Class #23: Monday, March 1, 2009 1 Class #23: Monday, March 2 Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone
Jan 18, 2018
Class #23: Monday, March 1, 2009
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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
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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
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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%
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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
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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
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