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Class #23: Monday, March 2

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

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Class #23: Monday, March 2

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

  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 = TD Class #23: Monday, March 1, 2009

  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

  4. Class #23: Monday, March 1, 2009

  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

  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

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

  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

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

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

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

  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

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