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FIGURE 5.14. Warm columns of air expand, cold columns contract, leading to a tilt of pressure surfaces, a tilt which typically increases with height in the troposphere. In Section 7.3, we will see that the corresponding winds are out of the paper, as marked by © in the figure.

FIGURE 5.14. Warm columns of air expand, cold columns contract, leading to a tilt of pressure surfaces, a tilt which typically increases with height in the troposphere. In Section 7.3, we will see that the corresponding winds are out of the paper, as marked by © in the figure.

to much lower values (around 1-2gkg-1) near the poles. At upper levels there is very little water vapor. This broad pattern can be understood by noting the striking correlation between q (Fig. 5.15) and T (Fig. 5.7). Air colder than 0°C can hold very little water vapor (see Fig. 1.5 and discussions in Sections 1.3 and 4.5).

Zonal-Average Saturated Specific Humidity (g/kg)

FIGURE 5.16. Zonally averaged saturated specific humidity, q„, in gkg 1, for annual-mean conditions.
FIGURE 5.17. Zonal mean relative humidity (%), Eq. 4-25, under annual mean conditions. Note that data are not plotted above 300 mbar, where q is so small that it is difficult to measure accurately by routine measurements.

in a broad downdraft. Now, within the updraft, the air becomes saturated (whence the cloud) and will frequently produce precipitation: the excess water will rain out. Therefore, even though the air is saturated within the cloud, by the time the air flows out from the top of the cloud, it has lost most of its water (since the cloud top is at much lower temperature than the ground, and hence its saturation specific humidity is very low). As this air descends and warms within the downdraft, it conserves its specific humidity. Since, once it has warmed, the saturation specific humidity at the air temperature has increased, the air becomes very dry in the sense that its relative humidity is very low. Hence, even though the air is saturated within the updraft, the average dry outflow

k k dry outflow

FIGURE 5.18. Drying due to convection. Within the updraft, air becomes saturated and excess water is rained out. The descending air is very dry. Because the region of ascent is rather narrow and the descent broad, convection acts as a drying agent for the atmosphere as a whole.

(at a fixed height) over the system as a whole is low. Convection, by lifting air to saturation and thus causing precipitation of the air's water, acts as a drying agent for the atmosphere. This can be vividly seen in the satellite mosaic of the water vapor distribution over the globe between heights of 6-10 km shown in Fig. 3 of the Preface. The regions of relatively dry descent (dark regions) on either side of the equatorial moist band (light), mark the latitude of the deserts. (These issues will be discussed further in Chapter 8.)

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