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

2. 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 = T D Class #23: Monday, March 1, 2009 2

3. 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

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5. 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

6. 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

7. How saturation vapor pressure varies with temperature Class #23: Monday, March 1, 2009 7

8. 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

9. Lifting processes in the atmosphere produce clouds Class #23: Monday, March 1, 2009 9

10. Convection is enhanced in saturated air Class #23: Monday, March 1, 2009 10

11. Conditional instability is very common in the atmosphere Class #23: Monday, March 1, 2009 11

12. 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 12