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ATMO 1300-006 Class #7 Monday, September 13, 2010 Chapter 4, Water continued Monday, 9/13/10 1 Saturation vapor pressure depends only on Temperature Monday, 9/13/10 2 Another way to measure humidity: the relative humidity • Relative humidity is expressed as a percentage, where 100% is saturation • Relative humidity can be defined in terms of the vapor pressure • Relative humidity = 100% x vapor pressure ÷ saturation vapor pressure • A low relative humidity allows a higher evaporation rate Monday, 9/13/10 3 Relative humidity has disadvantages • Relative humidity tells how the air is from saturation • 0% relative humidity: No water vapor • 100% relative humidity is saturated • But air at a high temperature with relative humidity of 50% may have more water vapor than air at a lower temperature with a relative humidity of 90% Monday, 9/13/10 4 The saturation water vapor content varies greatly with T Monday, 9/13/10 5 Relative humidity changes with Temperature for the same air Monday, 9/13/10 6 Another way to measure humidity: dew point • Dew point is also called the dewpoint temperature, abbreviated as Td or TD • Dew point is defined as the temperature to which air must be cooled (without changing the pressure) to become saturated • Dew point does not exceed the temperature Monday, 9/13/10 7 More on dewpoint • When the dewpoint is below 0°C (32°F), it is called the frost point, because deposition (water vapor to ice) in the form of frost will occur when the air becomes saturated • When air cools to the dewpoint, condensation occurs • On surfaces, this condensation is called dew (or frost) Monday, 9/13/10 8 Frozen Dew • Occurs in two steps • First, condensation occurs and the temperature is above freezing (32ºF); that is, the dew point is above freezing • The condensation is dew • Second, the temperature falls below freezing. The dew freezes to frozen drops • This ice is called frozen dew or black ice Monday, 9/13/10 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 Monday, 9/13/10 10 Monday, 9/13/10 11 Fig. 4-5, p. 91 Monday, 9/13/10 12 Fig. 4-6, p. 92 Monday, 9/13/10 13 Table 4-1, p. 92 Condensation in air (not on a surface like frost or dew) • In a lab with perfectly clean air, saturation requires 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 Monday, 9/13/10 14 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 Monday, 9/13/10 15 Monday, 9/13/10 16 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 Monday, 9/13/10 17 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 Monday, 9/13/10 18 Condensation in the atmosphere (continued) • Supersaturation (RH > 100%) does not occur in the atmosphere. The solute effect cancels the curvature effect. • 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 Monday, 9/13/10 19 Fog in a city Monday, 9/13/10 20 Fig. 4-8, p. 95 Monday, 9/13/10 21 Fig. 4-9, p. 96 Radiation fog forms at night in valleys due to cooling Monday, 9/13/10 22 Fig. 4-10, p. 97 Advection fog: warm moist air and a cool surface current Monday, 9/13/10 23 Fig. 4-11, p. 97 Steam fog/evaporation fog: cold air and warm water Monday, 9/13/10 24 Fig. 4-12, p. 98 Ice in clouds • Deposits (deposition) on small particles called ice nuclei (clay minerals, tiny ice crystals) • There is a scarcity of ice nuclei at high subfreezing temperatures near but < 32ºF • Many water droplets do not freeze at subfreezing temperatures, called supercooling. • Below -40ºC (or F), all water drops freeze Monday, 9/13/10 25 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) Monday, 9/13/10 26 Monday, 9/13/10 27 Fig. 4-30, p. 112 Monday, 9/13/10 28 Fig. 4-35, p. 117 How clouds form above the surface: lifting and cooling Monday, 9/13/10 29 Fig. 4-13, p. 99