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

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

Class #23: Monday, March 1, 2009

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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

Class #23: Monday, March 1, 2009

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Convection is enhanced in saturated air

Class #23: Monday, March 1, 2009

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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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