the dew point is reached, anextensive layer offog may form. In the United States, the Great Plainsoffers an excellent example. When humid easterly or southeasterlywinds blow westward from the Mississippi River upslope towardthe Rocky Mountains, the air gradually rises, resulting in an adia-batic temperattue decrease of about 13° C. When the differencebetween the air temperature and dew point of westward-movingair is less than 13° C, an extensive fog can result in the western plains.

Evaporation FogsWhen the saturation of air occurs primarily because of the addi-tion ofwater vapor, the resulting fogs are called evaporationfogs.Two types of evaporation fogs are recognized: steam fog andfrontal, or precipitation, fog.

Steam Fog When cool air moves over warm water, enoughmoisture may evaporate from the water surface to produce satu-ration. As the risingwater vapor meets the cold air, it immediatelyrecondenses and rises with the air that is being warmed frombelow. Because the water has a steaming appearance, the phe-

'tY‘it3ii_i§i_?i%§ ‘Lt? A. Satellite image of dense fog in California's San Joaquin Valley on November20, 2002. This early morning radiation fog was responsible for several car accidents in theregion, including a 14-car pileup. The white areas to the east of the fog are the snowcappedSierra Nevadas. (NASA image) B. Radiation fog can make the morning commute hazzardous. (Photoby Jeremy Walker/Getty Images Inc.—Stone Allstock)

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nomenon is called steam fog. Steam fog is fairly common overlakes and rivers in the fall and early winter, when the water maystill be relatively warm and the air is rather crisp (Figure W213).Steam fog is often shallow because as the steam rises, it reevapo-rates in the unsaturated air above.

Frontal Fog When frontal wedging occurs, warm air is lifted overcolder air. If the resulting clouds yield rain, and the cold air belowis near the dewpoint, enough rain will evaporate to produce fog. A

Titifiilitifi 'i‘i.§?;Ii Steam fog rising from Sparks Lake near Bend, Oregon.(Photo by Warren Morgan/CORBIS)

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5'12 CHAPTER 17 Moisture, Clouds, and Precipitation

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1"; ‘i: .=; 1' L . Map showing average number of days per year with heavy fog. Notice that thefrequency of dense fog varies considerably from place to place. Coastal areas, particularly thePacific Northwest and New England where cold currents prevail, have high occurrences ofdense fog.

fog formed in this manner is called frontal fog, or precipitationfog. The result is a more or less continuous zone ofcondensedwaterdroplets reaching from the ground up through the clouds.

Both steam fog and frontal fog result from the addition ofmoisture to a layer of air. The air is usually cool or cold and alreadynear saturation. Because air’s capacity for water vapor at low tem-peratures is small, only a relatively modest amount of evapora-tion is necessary to produce saturated conditions and fog.

The frequency of dense fog varies considerably from place toplace (ii; I. . 2 __ .;).As might be expected, fog incidence is highestin coastal areas, especiallywhere cold currents prevail, as along thePacific and New England coasts. Relatively high frequencies arealso found in the Great Lakes region and in the hurnid AppalachianMotmtains of the East. By contrast, fogs are rare in the interior of thecontinent, especially in the arid and semiarid areas of the West.

Students Sometimes Ask...Why does it often seem like the roads are slippery whenit rains after a long dry period?

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It appears that a buildup ofdebris on roads during dryweather causes more slipperyconditions after a rainfall. Onerecent traffic study indicatesthat if it rains today, there willbe no increase in risk of fatalcrashes if it also rained yester-

day. However, if it has been 2days since the last rain, thenthe risk for a deadly accidentincreases by 3.7 percent. Fur-thermore, if it has been 21days since the last rain, therisk increases to 9.2 percent.

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How PrecipitationFormsAlthough all clouds contain water, why dosome produce precipitation and othersdrift placidly overhead? This seeminglysimple question perplexed meteorologistsfor many years. Before examining theprocesses that generate precipitation, weneed to examine a couple of facts.

First, cloud droplets are very tiny, aver-aging under 20 micrometers (0.02 milli-meter) in diameter Forcomparison, a human hair is about 75micrometers in diameter. The small size ofcloud droplets results mainly because con-densation nuclei are usuallyvery abundantand the available water is distributed

among numerous droplets rather than concentrated into fewerlarge droplets.

Second, because of their small size, the rate at which clouddroplets fall is incredibly slow. An average cloud droplet fallingfrom a cloud base at 1,000 meters would require several hours toreach the ground. However, it would never complete its journey.

Days withheavy fog

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How Precipitation Forms 5'13

This cloud droplet would evaporate before it fell a few meters fromthe cloud base into the unsaturated air below.

How large must a droplet grow in order to fall as precipita-tion? A typical raindrop has a diameter of about 2,000 microme-ters [2 millimeters) or 100 times that of the average cloud droplethaving a diameter of 20 micrometers (0.02 millimeter). However,the volume of a typical raindrop is a million times that of a clouddroplet. Thus, for precipitation to form, cloud droplets must growin volume by roughly one million times. Two mechanisms thatgive rise to these “massive” drops are the Bergeron process andthe collision-coalescence process.

