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15qkq 20 Pressure (in mbar) y y

FIGURE 3.3. The global average vertical distribu

15qkq 20 Pressure (in mbar) y y

FIGURE 3.3. The global average vertical distribu tion of water vapor (in g kg (in mbar).

) plotted against pressure be calculated, using the methods outlined in Chapter 2. This profile was shown in Fig. 2.11. The troposphere is warmed in part through absorption of radiation by H2O and CO2; the stratosphere is warmed, indeed created, through absorption of radiation by O3.

It is within the troposphere that almost everything we classify as "weather" is located (and, of course, it is where we happen to live); it will be the primary focus of our attention. As we shall see in Chapter , its thermal structure cannot be satisfactorily explained solely by radiative balances. The troposphere is in large part warmed by convection from the lower surface. Temperature profiles observed in the troposphere and as calculated from radiative equilibrium are illustrated schematically in Fig. 3. . The observed profile is rather different from the radiative equilibrium profile of Fig. 2.11. As noted at the end of Chapter 2, the temperature discontinuity at the surface in the radiative equilibrium profile is not observed in practice. This discontinuity in temperature triggers a convective mode of vertical heat transport, which is the subject of Chapter .

Having described the observed T profile, we now go on to discuss the associated p and p profiles.

FIGURE 3.4. A schematized radiative equilibrium profile in the troposphere (cf. Fig. 2.11) (solid) and a schematized observed profile (dashed). Below the tropopause, the troposphere is stirred by convection and weather systems and is not in radiative balance. Above the tropopause, dynamical heat transport is less important, and the observed T is close to the radiative profile.

FIGURE 3.4. A schematized radiative equilibrium profile in the troposphere (cf. Fig. 2.11) (solid) and a schematized observed profile (dashed). Below the tropopause, the troposphere is stirred by convection and weather systems and is not in radiative balance. Above the tropopause, dynamical heat transport is less important, and the observed T is close to the radiative profile.

3.2. THE RELATIONSHIP BETWEEN PRESSURE AND DENSITY: HYDROSTATIC BALANCE_

Let us imagine that the atmospheric T profile is as observed, for example, in Fig. 3.1. What is the implied vertical distribution of pressure p and density p? If the atmosphere were at rest, or static, then pressure at any level would depend on the weight of the fluid above that level. This balance, which we now discuss in detail, is called hydrostatic balance.

Consider Fig. 3.5, which depicts a vertical column of air of horizontal cross-sectional area SA and height Sz. Pressure p(z) and density p(z) of the air are both expected to be functions of height z (they may be functions of x, y, and t also). If the pressure at the bottom of the cylinder is pB = p(z), then that at the top is

3.2. THE RELATIONSHIP BETWEEN PRESSURE AND DENSITY: HYDROSTATIC BALANCE

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