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ATMO 1300-006 Class #8 Wednesday, September 16, 2009 Chapter 4, Water in the atmosphere Wednesday, 9/16/09 1 Saturation vapor pressure depends only on Temperature Wednesday, 9/16/09 2 Wednesday, 9/16/09 3 Table 4-1, p. 92 Condensation • In the lab with perfectly clean air (no aerosol) takes a relative humidity of more than 200%. RH > 100% is supersaturation • Condensation is inhibited by the curvature effect • Small, very curved droplets have molecules with few neighbors, and are quick to evaporate Wednesday, 9/16/09 4 Wednesday, 9/16/09 5 Fig. 4-7, p. 93 Condensation in the atmosphere • Is inhibited by the curvature effect • Is enhanced by the solute effect • Some aerosol, salt particles for example, dissolve and have the ability to hold on to water molecules and suppress evaporation • Other aerosol particles form nuclei, or small surfaces for condensation Wednesday, 9/16/09 6 Condensation in the atmosphere (continued) • A cloud nucleus gives water molecules more neighbors, by acting like a small flat surface • There are always abundant cloud condensation nuclei in the atmosphere— dust, salt, pollen, pollutants • The solute effect permits condensation at RH < 100%. This is called haze Wednesday, 9/16/09 7 Condensation in the atmosphere (continued) • Supersaturation (RH > 100%) does not occur in the atmosphere • When the relative humidity reaches 100%, cloud particles form. • Cloud at the surface is called fog. • Fog reduces visibility to less than 1 km or 0.6 miles • Heavy fog is a travel hazard Wednesday, 9/16/09 8 Frozen dew • When the dew point is above the freezing point of water and cooling occurs, dew forms when the temperature reaches the dew point • Sometimes further cooling occurs after the dew forms • If the temperature falls below 0°C, the dew can freeze. When liquid dew freezes, it is called frozen dew Wednesday, 9/16/09 9 Frozen dew (continued) • Frozen dew is also called “black ice” • It is a major traffic hazard • It also causes slips and falls for people on foot • Frozen dew is hard to see • Frozen dew frequently forms on roads where there is a significant slope, as well as bridges and overpasses Wednesday, 9/16/09 10 Fog in a city Wednesday, 9/16/09 11 Fig. 4-8, p. 95 Wednesday, 9/16/09 12 Fig. 4-9, p. 96 Radiation fog forms at night in valleys due to cooling Wednesday, 9/16/09 13 Fig. 4-10, p. 97 Advection fog: warm moist air and a cool surface current Wednesday, 9/16/09 14 Fig. 4-11, p. 97 Steam fog/evaporation fog: cold air and warm water Wednesday, 9/16/09 15 Fig. 4-12, p. 98 Ice in clouds • Deposits (deposition) on small particles called ice nuclei • There is a scarcity of ice nuclei at high temperatures (near but below 32ºF) • Many water droplets do not freeze at subfreezing temperatures. This is called supercooling. • Below -40ºC (or F), all water drops freeze Wednesday, 9/16/09 16 Ice in clouds (continued) • Ice takes on different crystal shapes in clouds, depending on temperature and supersaturation • Clouds are saturated with respect to water, supersaturated with respect to ice • The saturation vapor pressure over ice is less than that over water • There can be ice fog (inland Alaska) Wednesday, 9/16/09 17 Wednesday, 9/16/09 18 Fig. 4-30, p. 112 Wednesday, 9/16/09 19 Fig. 4-35, p. 117 How clouds form above the surface: lifting and cooling Wednesday, 9/16/09 20 Fig. 4-13, p. 99 Wednesday, 9/16/09 21 Fig. 4-14, p. 100 Wednesday, 9/16/09 22 Fig. 4-15, p. 101