Precipitation from Cold Clouds:The Bergeron ProcessYou have probably watched a TV documentary in which moun-tain climbers brave intense cold and a ferocious snowstorm toscale an ice-covered peak. Although it is hard to imagine, verysimilar conditions exist in the upper portions of towering cumu-lonimbus clouds, even on sweltering summer days. It turns outthat the frigid conditions high in the troposphere provide an idealenvironment to initiate precipitation. In fact, in the middle lati-tudes much of the rain that falls begins with the birth ofsnowflakes high in the cloud tops where temperatures are con-siderably below freezing. Obviously, in the winter, even low cloudsare cold enough to trigger precipitation.

The process that generates much of the precipitation in themiddle latitudes is named the Bergeron process for its discov-erer, the highly respected Swedish meteorologist, Tor Bergeron.The process relies on two interesting phenomena: supercoolingand supersaturation.

Supercooling Cloud droplets do not freeze at 0° C (32° F) asexpected. In fact, pure water suspended in air does not freeze untilit reaches a temperature of nearly —40° C [—40° F). Water in theliquid state below 0° C is referred to as supercooled. Supercooledwater will readily freeze if it impacts an object. This explains whyairplanes collect ice when they pass through a cloud composed ofsupercooled droplets.

In addition, supercooled water droplets will freeze uponcontact with solid particles that have a crystal form closelyresembling that of ice. Such materials are termed freezingnuclei. The need for freezing nuclei to initiate the freezingprocess is similar to the requirement for condensation nuclei inthe process of condensation.

Students Sometimes Ask...What is the snowiest city in the United States?

According to National Weather inches) of snow annually, is theService records, Rochester, snowiest city in the UnitedNew York, which averages States. However, Buffalo, Newnearly 239 centimeters (94 York, is a close runner-up.

In contrast to condensation nuclei, freezing nuclei are verysparse in the atmosphere and do not generally become activeuntil the temperature reaches — 10° C ( 14° P) or less. Only at tem-peratures well below freezing will ice crystals begin to form inclouds, and even at that, they will be few and far between. Onceice crystals form, they are in direct competition with the super-cooled droplets for the available water vapor.

Supersaturation I/Vhen air is saturated (100% relative humid-ity) with respect to water, it is supersaturated (relative humidityis greater than 100%) with respect to ice. Table 17.3 shows that at—10° C (14° F), when the relative humidity is 100 percent withrespect to water, the relative humidity with respect to ice is nearly1 I0 percent. Thus, ice crystals cannot coexist with water dropletsbecause the air always “appears” supersaturated to the ice crys-tals. Hence, the ice crystals begin to consume the “excess” watervapor, which lowers the relative humidity near the surroundingdroplets. In turn, the water droplets evaporate to replenish thediminishing water vapor, thereby providing a continual source ofvapor for the growth of the ice crystals yI 1 ).

Because the level of supersaturation with respect to ice canbe quite great, the growth of ice crystals is generally rapid enoughto generate crystals large enough to fall. During their descent,these ice crystals enlarge as they intercept cloud drops, whichfreeze upon them. Air movement will sometimes break up thesedelicate crystals and the fragments will serve as freezing nuclei. Achain reaction develops, producing many ice crystals, which byaccretion form into large crystals called snowflakes.

In summary, the Bergeron process can produce precipitationthroughout the year in the middle latitudes, provided at least theupper portions of clouds are cold enough to generate ice crystals.The type of precipitation (snow, sleet, rain, or freezing rain) thatreaches the ground depends on the temperature profile in thelower few kilometers of the atmosphere. When the surface

TABLE 17.3 Relative Humidity with Respect to Ice When Relative Humiditywith Respect to Water is 100 Percent

Temperature (°C)0

-5-10-15-20

Relative Humidity with Respect to:

Water (%) Ice (%)100 100IISHZI 105

100 110

1011! I 16

ion 121

5'14 CHAPTER 1'7 Moisture, Clouds, and Precipitation

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temperature is above 4° C (39° F), snowflakes usually melt beforethey reach the ground and continue their descent as rain. Evenon a hot summer day, a heavy downpour may have begun as asnowstorm high in the clouds overhead.

Precipitation from Warm Clouds:The Col1ision—Coalescence ProcessA few decades ago, meteorologists believed that the Bergeronprocess was responsible for the formation of most precipitation.However, it was discovered that copious rainfall can be associ-ated with clouds located well below the freezing level (warmclouds), particularly in the tropics. This led to the proposal of asecond mechanism thought to produce precipitation—thecollision-coalescence process.

