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Meteo 3: Chapter 4
Water Vapor and Clouds
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Who Cares About Water Anyways?
 Phase changes of water important for
energy transport in atmosphere
 Clouds!
 Precipitation!
Overview
 Cloud: Collection of liquid water drops or ice
crystals
 Clouds form as
– 1) Water vapor condenses onto small particles known
as condensation nuclei to form liquid water drops, or
– 2) Water vapor deposits onto small particles known as
ice nuclei that allow for ice crystal formation
 In a cloud, water can be present in all three
phases at the same time
Hydrologic Cycle
 Global precipitation = Global evaporation
Terms
 Evaporation: liquid water molecules break bonds
with other molecules to escape to gaseous phase
 Condensation: Water vapor returns to liquid state
 Sublimation: Ice changes directly to water vapor
 Deposition: Water vapor changes directly to ice
 Transpiration: Plants releasing water vapor into
air
Temperature and Evaporation
 Evaporation occurs when liquid water
molecules gain enough kinetic energy to
break bonds
– The higher the temperature of water, the higher
the kinetic energy of its molecules, thus the
higher the evaporation rate
 Evaporation is a cooling process
Balance of Evaporation & Condensation
 Net Condensation: Condensation exceeds
evaporation
 Net Evaporation: Evaporation exceeds
condensation
 Vapor Pressure: Water vapor’s contribution to
total pressure
Equilibrium Vapor Pressure & Temperature
Birth of Clouds
 Relative Humidity: (vapor pressure / equilibrium
vapor pressure) * 100
– At saturation (rate of condensation = rate of
evaporation), RH = 100%
– Clouds form when RH exceeds 100% by a few tenths of
a percent
– Water vapor condenses onto CCN…some hygroscopic,
meaning they attract water vapor
– In summer, when RH exceeds 80%, net condensation
occurs on some particles (pollution), leading to
haze…associated with poor air quality
Mechanisms to induce cloud formation
 For clouds to form, there must be net
condensation
 We can get this by cooling the air
– As temperature lowers, molecular speeds
decrease, and water vapor gathers near CCN
– The amount of cooling needed is related to ratio
of vapor pressure / equilibrium vapor pressure
(RH)
Fog Formation by Cooling Air
Cooling via lifting
 Air pressure (density)
decreases with height
 Rising air parcels
expand, cooling as
they do work on
environment
 If vapor pressure >
equilibrium vapor
pressure =>
condensation
Clouds due to Lifting
Orographic Lifting: Lifting by Terrain
 Windward side of mountain, facing prevailing wind,
is extremely wet
 Leeward side, sheltered from wind, very
dry…known as rain shadow
Clouds due to Terrain
Orographic Lifting: California
Mixing Warm & Cold Air Masses
Assessing Air’s Moisture Content
 Problems with RH because denominator depends
on temperature
– Cold, dry air masses can have a high RH, even if they
hold little water vapor
– Relative humidity varies with time of day
– http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.html
Dew Point: Absolute measure of water vapor
 Dew Point: Temperature air must be cooled (at
constant pressure) to reach saturation
–
–
–
–
–
Less than or equal to temperature
Higher the dew point, more water vapor in air
Frost point if air temperature below 32ºF
Measured with a hygrometer or sling psychrometer
Changes by evaporating water into air, mixing drier air
from above, wind blowing in moist or dry air from
another region (air dries behind cold front, moistens
before cold front)
Applying Dew Point to Weather Forecasting
 1) Cloud Base Height
– Temperature of rising air decreases faster than
dew point…has a decent chance of eventually
reaching dew point
 2) First-Guess Low Temperature
 3) Severe Weather
– High dew points indicate enhanced risk