Jon Ahlquist 10/2/2006MET1010 Intro to the Atmosphere 1Chapter 5CondensationFormation of dew and frostCondensation nucleiHazeFog: radiation, advection, mixingCloud classification & examples of main typesCloud ceilingSatellite orbits: geostationary and polarCondensation 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 humidityHaze (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 hazeWet 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 ground, increasing amount of condensation & fog Strong wind inhibits fog formation by even greater mixing, which brings down drier from above Light winds and clear skies common near center of high pressure, so that is where fog often occurs. Thickest at sunrise after cooling all night. As the sun heats the air, the fog evaporates (“burns off”), starting at thin edge of fog.Jon Ahlquist 10/2/2006MET1010 Intro to the Atmosphere 2Radiation fog (fig. 5.5, p. 111)Drains into in valley. Clear above.Advection Fog (pp. 112-113) Advection fog = warm moist air advects over cold surface Advection means that a wind is required,in contrast to radiation fog. Occurs along Pacific Coast when warm moist air from Pacific blows over cold ocean current next to California coast. Example: Fog by San Francisco’s Golden Gate Bridge. Occurs over Grand Banks when wam moist air from Gulf Stream blows over cold water of Labrador Current. In winter, occurs along U.S. Gulf Coast and in England when warm moist air blows onto land cooled by radiation (called advection-radiation fog).Advection fog (fig. 5.7, p. 113) Summary of Radiation and Advection Fog Both radiation fog and advection fog involve warm moist air over cold surface Cold fog drains into low lying areas In these cases, air is stable (warm air has risen, cold air has sunk) so fog has clearly defined upper surface. Radiation and advection fog occur in stable environments (covered in chapter 6). Are fairly uniform and featureless in appearance.Upslope Fog (pp. 112-113) Fog can form when air cools to saturation as it blows up the side of a mountain. (Recall: Air does work as it expands into lower pressure.) Example: Upslope fog on the east side of the Rocky Mountains (fig. 5.9, p. 113)Steam fog (also called mixing fog, evaporation fog, pp. 113-116) Water evaporates from warm surface; warm, moist air rises; mixes with cold air above; air cools to dewpoint Air in steam fog not stable near surface (warm air rises), looks wispy with no definite top Occurs over: pot of boiling water (hence name “steam fog”) outdoor heated pool on cold day hot asphalt after summer rain warm lake in mid-latitudes when cold air in early fall blows inJon Ahlquist 10/2/2006MET1010 Intro to the Atmosphere 3How steam fog (mixing fog) forms(Special topic, p. 115) Imagine two equal-sized blobs of air, both nearly saturated. Recall fig. 4.10, p. 91, showing saturation vapor pressure versus temperature When the blobs are mixed together, their temperatures and dew points are averaged Even if the initial dew points were below saturation, the mixed partial can be above saturation. If so, condensation will result, i.e., mixing fog.Midpoint temp &vapor pressurefor mixed airInitial temperatures& vapor pressurefor 2 moist blobs of airSteam fog (fig. 5.10, p. 116): wispy!Fog Review & Fog in Florida (pp. 116-117) Look at the map on page 116 and explain why coastal California, the Appalachians, New England, and the Gulf Coast are so foggy. Fog is a problem on Florida highways. For info on what other states have done to monitor their highways, see http://garnet.acns.fsu.edu/~jelsner/fdot/OtherRWIS.html Florida even gets “superfog,” which is especially dense fog caused by the abundance of condensation nuclei in forest fire smoke. For more information, do an Internet search on: superfog FloridaCloud classification (pp. 117-119) Classification proposed in 1803 by English scientist Luke Howard Uses Latin words to describe cloud appearance Stratus = layer (same root as “stratosphere”) Cumulus = heap (root as in “accumulate”) Cirrus = lock of hair Nimbus = rain cloud Alto = prefix used to denote middle clouds,just as an alto voice is a middle-pitched voice For more info and great pictures, see The Audubon Society Field Guide to North
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