Research has shown that clouds composed entirely of liquiddroplets must contain some droplets larger than 20 micrometers(0.02 millimeters) ifprecipitation is to form. These large dropletsform when “giant” condensation nuclei are present, or whenhygroscopic particles such as sea salt exist. Hygroscopic particlesbegin to remove water vapor from the air at relative humiditiesunder 100 percent and can grow quite large. Because the rate atwhich drops fall is size-dependent, these “giant” droplets fall mostrapidly. As they plummet, they collide with smaller, slowerdroplets and coalesce. Growing larger in the process, they fall

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»f'iI;=i: The collision-coalescence process. A. Most clouddroplets are so small that the motion of the air keeps them suspended.Because large cloud droplets fall more rapidly than smaller droplets,they are able to sweep up the smaller ones in their path and grow.B. As these drops increase in size, their fall velocity increases, resultingin increased air resistance, which causes the raindrop to flatten. C. Asthe raindrop approaches 4 millimeters in size, it develops a depressionin the bottom. D. Finally, when the diameter exceeds about 5 millimeters,the depression grows upward almost explosively, forming a donutlikering of water that immediately breaks into smaller drops. (Note thatthe drops are not drawn to scale—a typical raindrop has a volumeequal to roughly 1 million cloud droplets.)

even more rapidly (or in an updraft, rise more slowly), increasingtheir chances of collision and rate of growth -I. is Aftera great many such collisions they are large enough to fall to thesurface without completely evaporating.

Because of the number of collisions required for growth toraindrop size, droplets in clouds that have great vertical thicknessand abundant moisture have a better chance of reaching therequired size. Updrafts also aid this process, for they allow thedroplets to traverse the cloud repeatedly.

Forms of Precipitation 5'15

Raindrops can grow to a maximum size of 5 millimeters, atwhich point they fall at the rate of 33 kilometers (20 miles) perhour. At this size and speed, the water’s surface tension, whichholds the drop together, is overcome by the drag imposed by theair, which in turn pulls the drops apart. The resulting breakup ofa large raindrop produces numerous smaller drops that beginanew the task of sweeping up cloud droplets. Drops that are lessthan 0.5 millimeter upon reaching the ground are termed drizzleand require about I0 minutes to fall from a cloud 1,000 meters(3,300 feet) overhead.

The collision-coalescence process is not quite as simple asdescribed. First, as the large droplets descend, they produce an airstream around them similar to that produced by a fast-movingautomobile. If an automobile is driven at night and we use thebugs that are often out to be analogous to the cloud droplets, it iseasy to visualize how most cloud droplets are swept aside. Thelarger the cloud droplet (or bug), the better its chance of collidingwith the giant droplet (or car). Second, collision does not guar-antee coalescence. Experiments indicate that atmospheric elec-tricity may hold these droplets together once they collide. If adroplet with a negative charge should collide with a positivelycharged droplet, electrical attraction of opposite charges may bindthem together.

In summary, two mechanisms are known to generate pre-cipitation: the Bergeron process and the collision-coalescenceprocess. The Bergeron process is dominant in the middle latitudeswhere cold clouds (or cold cloud tops) are the rule. In the tropics,abundant water vapor and comparatively few condensationnuclei are the norm. This leads to the formation of fewer, largerdrops with fast fall velocities that grow by collision andcoalescence.

TABLE 17.4 Forms of Precipitation

CONCEPT CHECK 1 7. 80 What is the difference between precipitation and condensa-

tion?Q Describe the Bergeron process in your own words.Q What is meant by supersatm'a.ti0n.?Q What conditions favor the collision-coalescence process?

Forms of PrecipitationMuch of the world’s precipitation begins as snow crystals or othersolid forms, such as hail or graupel (Table 17.4). Entering thewarmer air below the cloud, these ice particles often melt andreach the ground as raindrops. In some parts of the world, par-ticularly the subtropics, precipitation often forms in clouds thatare warmer than 0° C (32° P). These rains frequently occur over theocean where cloud condensation nuclei are not plentiful andthose that exist vary in size. Under such conditions, cloud dropletscan grow rapidly by the collision-coalescence process to producecopious amounts of rain.

Because atmospheric conditions vary greatly from place toplace as well as seasonally, several different forms of precipita-tion are possible. Rain and snow are the most common and famil-iar, but the other forms of precipitation listed in Table 17.4 areimportant as well. The occurrence of sleet, glaze, or hail is oftenassociated with important weather events. Although limited inoccurrence and sporadic in both time and space, these forms,especially glaze and hail, can cause considerable damage.

State ofMatter

Liquid

Type Appropriate Size DescriptionM131; 0.005—0.05 mm

clouds.

Drizzle Less than 0.5 mm LiquidRain 0.5-5 mm Liquid

from one place to another.

Sleet 0.5-5 mm Solid

Droplets large enough to be felt on the face when air is moving 1 meter/second. Associated with stratus

Small uniform drops that fall from stratus clouds, generally for several hours.Generally produced by nimbostratus or cumulonimbus clouds. When heavy, it can show high variability

Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer ofsubfreezing air. Because the ice particles are small, damage, if any, is generally minor. Sleet can maketravel hazardous.

