The Water CycleEcclesiastes 1:7 All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again

Fig. 4-1, p.78

Structure of WaterStructure of Water

Water's unique molecular structure and hydrogen bonds enable Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.all 3 phases to exist in earth's atmosphere.

Sublimation & deposition describe the non-incremental changes Sublimation & deposition describe the non-incremental changes between solid and vapor phases.between solid and vapor phases.

Figure 5.1Figure 5.1

Figure 5.2Figure 5.2

Evaporating Water into AirEvaporating Water into Air

Figure 5.3AFigure 5.3A

Evaporation:departures of Evaporation:departures of water molecules from its water molecules from its surfacesurfaceCondensation : arrivals of Condensation : arrivals of molecules from adjacent molecules from adjacent vaporvapor

When air is saturated, When air is saturated, evaporation and evaporation and condensation are in condensation are in equilibrium.equilibrium.

NRG is taken from the environment during evaporation and NRG is released to the environment by condensation

‘‘Latent Heat’ is the term that describes the NRG transferred/stored Latent Heat’ is the term that describes the NRG transferred/stored

by water changing phaseby water changing phase

Water in the atmosphere…Water in the atmosphere…

Absolute & Specific HumidityAbsolute & Specific Humidity

For a given mass of water vapor For a given mass of water vapor in an air parcel, the absolute in an air parcel, the absolute humidity humidity changes changes as the parcel as the parcel volume changes (e.g., lifts or volume changes (e.g., lifts or descends).descends).

Specific humidity is concerned Specific humidity is concerned with the mass of vapor to mass of with the mass of vapor to mass of air, and air, and is not affected by changes is not affected by changes in parcel volumein parcel volume..

Absolute Humidity: mass of water vapor/volume of air

Specific Humidity: mass of water vapor/total mass of air

Mixing ratio:Mixing ratio:

Mass of water vapor/mass of dry airMass of water vapor/mass of dry air g/kgg/kg Mixing ratio and specific humidity do not Mixing ratio and specific humidity do not

change as long as water vapor is not added change as long as water vapor is not added or subtracted from the parcel of air.or subtracted from the parcel of air.

Specific Humidity vs. SaturationSpecific Humidity vs. Saturation

Figure 5.9Figure 5.9

Warm air can absorb more Warm air can absorb more vapor than cold air.vapor than cold air.

For a given parcel of air, For a given parcel of air, specific humidity declines specific humidity declines from its highest in the from its highest in the tropics to its lowest in the tropics to its lowest in the colder poles.colder poles.

Determining Vapor PressureDetermining Vapor Pressure

Average atmospheric pressure Average atmospheric pressure of 1013 mb is comprised in part of 1013 mb is comprised in part by the weight of vapor by the weight of vapor molecules.molecules.

Warmer air can absorb more Warmer air can absorb more vapor than cooler air before it vapor than cooler air before it saturates.saturates.

Figure 5.10Figure 5.10

Relative HumidityRelative Humidity

Water vapor/capacity to hold water vaporWater vapor/capacity to hold water vapor This is a measure of how much water vapor the air This is a measure of how much water vapor the air

is holding divided by the amount of water vapor is holding divided by the amount of water vapor the a given parcel of air CAN hold.the a given parcel of air CAN hold.

Expressed as a percentageExpressed as a percentage Changes in RH occur by changing the water vapor Changes in RH occur by changing the water vapor

content or changing the temperature of the air content or changing the temperature of the air parcel parcel

Relative Humidity TrendsRelative Humidity Trends

Relative humidity (RH) indicates air parcel proximity to saturation.Relative humidity (RH) indicates air parcel proximity to saturation.

Saturation can be achieved, or RH increased, by adding more Saturation can be achieved, or RH increased, by adding more water or dropping the air temperature.water or dropping the air temperature.

Dew point is the temperature at which saturation occurs.Dew point is the temperature at which saturation occurs.

Figure 5.11Figure 5.11

Seasonal Dew Point MapsSeasonal Dew Point Maps

January, July, dew pointJanuary, July, dew point

Dew Point vs. Relative HumidityDew Point vs. Relative Humidity

Dew point is the temperature for Dew point is the temperature for saturation, and used with a vapor saturation, and used with a vapor pressure curve reveals the mass of pressure curve reveals the mass of vapor in the air.vapor in the air.

While relative humidity may be While relative humidity may be higher in polar air, more water is higher in polar air, more water is actually absorbed in desert air actually absorbed in desert air (warm air holds more water).(warm air holds more water).

