class #23: monday, march 1, 2009 1 class #23: monday, march 2 clouds, fronts, precipitation...
DESCRIPTION
How condensation happens in the real atmosphere Small drops are very curved and evaporate very easily –Called the curvature effect –In clean air in the laboratory drops form when relative humidity reaches 400% The real atmosphere has lots of small aerosol particles –Some attract water molecules (hygroscopic) –Some are flatter surfaces for condensation Class #23: Monday, March 1,TRANSCRIPT
Class #23: Monday, March 1, 2009
1
Class #23: Monday, March 2
Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone
Brief review of how clouds form
• This material comes from Chapter 4• Condensation occurs when air becomes
saturated• Saturation occurs when the rate of
condensation = the rate of evaporation• Saturation occurs when the relative
humidity is 100%• Saturation occurs when T = TD
Class #23: Monday, March 1, 2009
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How condensation happens in the real atmosphere
• Small drops are very curved and evaporate very easily– Called the curvature effect– In clean air in the laboratory drops form when
relative humidity reaches 400%• The real atmosphere has lots of small
aerosol particles– Some attract water molecules (hygroscopic)– Some are flatter surfaces for condensation
Class #23: Monday, March 1, 2009
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Class #23: Monday, March 1, 2009
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Condensation (continued)
• The small particles are called cloud condensation nucleii or CCN– There are always plenty of CCN
• The CCN are able to negate the curvature effect
• The result: Condensation occurs at a relative humidity of 100%
• Exception: Haze, tiny drops, RH<100%
Class #23: Monday, March 1, 2009
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Making a cloud
• Requires saturating the air• How to saturate the air
– There are 3 processes in the atmosphere– First: Add moisture to the air until it becomes
saturated• How? By evaporation. Occurs, but not so
common (over water surface and light precip)– Second: Mix warm moist air with cold air
• Occurs, but not so common
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How saturation vapor pressure varies with temperature
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Making a cloud (continued)
• Third, most important, and most common:• Cooling the air until it becomes saturated
– At the surface, cooling at the same pressure until the temperature equals the dew point. This produces a cloud at the ground called fog.
– Lifting the air, which produces cooling at the DALR of 10 degrees C per 1000m
• Lower pressure, expansion, energy loss, T falls
Class #23: Monday, March 1, 2009
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Lifting processes in the atmosphere produce clouds
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Convection is enhanced in saturated air
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Conditional instability is very common in the atmosphere
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Lifting, fronts and cloud formation
• At fronts, one, two, three or all four lifting processes can be acting at the same time
• Frontal lifting forces the warmer air over the colder air, and an upslope enhances lifting
• Convergence occurs because the wind direction changes at the front
• Convection can occur with surface heating
Class #23: Monday, March 1, 2009
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