Glaze Layers 1 mm—2 cm Solid_ thick _ _

Rime Variable Solidaccumulations

Snow 1 mm—2 cm Solid

Produced when supercooled raindrops freeze on contact with solid objects. Glaze can form a thickcoating of ice having sufficient weight to seriously damage trees and power lines.Deposits usually consisting of ice feathers that point into the wind. These delicate, frostlikeaccumulations form as supercooled cloud or fog droplets encounter objects and freeze on contact.The crystalline nature of snow allows it to assume many shapes including six-sided crystals, plates, andneedles. Produced in supercooled clouds where water vapor is deposited as ice crystals that remain

Hail 5 mm-10 cm Solidor larger

Graupel 2—5 mm Solid

frozen during their descent.Precipitation in the form of hard, rounded pellets or irregular lumps of ice. Produced in largecumulonimbus clouds, where frozen ice particles and supercooled water coexist.Sometimes called soft hail, graupel forms when rime collects on snow crystals to produce irregularmasses of "soft" ice. Because these particles are softer than hailstones, they normally flatten out uponimpact.

516 CHAPTER 17 Moisture, Clouds, and Precipitation

RainIn meteorology, the term rain is restricted to drops ofwater thatfall from a cloud and have a diameter of at least 0.5 millimeter(0.02 inch). Most rain originates either in nimbostratus clouds orin towering cumulonimbus clouds that are capable ofproducingunusually heavy rainfalls known as cloudbursts. Raindrops rarelyexceed about 5 millimeters (0.2 inch) in diameter. Larger drops donot survive because surface tension, which holds the dropstogether, is exceeded by the frictional drag of the air. Conse-quently, large raindrops regularly break apart into smaller ones.

Fine, uniform drops of water having a diameter less than 0.5millimeter (0.02 inch) are called drizzle. Drizzle can be so finethat the tiny drops appear to float and their impact is almostimperceptible. Drizzle and small raindrops generally are pro-duced in stratus or nimbostratus clouds where precipitation canbe continuous for several hours or, on rare occasions, for days.

SnowSnow is precipitation in the form of ice crystals (snowflakes) or;more often, aggregates of crystals. The size, shape, and concen-tration of snowflakes depend to a great extent on the temperatureat which they form.

Recall that at very low temperatures, the moisture content ofair is small. The result is the formation of very light, fluffy snowmade up of individual six-sided ice crystals. This is the “powder”that downhill skiers talk so much about. By contrast, at tempera-tures warmer than about —5° C (23° F), the ice crystals jointogether into larger clumps consisting of tangled aggregates ofcrystals. Snowfalls composed of these composite snowflakes aregenerally heavy and have a high moisture content, which makesthem ideal for making snowballs.

Sleet and GlazeSleet is a wintertime phenomenon and refers to the fall of smallparticles of ice that are clear to translucent. For sleet to be pro-duced, a layer of air with temperatures above freezing must over-lie a subfreezing layer near the ground. When raindrops, which areoften melted snow, leave the warmer air and encounter the colderair below, they freeze and reach the ground as small pellets of icethe size of the raindrops from which they formed.

Students Sometimes Ask...What places in the United States receive the mostsnowfall?

western United States. Therecord for a single season goesto Mount Baker ski area northof Seattle, Washington, where2,896 centimeters (1,140inches) of snow fell during thewinter of 1998-1999.

Despite the impressive lake-effect snowstorms recorded incities such as Buffalo andRochester, New York, thegreatest accumulations ofsnow generally occur in themountainous regions of the

On some occasions, when the vertical distribution of tem-peratures is similar to that associated with the formation of sleet,freezing rain or glaze results instead. In such situations, the sub-freezing air near the ground is not thick enough to allow the rain-drops to freeze. The raindrops, however, do become supercooledas they fall through the cold air and turn to ice upon colliding withsolid objects. The result can be a thick coating of ice having suf-ficient weight to break tree limbs, down power lines, and makewalking or driving extremely hazardous

HailHail is precipitation in the form ofhard, rounded pellets or irreg-ular lumps of ice. Moreover, large hailstones often consist of aseries ofnearly concentric shells of differing densities and degreesof opaqueness Most hailstones have diameters

>1:-'.i==1i Glaze forms when supercooled raindrops freeze oncontact with objects. In January 1998 an ice storm of historicproportions caused enormous damage in New England andsoutheastern Canada. Nearly 5 days of freezing rain (glaze) leftmillions without electricity—some for as long as a month. (Photo bySyracuse Newspapers/The Image Works)

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Hailstones. A. Hailstones begin as small ice pelletsthat grow by adding supercooled water droplets as they movethrough a cloud. Strong updrafts may carry stones upward in severalcycles, increasing the size of the hail by adding a new layer witheach cycle. Eventually, the hailstones encounter a downdraft or growtoo large to be supported by the updraft. B. This cut hailstone fell atCoffeyville, Kansas, in 1970 and weighed 0.75 kilogram (1.67 pounds).Notice ii2S layered structure. (Photo courtesy of University Corporation forAtmospheric Research/National Science Foundation/NCAR).

between I centimeter (0.4-inch, pea size) and 5 centimeters(2-inch golf ball-size), although some can be as big as an orangeor larger. Occasionally, hailstones weighing a pound or more havebeen reported. Many of these were probably composites of severalstones frozen together.