Relative and Specific HumidityRelative and Specific Humidity

Relative humidity (RH) as an Relative humidity (RH) as an indicator of saturation reveals indicator of saturation reveals that desert air is far from that desert air is far from saturated, and that cold polar saturated, and that cold polar air nears saturation.air nears saturation. Graphs of RH contrast with Graphs of RH contrast with specific humidity in the deserts specific humidity in the deserts and poles.and poles.

Figure 5.14Figure 5.14

Figure 5.9Figure 5.9

Sources of MoistureSources of Moisture

Patterns of US humidity are strongly governed by wind direction Patterns of US humidity are strongly governed by wind direction and ocean temperatures.and ocean temperatures. Cooler Pacific waters create lower humidities in the west, while Cooler Pacific waters create lower humidities in the west, while warmer Gulf waters generate high humidity along the southeast and warmer Gulf waters generate high humidity along the southeast and east coast.east coast.

Figure 5.15Figure 5.15

Relative Humidity and ComfortRelative Humidity and Comfort

Unsaturated air may absorb more water from the Unsaturated air may absorb more water from the evaporation of human sweat.evaporation of human sweat.

The departure of fast moving, and by definition higher The departure of fast moving, and by definition higher temperature, water molecules into the vapor phase cools temperature, water molecules into the vapor phase cools the human skin.the human skin.

In winter, this process can make a dry house extra In winter, this process can make a dry house extra chilly.chilly.

Heat Index & SafetyHeat Index & Safety

Figure 5.16Figure 5.16

Human perception of Human perception of temperature is distinct temperature is distinct from measured air from measured air temperature, and is temperature, and is particularly different particularly different at higher humidities at higher humidities when the human body when the human body is less efficient at is less efficient at sweating and self-sweating and self-cooling.cooling.

On hot days, fans that On hot days, fans that move saturated air move saturated air away from the skin away from the skin help humans avoid help humans avoid unwanted heat unwanted heat syndromes.syndromes.

Measuring Relative HumidityMeasuring Relative Humidity

The simplest tool is the sling psycrometerThe simplest tool is the sling psycrometer This tool uses two thermometersThis tool uses two thermometers A standard scale is provided in your text in A standard scale is provided in your text in

AppendixDAppendixDOne is dry the other has a small ‘sock’ a cloth cover One is dry the other has a small ‘sock’ a cloth cover

that is soaked with waterthat is soaked with waterWhen the instrument is spun around, liquid water When the instrument is spun around, liquid water

evaporates from the ‘sock’ lowering the evaporates from the ‘sock’ lowering the temperature… the amount of water that evaporates temperature… the amount of water that evaporates indicates the amount of water that CAN indicates the amount of water that CAN evaporate… thus indicating the relative humidity.evaporate… thus indicating the relative humidity.

Sling PsychrometerSling Psychrometer

Figure 5.17Figure 5.17

Wet bulb temperature indicates how cool a surface will become by Wet bulb temperature indicates how cool a surface will become by evaporating water into the air, and when compared with the dry evaporating water into the air, and when compared with the dry bulb, or regular, air temperature it indicates relative humidity.bulb, or regular, air temperature it indicates relative humidity.

These two temperatures are measured by this instrument.These two temperatures are measured by this instrument.

Hair & Other HygrometersHair & Other Hygrometers

Human and horse hair becomes roughly 2.5% shorter as relative Human and horse hair becomes roughly 2.5% shorter as relative humidity drops from 100% to 0%, which is the principle operating humidity drops from 100% to 0%, which is the principle operating the hair hygrometer.the hair hygrometer. Other hygrometers are based on electrical resistance, infrared Other hygrometers are based on electrical resistance, infrared absorption, and dew point condensation.absorption, and dew point condensation.

Figure 5.18Figure 5.18

Formation of Dew & FrostFormation of Dew & Frost

As air cools to its saturation, or dew point, vapor molecules slow As air cools to its saturation, or dew point, vapor molecules slow down and can adhere as dew on the ground surface or as frost down and can adhere as dew on the ground surface or as frost when air temperature drops below freezing.when air temperature drops below freezing.

Daily temperature lows often occur by radiational cooling, Daily temperature lows often occur by radiational cooling, forming dew at night or early morning.forming dew at night or early morning.

Figure 6.1Figure 6.1Figure 6.2Figure 6.2

Haze & Water Seeking NucleiHaze & Water Seeking Nuclei

Above the ground surface, cooling and slowing water vapor instead Above the ground surface, cooling and slowing water vapor instead condenses upon condensation nuclei.condenses upon condensation nuclei.Hygroscopic particles such as salt or dust seek condensing vapor, Hygroscopic particles such as salt or dust seek condensing vapor, and can form a wet white haze when relative humidity is above and can form a wet white haze when relative humidity is above 75%, or a dry blue haze when drier.75%, or a dry blue haze when drier.

