moisture clouds and precipitation
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DESCRIPTIONChapter 17 of Earth Science 13th Edion by Tarbuck and Lutgens
Waters Changes of State 491
i?iIttll1i'. 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 meanshidden, 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 reducedhence 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 skinhence, 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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Water molecules in iceare bonded together ina configuration that has
lots of empty space,which accounts for icebeing less dense than
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too 6 *> Q GAS6 o . iQO6_ Ga? water vapor)
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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 17.2sublimationand 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 orhoarrost 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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- "15 *3. "e%-\,. -.. _.' '*;'t\2T;~*.-_Q '-, "_--_= ~.!.-:L.:.t-;;-."la:Z-lit: I11 P Moisture and Cloud Formation
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)l ) 0 1rib- Q ill-C3
.13 BT22) _ 0-3p -20(-4) ovs
-10(14) 20 (32) 3.55l41) 5
l0l50) ll? 7l5l59) 10zolss) 1425)9) C O 2030 (86) 26.5sslss) 3540 (104) 4?
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 humidit