The record for the largest hailstone ever found in the UnitedStates was set on Iuly 23, 2010, inVivian, South Dakota. The stonewas over 20 centimeters (8 inches) in diameter and weighednearly 900 grams (2 pounds). The stone that held the previousrecord of 766 grams (1.69 pounds) fell in Coffeyville, Kansas, in1970 (Figure l7.30B). The diameter of the stone found in SouthDakota also surpassed the previous record of a 17.8-centimeter(7-inch) stone that fell in Aurora, Nebraska, in 2003. Even largerhailstones have reportedly been recorded in Bangladesh, wherea I987 hailstorm killed more than 90 people. It is estimated that

large hailstones hit the ground at speeds exceeding I60 kilome-ters (100 miles) per hour.

The destructive effects of large hailstones are well known,especially to farmers whose crops have been devastated in a fewminutes and to people whose windows and roofs have been dam-aged ?.3’;l.). In the United States, hail damage each year isin the hundreds of millions of dollars.

Hail is produced only in large cumulonimbus clouds whereupdrafts can sometimes reach speeds approaching I60 kilometers(I00 miles) per hour, and where there is an abundant supply ofsupercooled water. Hailstones begin as small embryonic ice pelletsthat grow by collecting supercooled water droplets as they fallthrough the cloud (Figure 17 .30A). Ifthey encounter a strong updraft,they maybe carried upward again and begin the downward journeyanew. Each trip through the supercooled portion of the cloud maybe represented by an additional layer of ice. Hailstones can also formfrom a single descent through an updraft. Either way, the processcontinues until the hailstone encounters a downdraft or grows tooheavy to remain suspended by the thunderstorm’s updraft.

RimeRime is a deposit of ice crystals formed by the freezing of super-cooled fog or cloud droplets on objects whose surface tempera-ture is below freezing. When rime forms on trees, it adorns themwith its characteristic ice feathers, which can be spectacular tobehold (i.?lr3ui'o In these situations, objects such as pineneedles act as freezing nuclei, causing the supercooled dropletsto freeze on contact. l/Vhen the wind is blowing, only the wind-ward surfaces of objects will accumulate the layer of rime.

CONCEPT cnzcx 1 7. 9Q List the forms ofprecipitation and the circumstances of their

formation.Q Explain why snow can sometimes reach the ground as rain,

but the reverse does not occur.Q How is sleet different from glaze? Which is usually more

hazardous‘?

ii‘}?.l:€i§. Hail damage at an auto dealership in Amarillo, Texas,following a severe thunderstorm in June 2004. (Photo by HenryBargas/Amarillo Globe News/AP Photo)

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i¥‘lt1;ili.i-in il_‘i.fitII§ Rime consists of delicate ice crystals that form whensupercooled fog or cloud droplets freeze on contact with objects.(Photo by Siepman/Photolibrary)

Measuring PrecipitationThe most common form of precipitation—rain—is the easiest tomeasure. Any open container having a consistent cross sectionthroughout can be a rain gauge. In general practice, however,more sophisticated devices are used so that small amounts ofrainfall can be measured accurately and losses from evaporationcan be reduced. The standard rain gauge (.Firle.rn '.l‘iF.3I’l) has adiameter of about 20 centimeters (8 inches) at the top. Once thewater is caught, a funnel conducts the rain into a cylindrical mea-suring tube that has a cross-sectional area only one-tenth as largeas the receiver. Consequently, rainfall depth is magnified 10 times,which allows for accurate measurements to the nearest 0.025 cen-timeter (0.01 inch). The narrow opening also minimizes evapo-ration. When the amount of rain is less than 0.025 centimeter, itis reported as a trace of precipitation.

Measurement ErrorsIn addition to the standard rain gauge, several types of recordinggauges are routinely used. These instruments record not only theamount of rain but also its time of occurrence and intensity(amount per unit of time).

No matter which rain gauge is used, proper exposure is criti-cal. Errors arise when the gauge is shielded from obliquely fallingrain by buildings, trees, or other high objects. Hence, the instru-ment should be at least as far away from such obstructions as theobjects are high. Another cause of error is the wind. It has been

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i?‘§i@E§il;ii~‘iii Tl‘i.i%I'~i Precipitation measurement. The standard rain gaugeallows for accurate rainfall measurement to the nearest 0.025centimeter (0.01 inch). Because the cross-sectional area of themeasuring tube is only one-tenth as large as the collector, rainfall ismagnified 10 times.

shown that with increasingwind and turbulence, it becomes moredifficult to collect a representative quantity of rain.

