Determining Bases of Convective Type Clouds

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Determining Bases of Convective Type Clouds

You have seen from our foregoing discussion that moisture is important in determining certain

Figure 2-4-10.—Degrees of stability in relation to temperature changes with height.

stability conditions in the atmosphere. You know, too, that the difference between the temperature and the dewpoint is an indication of the rela-tive humidity. When the dewpoint and the temperature are the same, the air is saturated and some form of condensation cloud may be ex-pected. This lends itself to a means of estimating the height of the base of clouds formed by sur-face heating when the surface temperature and dewpoint are known. You know that the dewpoint decreases in temperature at the rate of 1°F per 1,000 feet during a lifting process. The ascending parcel in the convective current experiences a decrease in temperature of about 5 1/2°F per 1,000 feet. Thus the dewpoint and the temperature approach each other at the rate of 4 1/2°F per 1,000 feet. As an example, consider the surface temperature to be 80°F and the surface dewpoint 62°F, a difference of 18°F. This difference, divided by the approximate rate the temperature approaches the dewpoint (4 1/2°F per 1,000 ft) indicates the approximate height of the base of the clouds caused by this lifting process (18 ÷ 4 1/2) x 1000 = 4,000 feet). This is graph-ically shown in figure 2-4-11.

This method cannot be applied to all cloud types. It is limited to clouds formed by convection currents, such as summertime cumulus clouds, and only in the locality where the clouds form. It is not valid around maritime or moun-tainous areas.

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