Figure 6.3Figure 6.3

Fog & Human SafetyFog & Human Safety

Figure 6.8Figure 6.8

Foggy days in the US Foggy days in the US have a predictable have a predictable distribution due to distribution due to ocean and mountain ocean and mountain influences.influences.

Fog can help crop Fog can help crop growth in California, growth in California, but can also cause but can also cause severe automobile, severe automobile, airplane, and boating airplane, and boating accidents.accidents.

As a result, there are As a result, there are several fog dispersal several fog dispersal experiments.experiments.

Condensation & Moisture SourcesCondensation & Moisture Sources

Figure 6.6Figure 6.6

Fog that is Fog that is filtered by tree filtered by tree branches, or branches, or condensation of condensation of vapor directly vapor directly onto vegetation onto vegetation or the ground or the ground surface, surface, provides an provides an important water important water source for source for ecological ecological processes.processes.

FOG DRIP

Ground Based Radiation FogGround Based Radiation Fog

Figure 6.4Figure 6.4

Fog is condensed vapor droplets at a density that severely restricts Fog is condensed vapor droplets at a density that severely restricts visibility.visibility.It may form by radiation fog, which occurs at the ground when dew It may form by radiation fog, which occurs at the ground when dew point temperature is reached by radiational cooling.point temperature is reached by radiational cooling.Acid fog threatens humans health because the droplets combine Acid fog threatens humans health because the droplets combine with gaseous pollutants.with gaseous pollutants.

Advection FogAdvection Fog

Warm moist air that moves, or advects, above a cold surface may Warm moist air that moves, or advects, above a cold surface may become cooled to its dew point temperature, creating an advection become cooled to its dew point temperature, creating an advection fog.fog.This fog often forms above the ocean due to mixing currents, or This fog often forms above the ocean due to mixing currents, or when warm ocean air rolls into the cooler waters at the Pacific when warm ocean air rolls into the cooler waters at the Pacific Coastline.Coastline.

Figure 6.5Figure 6.5

Evaporation or Mixing FogEvaporation or Mixing Fog

Fog can form by Fog can form by mixing warm mixing warm unsaturated air unsaturated air with cool with cool unsaturated air, unsaturated air, which can occur which can occur during evaporation.during evaporation.

Steam fog is on Steam fog is on example of this example of this mixing process and mixing process and occurs when warm occurs when warm pools of water are a pools of water are a source for vapor source for vapor that condense into that condense into the cooler air abovethe cooler air above..

Figure 6.7Figure 6.7

Cloud Groups & TypesCloud Groups & Types

Clouds are water droplets suspended in the Clouds are water droplets suspended in the atmosphere.atmosphere.

Clouds are grouped by their elevation as high, middle, Clouds are grouped by their elevation as high, middle, low, and those that vertically stretch across many low, and those that vertically stretch across many altitudes.altitudes.

There are several cloud types in these 4 groups.There are several cloud types in these 4 groups.

Condensation onto NucleiCondensation onto NucleiCondensation of Condensation of water vapor into water vapor into liquid water is more liquid water is more likely to occur when likely to occur when the vapor cools and the vapor cools and slows, and attaches to slows, and attaches to nuclei.nuclei.

Heated vapor moves Heated vapor moves so fast that it bounces so fast that it bounces away from away from condensation nucleicondensation nuclei

Figure 5.4AFigure 5.4A

Common condensation nuclei in the atmosphere: dust, salt, smoke (acid rain)

Cooling the Air….Cooling the Air….

Cool air can carry less moistureCool air can carry less moisture When air is cooled, water vapor condensesWhen air is cooled, water vapor condenses Condensation forms clouds and is the source of Condensation forms clouds and is the source of

precipitation.precipitation. When air is lifted, it cools.When air is lifted, it cools. Sources of lifting:Sources of lifting:

– Orographic liftOrographic lift– Frontal liftingFrontal lifting– Convective liftingConvective lifting– Convergent liftingConvergent lifting

Orographic Lift

Jet ContrailsJet Contrails

Figure 6.25Figure 6.25

Jet engine Jet engine exhaust provides exhaust provides vapor and nuclei vapor and nuclei for condensation for condensation trails (contrails), trails (contrails), which evaporate which evaporate quickly in dry quickly in dry air, but linger air, but linger with higher with higher relative relative humidities.humidities.

P-51 Mustangs of the 325th Fighter Group Fly top cover for B-24s en route to targets in German occupied Europe (photo by John Gaston)

The much faster fighters flew long curving flight paths in order to stay back with the bombers they were tasked to escort.