Measuring SnowfallWhen snow records are kept, two measurements are normallytaken: depth and water equivalent. Usually the depth of snow ismeasured with a calibrated stick. The actual measurement is sim-ple, but choosing a representative spot often poses a dilemma.Even when winds are light or moderate, snow drifts freely. As arule, it is best to take several measurements in an open place awayfrom trees and obstructions and then average them. To obtain thewater equivalent, samples can be melted and then weighed ormeasured as rain.

The quantity of water in a given volume of snow is not con-stant. A general ratio of 10 units of snow to 1 unit ofwater is oftenused when exact information is not available, but the actual watercontent of snow may deviate widely from this figure. It may takeas much as 30 centimeters of light, fluffy dry snow or as little as4 centimeters ofwet snow to produce I centimeter ofwater.

Precipitation Measurementby Weather RadarToday’s TV weathercasts show helpful maps like the one in

rs.-o fi.:§~-4 to depict precipitation patterns. The instrument thatproduces these images is called weather radar.

The development ofradar has given meteorologists an impor-tant tool to probe storm systems that may be up to a few hundred

kilometers away. All radar units have a transmitter that sends outshort pulses of radio waves. The specific wavelengths that are useddepend on the objects the user wants to detect. When radar isused to monitor precipitation, wavelengths between 3 and I0 cen-timeters are employed.

These wavelengths can penetrate small cloud droplets, butare reflected by larger raindrops, ice crystals, or hailstones. Thereflected signal, called an echo, is received and displayed on a TVmonitor. Because the echo is “brighter” when the precipitation ismore intense, modern radar is able to depict not only the regionalextent of the precipitation but also the rate ofrainfall. Figure 17.34is a typical radar display in which colors show precipitation inten-sity. Weather radar is also an important tool for determining therate and direction of storm movement.

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a trace. When this occurs, how much (or little) rain hasfallen?

Q Why is rainfall easier to measure than snowfall?

GIVE IT SOME

1. Refer to Figure 17.2 to complete the following:a. In which state of matter is water densest?b. In which state of matter are water molecules most energetic‘?c. In which state of matter is water compressible?

2. The accompanying photo shows a cup of hot coffee. Vlfhat state of matter IS the steam risingfrom the liquid? Explain your answer.

3. The primary mechanism by which the human body cools itself is perspirationa. Explain how perspiring cools the skin.b. Referring to the data for Phoenix, Arizona, and Tampa, Florida (Table A), in which city

would it be easier to stay cool by perspiring? Explain your choice

Table ACity Temperature Dew point temperature

Phoenix. AZ IOI'F 47'FTampa. FL IOI'F 77'F

Photo by Dmitry Kolmakov/Shutterstock

4. During hot summer weather, many people put “koozies” around their beverages to keep the drinkscold. In addition to preventing a warm hand from heating the container through conduction, whatother mechanism(s) slow the process ofwarming beverages?

5. Refer to Table 17.1 to answer this question. How much more water 1S contained in saturated arr at atropical location with a temperature of 40° C compared to a polar location with a temperature of~10° C?

6. Refer to the data for Phoenix, Arizona, and Bismarck, North Dakota (Table B) Tabje 5to Complete the following: Cr Temperature Dew pomt temperaturea. Which city has a higher relative humidity? p,,oen,x AZ 0 F Fb. Vlfhich city has the greatest quantity ofwater vapor in the air? BISMI‘/< N9 39 F 35 Fc. In which city is the air closest to its saturation point with respect to water

vapor?d. In which city does the air have the greatest holding capacity for water vapor‘?

g 520 CHAPTER 17 Moisture, Clouds, and Precipitation_

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The accompanying graph shows how air temperature and relative humiditychange on a typical summer day in the Midwest. Assuming the dew-pointtemperature remained constant, what would be the best time of day towater a lawn to minimize evaporation of the water spread on the grass?The accompanying diagram shows air flowing from the ocean over a coastalmountain range. Assume that the dew-point temperature remains constant ,

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in dry air (relative humidity less than 100%). If the air parcel becomes satu- j _ _ _ H g _ if --____, grated, the dew-point temperature will cool at the wet adiabatic rate as it (Dew Pol”? T"”lP“"”l“"’eascends, but will not change as the air parcel descends. Use this informa- %”“ 7 I EV‘ W F Sq iftion to complete the following:a. Determine the air temperature and dew-point temperature for

the air parcel at each location shown (B-G) on the diagram.b. At what elevation will clouds begin to form (relative humidity

= 100%)? Q3 kmc. Compare the air temperatures at points A and G. Why are they / \different? C. E-

2 kmd. How did the water vapor content of the air change as the par- _cel of air traversed the mountain? (Hint: Comp are dew-point I km Windward I Leewardtemperatures.)

