Meteo 3: Chapter 16 Winter Weather Read pages 619-630, 632-640, 643-648.
Meteo 3: Chapter 4
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Transcript of Meteo 3: Chapter 4
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