Summary of Cloud TypesSummary of Cloud Types

Figure 6.20Figure 6.20

Stratocumulus CloudsStratocumulus Clouds

Low clouds with rounded patches that range in color from light to Low clouds with rounded patches that range in color from light to dark gray.dark gray.With your hand extended overhead, they are about the size of your With your hand extended overhead, they are about the size of your palm.palm.

Figure 6.15Figure 6.15

Cirrus CloudsCirrus Clouds

High clouds (above 6000 m in middle High clouds (above 6000 m in middle latitudes) that are thin and wispy latitudes) that are thin and wispy and comprised mostly of ice crystals.and comprised mostly of ice crystals.

Figure 6.9Figure 6.9

Cirrocumulus CloudsCirrocumulus Clouds

Figure 6.10Figure 6.10

High clouds High clouds that are that are rounded rounded puffs, puffs, possibly in possibly in rows, are rows, are less common less common than cirrus.than cirrus.

Cirrostratus CloudsCirrostratus Clouds

High clouds High clouds that thinly that thinly cover the cover the entire sky entire sky with ice with ice crystals.crystals.

Light Light passing passing through through these these crystals may crystals may form a halo.form a halo.

Figure 6.11Figure 6.11

Altocumulus CloudsAltocumulus Clouds

Figure 6.12Figure 6.12

Middle clouds Middle clouds (between 2000 (between 2000 and 7000m in and 7000m in middle latitudes) middle latitudes) that are puffy that are puffy masses of white masses of white with gray edges.with gray edges.

With your hand With your hand overhead, they overhead, they are about the are about the size of your size of your fingernail.fingernail.

Altostratus CloudsAltostratus Clouds

Middle Middle clouds that clouds that cover the cover the entire sky entire sky and may and may create a create a dimly visible dimly visible or watery or watery sun and sun and diminish diminish formation of formation of shadows.shadows.

Figure 6.13Figure 6.13

Nimbostratus CloudNimbostratus Cloud

Figure 6.14Figure 6.14

Low clouds Low clouds (below 2000m) (below 2000m) with with precipitation precipitation that reaches the that reaches the ground.ground.

Shredded parts Shredded parts of these clouds of these clouds are called are called stratus fractus stratus fractus or scud.or scud.

Stratus CloudsStratus Clouds

Figure 6.16Figure 6.16

Low clouds that resembles a fog, but does not reach the ground, and Low clouds that resembles a fog, but does not reach the ground, and can generate a light mist or drizzle.can generate a light mist or drizzle.

Cumulus Humilis CloudsCumulus Humilis Clouds

Clouds with vertical development that take a variety of shapes, Clouds with vertical development that take a variety of shapes, separated by sinking air and blue sky.separated by sinking air and blue sky.

Shredded sections are called cumulus fractus.Shredded sections are called cumulus fractus.

Figure 6.17Figure 6.17

Cumulus Congestus CloudsCumulus Congestus Clouds

Figure 6.18Figure 6.18

Clouds with vertical development that become larger in height, with Clouds with vertical development that become larger in height, with tops taking a ragged shape similar to cauliflower.tops taking a ragged shape similar to cauliflower.

Pileus CloudPileus Cloud

Figure 6.23Figure 6.23

An unusual An unusual cloud that cloud that forms above forms above a building a building cumulus by cumulus by deflected deflected moist winds.moist winds.

Mammatus CloudsMammatus Clouds

An unusual cloud that hang like sacks, formed by sinking air with a An unusual cloud that hang like sacks, formed by sinking air with a high water content.high water content.

Figure 6.24Figure 6.24

Nacreous CloudsNacreous Clouds

An unusual cloud best viewed at winter in the poles and forms in the An unusual cloud best viewed at winter in the poles and forms in the stratosphere.stratosphere.

Figure 6.26Figure 6.26

Noctilucent CloudsNoctilucent Clouds

Figure 6.27Figure 6.27

An unusual wavy cloud that is best viewed at the poles and forms in An unusual wavy cloud that is best viewed at the poles and forms in the upper mesosphere.the upper mesosphere.

Ground based Viewing of CloudsGround based Viewing of Clouds

Clouds directly overhead will appear to be less densely packed than Clouds directly overhead will appear to be less densely packed than clouds at the horizon due to viewing angles.clouds at the horizon due to viewing angles.

Figure 6.28Figure 6.28

Lenticular CloudsLenticular Clouds

Figure 6.21Figure 6.21

An unusual An unusual cloud that cloud that has a lens has a lens shape and shape and forms in the forms in the crest of a crest of a wave.wave.

A lenticular cloud that forms downwind of a mountain peak and is A lenticular cloud that forms downwind of a mountain peak and is regularly replenished by condensing water vapor.regularly replenished by condensing water vapor.

Figure 6.22Figure 6.22

Photo by Chris Terry