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Moist processes (Lapse rate (Different lapse rates of unsaturated and…
Moist processes
Lapse rate
The higher the air temp, the greater the saturation mixing ratio- more water vapour can be held in a parcel
A decrease in temp below dew point results in more water condensing out at higher temps than low- more latent heat released
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As gradient of saturation vapour pressure increases with temp, wet adiabatic lapse rate decreases as temp increases
Different lapse rates of unsaturated and saturated air mean air flowing down lee side of mountain range is frequently warmer than air on upwind side
In Alps, warm dry air called Fohn, Chinook in American Rockies
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Cloud radiative effects
Altitude, type and thickness of cloud determines whether local net effect is to warm or cool air and surface below
E.G: stratus emits almost as much IR as it receives, whereas cirrus emits much less IR than it receives (ice cold)
Clouds play important role in controlling global radiation budget: reflection of incoming radiation, absorption of solar and thermal IR radiation, emission of IR rad
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Clouds provide an environmental where aqueous phase chemical reactions can take place- aerosol particles can be modified in clouds
Adiabatic lifting
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As a parcel of air is lifted, pressure decreases and parcel expands and cools at the dry adiabatic lapse rate
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Radiative cooling
Once cloud has formed, radiative cooling of cloud droplets is much more efficient
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Radiative cooling -> reduced saturation vapour pressure -> more condensation -> higher cloud water content
Relative humidity
Mixing of two unsaturated air masses at diff temps, such that final humidity is above saturation point
On a vapour pressure vs temp graph: adding water vapid at constant temp, raises humidity to saturation point
Occurs at any surface: since temp decreases with altitude, evaporation into unsaturated surface layer can result in saturation of air in upper boundary layer
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Temo and vapour pressure resulting from mixing are averages of initial values in proportion to masses of each being mixed: Tmix= (T1xM1 + T2xM2)/ (M1 + M2)
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Sources
Evaporation of precipitation: latent heat supplied by cooling of air, never reaches ground as always evaporates before
Evaporation from surface: requires energy to supply latent heat of evaporation- sunlight or conduction from surface (cools surface)
Sinks
Precipitation: rain, snow, rail
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