e. On which side of the mountain might you expect lush vegeta- jA- - _. . . . . Sea Temperature at POIHT A = 27‘C -tion and on which side would you expect desert-like cond1- [eve] Dew ,,O,;,,f femperafure of Pam,» A = )7-C

figns? Dry adiabatic rate = 10:6‘/kmf. Where in the United States might you find such a situation? -------- ~ We’ adiab“?-?qfe : 5C/-kmThe dimensions of the cumulonimbus cloud pictured inFigure l7.2lH are roughly l2 km tall, 8 km wide, and 8 km long. Assume that the droplets in everycubic meter of the cloud total 0.5 cubic centimeter. How much liquid does the cloud contain? Howmany gallons is this (3785 cms = 1 gallon)?Cloud droplets form and grow as water vapor condenses onto a hygroscopic condensation nuclei.Research has shown that the maximum radius for cloud droplets is about 0.05 millimeter. However,typical raindrops have volumes thousands of times greater. How do cloud droplets become rain-drops?Why does radiation fog form mainly on clear nights as opposed to cloudy nights? -, Table CWhich winter storm is likely to produce deeper snowfall: a low-pressure system that passes convemon of radar reflecfimy ,through the Midwestern states of Nebraska, Iowa, and Illinois (26° F average temperature at to rainfall ratethe time of the storm) or the same exact system passing through North Dakota, Minnesota, l Radar Reflectivity Rainfall Rateand Wisconsin (l6° F average temperature at the time of the storm). (“'52) ("'”ches/hf) 1Weather radar provides information on the intensity of precipitation in addition to the total j 65 if;amount of precipitation that falls over a given time period. Table C shows the relationshipbetween radar reflectivity values and rainfall rates. If radar measured a reflectivity value of [347 dBZ for 2% hours over a location, how much rain will have fallen there? l ,What are the advantages and disadvantages of rain gauges compared to , weather radar in 'measuring rainfall?

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Water vapor, an odorless, colorless gas, changes from onestate of matter (solid, liquid, or gas) to another at the temper-atures and pressures experienced near Earth’s surface. Theprocesses involved are evaporation, condensation, melting,freezing, sublimation, and deposition. During each change,latent (hidden) heat is either absorbed or released.Humidity is the general term to describe the amount of watervapor in the air. The methods used to express humidity quan-titatively include (1) mixing ratio, the mass of water vapor in aunit of air compared to the remaining mass of dry air; (2)vapor pressure, that part of the total atmospheric pressureattributable to its water-vapor content; (3) relative humidity,the ratio of the air’s actual water-vapor content compared tothe amount ofwater vapor required for saturation at that tem-perature; and (4) dew point, the temperature to which a parcelof air would need to be cooled to reach saturation. X/Vhen air issaturated, the pressure exerted by the water vapor, called thesaturation vapor pressure, produces a balance between thenumber of water molecules leaving the surface of the waterand the number returning. Because the saturation vapor pres-sure is temperature-dependent, at higher temperatures morewater vapor is required for saturation to occur.Relative humidity can be changed in two ways. One is byadding or subtracting water vapor: The second is by changingthe airs temperature. When air is cooled, its relative humidityincreases.The cooling of air as it rises and expands owing to succes-sively lower air pressure is the basic cloud-forming process.Temperature changes in air brought about by compressing orexpanding the air are called adiabatic temperature changes.Unsaturated air warms by compression and cools by exp an-sion at the rather constant rate of 10° C per 1,000 meters ofaltitude change, a figure called the dry adiabatic rate. If airrises high enough, it will cool sufficiently to cause condensa-tion and form a cloud. From this point on, air that continuesto rise will cool at the wet adiabatic rate, which varies from5° C to 9° C per 1,000 meters of ascent. The difference in thewet and dry adiabatic rates is caused by the condensing watervapor releasing latent heat, thereby reducing the rate at whichthe air cools.Pour mechanisms that can initiate the vertical movement ofair are (1) orographic lifting, which occurs when elevated ter-rains, such as mountains, act as barriers to the flow of air;(2) frontal wedging, when cool air acts as a barrier over whichwarmer, less dense air rises; (3) convergence, which happens

