ch05 - Jon Ahlquist Chapter 5 Condensation Formation of dew...

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Jon Ahlquist 10/2/2006 MET1010 Intro to the Atmosphere 1 Chapter 5 Condensation Formation of dew and frost Condensation nuclei Haze Fog: radiation, advection, mixing Cloud classification & examples of main types Cloud ceiling Satellite orbits: geostationary and polar Condensation nuclei (p. 109) Condensation nuclei: tiny particles on which condensation can occur. Abundant! (100’s to 10,000’s in cubic centimeter) Small: less than 0.2 micrometers. Most abundant, least effective Large: 0.2 to 1 micrometer (wavelength of light) Giant: bigger than 1 micrometer. Least abundant, most effective All condensation nuclei in the atmosphere are too small to see well even with a microscope, especially small nuclei Consist of dust, smoke, salt, products of chemical reactions, etc. Condensation Nuclei (cont.) Some are “wettable”: water spreads out on them, as on an unwaxed car Some are not wettable (“hydrophobic”): water beads up on them, as on a waxed car Some (such as salts) are “hygroscopic”: they dissolve and allow condensation at < 100% relative humidity (can be as low as 75% relative humidity, p. 110) Table salt clumps in salt shaker because of this. “Damp Rid” is a kind of salt sold for drying air in closets Condensation usually forms in air around 100% relative humidity Haze (pp. 109-110) Dry haze: very small particles. Recall that very small particles scatter more blue light than red light. If dark background, dry haze looks blue. (Light from elsewhere scattered toward you, as blue of sky.) If light background, the light from background travels toward your eyes. As it passes through the dry haze, more blue light is scattered out, so haze looks yellowish, just as the sun does. Wet haze: condensation occurred, so particles are larger & scatter all colors. Wet haze looks whitish. Forms at relative humidity as low as 75% if salt is in air, typically from evaporated sea spray. Scatters more light than dry haze Wet haze example ( Fig. 5.4, p. 110 of text) Note white haze above water. Fog (pp. 110-116) Fog = cloud next to ground Fog in city usually thicker than fog over ocean. Cities are polluted and have very many condensation particles. Ocean fog is made of fewer but larger water droplets. Radiation fog (= ground fog, p. 111): infrared radiation from ground cools air next to it until condensation occurs. The longer the night, the more the cooling, so radiation fog is most common in late fall and winter Light breeze stirs air, bringing more air in contact with cold