Water is abundant on our planet, distinguishing Earth from all other planets in the solar system. More than 97% of Earth's water is in the oceans, with 2.1% in the polar ice caps and 0.6% in aquifers. The atmosphere contains only about one part in a hundred thousand (0.001%) of Earth's available water. However, the transport and phase distribution of this relatively small amount of water (estimated total liquid equivalent volume of 13,000 km3) are some of the most important features of Earth's climate. The existence of varying pressures and temperatures in the atmosphere and at the Earth's surface causes water to constantly transfer among its gaseous, liquid, and solid states. Clouds, fogs, rain, dew, and wet aerosol particles represent different forms of that water. Aqueous atmospheric particles play a major role in atmospheric chemistry, atmospheric radiation, and atmospheric dynamics.
The mass concentration of water vapor as a function of temperature and relative humidity in the atmosphere is presented in Figure 7.1. The maximum water vapor capacity of the atmosphere at a given temperature is given by the saturation line (100% RH). The ability of the atmosphere to hold water vapor decreases exponentially with temperature; that is, at 30°C it can contain as much as 30.3g(water) m~3 but at 0°C it becomes saturated at a concentration of 4.8g(water) m~3. This basic thermodynamic property of the atmosphere is the reason for the creation of liquid water. Consider an air parcel that has initially a temperature of 20°C and contains a water vapor concentration of 8.4 gm~3. The water vapor saturation concentration of the atmosphere at 20°C is 17.3 g(water) m"3 and the RH of the air parcel is equal to 100 x (8.4/17.3) = 48.5% (point A in Figure 7.1). Let us assume that the air parcel cools down to 5°C by rising and expanding in the atmosphere without any interaction with its surroundings, that is, maintaining its water vapor concentration. At the new state (point B in Figure 7.1) the water vapor saturation concentration of the atmosphere is 6.8g(water) m~3 and the air parcel is supersaturated by 1.6g(water) m~3. This extra water vapor then condenses on available aerosol particles forming a cloud with a liquid water content of 1.6g(water) m 3, and the air parcel's RH returns to 100% with a water vapor concentration of 6.8 g(water) m~3 (point C). This description of the creation of a cloud is highly simplified, but still conveys the main thermodynamic processes. A more detailed picture will be presented later.
Clouds cover approximately 60% of the Earth's surface. Average global cloud coverage over the oceans is estimated at 65% and over land at 52% (Warren et al. 1986, 1988).
Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Second Edition, by John H. Seinfeld and Spyros N. Pandis. Copyright © 2006 John Wiley & Sons, Inc.
FIGURE 7.1 Atmospheric water vapor concentration as a function of temperature and relative humidity.
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