Transcript Class #25: Friday, March 7

Class #23: Monday,
March 2
Clouds, fronts, precipitation
processes, upper-level waves,
and the extratropical cyclone
Class #23: Monday, March 1,
2009
1
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
2
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
3
Class #23: Monday, March 1,
2009
4
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
5
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
Class #23: Monday, March 1,
2009
6
How saturation vapor pressure
varies with temperature
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2009
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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
8
Lifting processes in the
atmosphere produce clouds
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2009
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Convection is enhanced in
saturated air
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2009
10
Conditional instability is very
common in the atmosphere
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2009
11
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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