Meteo 3: Chapter 4Meteo 3: Chapter 4

Water Vapor and CloudsWater Vapor and Clouds

Read Chapter 4Read Chapter 4

Who Cares About Water Anyways?Who Cares About Water Anyways?

Phase changes of water important for Phase changes of water important for energy transport in atmosphereenergy transport in atmosphere

Clouds!Clouds! Precipitation! Precipitation!

OverviewOverview

Cloud:Cloud: Collection of liquid water drops or ice Collection of liquid water drops or ice crystalscrystals

Clouds form asClouds form as– 1) Water vapor condenses onto small particles known 1) Water vapor condenses onto small particles known

as condensation nuclei to form liquid water drops, oras condensation nuclei to form liquid water drops, or– 2) Water vapor deposits onto small particles known as 2) Water vapor deposits onto small particles known as

ice nuclei that allow for ice crystal formation ice nuclei that allow for ice crystal formation

In a cloud, water can be present in all three In a cloud, water can be present in all three phases at the same timephases at the same time

Hydrologic CycleHydrologic Cycle Global precipitation = Global evaporationGlobal precipitation = Global evaporation

TermsTerms

Evaporation:Evaporation: liquid water molecules break bonds liquid water molecules break bonds with other molecules to escape to gaseous phasewith other molecules to escape to gaseous phase

Condensation:Condensation: Water vapor returns to liquid state Water vapor returns to liquid state Sublimation:Sublimation: Ice changes directly to water vapor Ice changes directly to water vapor Deposition:Deposition: Water vapor changes directly to ice Water vapor changes directly to ice Transpiration:Transpiration: Plants releasing water vapor into Plants releasing water vapor into

airair

Temperature and EvaporationTemperature and Evaporation

Evaporation occurs when liquid water Evaporation occurs when liquid water molecules gain enough kinetic energy to molecules gain enough kinetic energy to break bondsbreak bonds– The higher the temperature of water, the higher The higher the temperature of water, the higher

the kinetic energy of its molecules, thus the the kinetic energy of its molecules, thus the higher the evaporation ratehigher the evaporation rate

Evaporation is a cooling processEvaporation is a cooling process

Balance of Evaporation & CondensationBalance of Evaporation & Condensation

Net Condensation: Net Condensation: Condensation exceeds Condensation exceeds evaporationevaporation

Net Evaporation: Net Evaporation: Evaporation exceeds Evaporation exceeds condensationcondensation

Vapor Pressure:Vapor Pressure: Water vapor’s contribution to Water vapor’s contribution to total pressuretotal pressure

Equilibrium Vapor Pressure & TemperatureEquilibrium Vapor Pressure & Temperature

Birth of CloudsBirth of Clouds

Relative Humidity: (Relative Humidity: (vapor pressure / equilibrium vapor pressure / equilibrium vapor pressure) * 100 vapor pressure) * 100 – At saturation (rate of condensation = rate of At saturation (rate of condensation = rate of

evaporation), RH = 100%evaporation), RH = 100%– Clouds form when RH exceeds 100% by a few tenths of Clouds form when RH exceeds 100% by a few tenths of

a percenta percent– Water vapor condenses onto CCN…some hygroscopic, Water vapor condenses onto CCN…some hygroscopic,

meaning they attract water vapormeaning they attract water vapor– In summer, when RH exceeds 80%, net condensation In summer, when RH exceeds 80%, net condensation

occurs on some particles (pollution), leading to haze…occurs on some particles (pollution), leading to haze…associated with poor air qualityassociated with poor air quality

Mechanisms to induce cloud formationMechanisms to induce cloud formation

For clouds to form, there must be net For clouds to form, there must be net condensationcondensation

We can get this by cooling the airWe can get this by cooling the air– As temperature lowers, molecular speeds As temperature lowers, molecular speeds

decrease, and water vapor gathers near CCNdecrease, and water vapor gathers near CCN– The amount of cooling needed is related to ratio The amount of cooling needed is related to ratio

of vapor pressure / equilibrium vapor pressure of vapor pressure / equilibrium vapor pressure (RH)(RH)

Fog Formation by Cooling AirFog Formation by Cooling Air

Cooling via liftingCooling via lifting

Air pressure (density) Air pressure (density) decreases with heightdecreases with height

Rising air parcels Rising air parcels expand, cooling as expand, cooling as they do work on they do work on environmentenvironment

If vapor pressure > If vapor pressure > equilibrium vapor equilibrium vapor pressure => pressure => condensationcondensation

Clouds due to LiftingClouds due to Lifting

Orographic Lifting: Lifting by TerrainOrographic Lifting: Lifting by Terrain

Windward side of mountain, facing prevailing wind, Windward side of mountain, facing prevailing wind, is extremely wetis extremely wet

Leeward side, sheltered from wind, very dry…Leeward side, sheltered from wind, very dry…known as rain shadowknown as rain shadow

Clouds due to TerrainClouds due to Terrain

Orographic Lifting: CaliforniaOrographic Lifting: California

Mixing Warm & Cold Air MassesMixing Warm & Cold Air Masses

Assessing Air’s Moisture ContentAssessing Air’s Moisture Content Problems with RH because denominator depends Problems with RH because denominator depends

on temperatureon temperature– Cold, dry air masses can have a high RH, even if they Cold, dry air masses can have a high RH, even if they

hold little water vaporhold little water vapor– Relative humidity varies with time of dayRelative humidity varies with time of day– http://http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.htmlprofhorn.meteor.wisc.edu/wxwise/relhum/rhac.html

Dew Point: Absolute measure of water vaporDew Point: Absolute measure of water vapor

Dew Point: Dew Point: Temperature air must be cooled (at Temperature air must be cooled (at constant pressure) to reach saturationconstant pressure) to reach saturation– Less than or equal to temperature Less than or equal to temperature – Higher the dew point, more water vapor in airHigher the dew point, more water vapor in air– Frost point if air temperature below 32ºFFrost point if air temperature below 32ºF– Measured with a hygrometer or sling psychrometerMeasured with a hygrometer or sling psychrometer– Changes by evaporating water into air, mixing drier air Changes by evaporating water into air, mixing drier air

from above, wind blowing in moist or dry air from from above, wind blowing in moist or dry air from another region (air dries behind cold front, moistens another region (air dries behind cold front, moistens before cold front)before cold front)

Applying Dew Point to Weather ForecastingApplying Dew Point to Weather Forecasting

1) Cloud Base Height1) Cloud Base Height– Temperature of rising air decreases faster than Temperature of rising air decreases faster than

dew point…has a decent chance of eventually dew point…has a decent chance of eventually reaching dew pointreaching dew point

2) First-Guess Low Temperature2) First-Guess Low Temperature 3) Severe Weather3) Severe Weather

– High dew points indicate enhanced riskHigh dew points indicate enhanced risk