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Unformatted text preview: Jon Ahlquist 10/5/2006 MET1010 Intro to the Atmosphere 1 Chapter 7: Precipitation The size of cloud droplets and raindrops Increased saturation vapor pressure around a curved surface: curvature effect Terminal velocity of droplets of various sizes Growth of cloud droplets to rain drops by collision- coalescence Formation of ice in the atmosphere Growth of ice crystals via the ice-crystal process Rain from ice Ice: different shapes at different temperatures Measuring precipitation Precipitation (p. 164, column 1) ¡ Is it ever too cold to snow? No, but little snow when it is very cold for 2 reasons: ¢ Capacity for water vapor is small at low temperatures, so even saturated air can condense little moisture. ¢ Coldest temperatures occur when skies are clear. (Clouds are “greenhouse” blankets.) ¡ Is it ever too warm to snow? Snow flakes possible with surface temperature at 50° F. See p. 177. ¡ Precipitation = any form of water (solid or liquid) that falls from a cloud and reaches the ground, including rain, drizzle, sleet, snow, snow grains, snow pellets, ice pellets, and hail. (Precip types on pp. 174-184). How Do Cloud Droplets Grow? (p. 164) ¡ Formation of raindrops from cloud droplets is complex ¡ Typical condensation nucleus: 0.2 micrometers Typical cloud droplet: 20 micrometers Typical rain droplet: 2000 micrometers = 2 mm (See fig 7.1, p. 164) ¡ Rain droplet radius is about 100 times greater than cloud droplet radius, so volume is about 100x100x100 = 1 million times greater ¡ How can cloud droplets become raindrops with a million times the volume in a few tens of minutes? ¡ We’ll see they grow not by continued condensation but by many small droplets merging to form a rain drop. ¡ First, we’ll consider small droplets. Saturation around a cloud droplet ¡ Curvature effect: water evaporates more easily from a tiny droplet than from a flat surface because a molecule on the surface of a droplet has fewer neighboring water molecules in the liquid to hold it. (See next slide.) ¡ Thus, saturation vapor pressure is higher around tiny cloud droplets than over pan of water. Note the larger number of water molecules (shown in red) around the droplet than above the flat water surface. (The difference is exaggerated.) Fig. 7.2, p.165: Red balls indicate water vapor molecules. More exist around curved cloud droplet than over flat surface at saturation. Saturation around a cloud droplet ¡ Compare the number of neighboring molecules for a molecule on a flat surface versus the number of neighbors when the surface is curved. (Figure below not in textbook.) Light blue rectangle represents water with a flat surface Extra neighbors for flat surface Light blue circle represents water molecule on surface Tiny spherical water droplet Saturation around a droplet (p. 165) ¡ Saturation vapor pressure is largest for smallest droplets ¡ Droplets 1 μm or larger behave almost like flat surface Fig. 7.3, p. 165 if relative humidity is above saturation vapor pressure line...
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This note was uploaded on 07/21/2011 for the course MET 1010 taught by Professor Staff during the Fall '08 term at FSU.
- Fall '08