In Review Chapter 17 Moisture, Clouds and Precipitationwhen air flows together and a general upward movement ofair occurs; and (4) localized convective lzfifing, which occurswhen unequal surface heating causes pockets of air to risebecause of their buoyancy.The stability ofair is determined by examining the tempera-ture of the atmosphere at various altitudes. Air is said to beunstable when the environmental lapse rate (the rate of tem-perature decrease with increasing altitude in the tropo-sphere) is greater than the dry adiabatic rate. Stateddifferently, a column of air is unstable when the air near thebottom is significantly warmer (less dense) than the air aloft.For condensation to occur, air must be saturated. Saturationtakes place either when air is cooled to its dew point, whichmost commonly happens, or when water vapor is added tothe air. There must also be a surface on which the water vaporcan condense. In cloud and fog formation, tiny particlescalled condensation nuclei serve this purpose.Clouds are classified on the basis of their appearance andheight. The three basic forms are cirrus (high, white, thin,wispy fibers), cumulus (globular, individual cloud masses),and stratus (sheets or layers that cover much or all of the sky).The four categories based on height are high. clouds (basesnormally above 6,000 meters), middle clouds (from 2,000 to6,000 meters), low clouds (below 2,000 meters), and clouds ofvertical development.Fog is defined as a cloud with its base at or very near theground. Fogs form when air is cooled below its dew point orwhen enough water vapor is added to the air to bring aboutsaturation. Various types of fog include advectionfog, radi-ationfog, upslopefog, steamfog, andfrontal (or precipitation)fes-For precipitation to form, millions of cloud droplets mustsomehow join together into large drops. Two mechanisms forthe formation of precipitation have been proposed. (1) Inclouds where the temperatures are below freezing, ice crys-tals form and fall as snowflakes. At lower altitudes thesnowflakes melt and become raindrops before they reach theground. (2) Large droplets form in warm clouds that containlarge hygroscopic (“water-seeking”) nuclei, such as salt parti-cles. As these big droplets descend, they collide and join withsmaller water droplets. After many collisions, the droplets arelarge enough to fall to the ground as rain.The forms of precipitation include rain, snow, sleet, freezingrain (glaze), hail, and rime.

522 CHAPTER 17 Moisture, Clouds, and Precipitation

Kev Termsabsolute instability (p. 504) fog (p. 510)absolute stability (p. 503) freezing nuclei (p. 5 13)adiabatic temperature change (p. 499) front (p. 500)advection fog (p. 510) frontal fog (p. 512)Bergeron process (p. 513) frontal wedging (p. 500)calorie (p. 490) glaze (p. 516)cirrus (p. 506) hail (p. 516)cloud (p. 506) high cloud (p. 506)cloud ofvertical development (p. 508) humidity (p. 492)collision-coalescence process (p. 514) hygrometer (p. 496)condensation (p. 492) hygroscopic nuclei (p. 506)condensation nuclei (p. 506) latent heat (p. 491)conditional instability (p. 504) localized convective lifting (p. 501)convergence (p. 500) low cloud (p. 506)cumulus (p. 506) middle cloud (p. 506)deposition (p. 492) mixing ratio (p. 493)dew-point temperature (p. 495) orographic lifting (p. 500)dry adiabatic rate (p. 499) parcel (p. 499)evaporation (p. 491) precipitation fog (p. 512)

Examining the Earth SystemThe interrelationships among Earth’s spheres have producedthe Great Basin area of the western United States, whichincludes some of the driest areas in the world. Examine themap of the region in Appendix D. Although the area is only afew hundred miles from the Pacific Ocean, it is a desert.1/Vhy? Did any geologic factor(s) contribute to the formationof this desert environment? Do any major rivers have theirsource in the Great Basin? Explain. (For information aboutdeserts in the United States, visit the United States Geologi-cal Survey [USGS] Deserts: Geology and Resources Websiteat http: //pubs.usgs.gov/gip/deserts/ .)

Phoenix and Flagstaff, Arizona, are located nearby eachother in the southwestern United States. Using the climatediagrams for the two cities found in Figure 20.16, describethe impact that elevation has on the precipitation and tem-

psychrometer (p. 496)radiation fog (p. 510)rain (p. 516)rain shadow desert (p. 500)relative humidity (p. 493)rime (p. 517)saturation (p. 492)sleet (p. 516)snow (p. 516)stable air (p. 502)steam fog (p. 511)stratus (p. 506)sublimation (p. 492)supercooled (p. 513)supersaturation (p. 513)unstable air (p. 502)upslope fog (p. 511)vapor pressure (p. 492)wet adiabatic rate (p. 499)

perature of each city. Compare the natural vegetation ofthese nearby locations. (To examine the current weatherconditions and forecasts for Phoenix and Flagstaff, Arizona,contact USA Today at http://www.usatoday.com/weather/and/or The Weather Channel at http://www.weather.com.)

The amount of precipitation that falls at any particular placeand time is controlled by the quantity of moisture in the airand many other factors. How might each of the followingalter the precipitation at a particular locale? (a) An increasein the elevation of the land. (b) A decrease in the area cov-ered by forests and other types ofvegetation. (c) Lowering ofaverage ocean-surface temperatures. (d) An increase in thepercentage of time that the winds blow from an adjacentbody ofwater. (e) A major episode of global volcanism last-ing for decades.

Mastering Geology 523

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0 GEODe: Earth Science: An interactive visual wallcthrough ofkey concepts

Geoscience Animation Library: More than 100 animationsilluminating many difficult-to-understand Earth scienceconceptsIn The News RSS Feeds: Current Earth science events andnews articles are pulled into the site with assessmentPearson eTextOptional Self Study QuizzesWeb LinksGlossaryFlashcards


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