Saturation vapour pressure over water, as normally defined, varies with the temperature (Fig. 4.3). Hence if droplets of different temperatures co-exist, a difference of vapour pressure exists between them which would cause evaporation from the warmer particle and condensation on the cooler ones till the equilibrium
vapour pressure between them is adjusted to a value intermediate between the original vapour pressures. The difference in saturation vapour pressure for the same temperature difference between cloud elements varies, however, with air temperatures. Fig. 5.5 shows the effect in the form of three curves.
Curve A shows the temperature differences that would produce a saturation vapour pressure difference of 0.27 mb at different air temperatures; Curve B shows the same effect for a saturation vapour pressure difference of 0.13 mb; and Curve C shows the temperature differences required for a saturation vapour pressure difference of 0.054 mb at different air temperatures. It will be seen from Fig. 5.5 that at air temperatures above freezing, a smaller difference in temperature between cloud elements is required to produce a given vapour pressure difference than at sub-freezing temperatures.
In the actual atmosphere, temperature difference between cloud elements occurs when different portions of the cloud are brought together by vertical mixing, or when drops from the upper part of the cloud descend through the cold lower part. Osborne Reynolds showed that the difference may result from the radiative cooling of the top surface of the cloud or the differential heating by the Sun of the different portions of the cloud. This so-called Reynolds effect is probably an important factor in the formation of rain in the tropics where a temperature difference of the order of 2 °C or 3 °C between neighboring cloud elements and intense vertical mixing through great heights occur frequently in large cumulus, stratocumulus, or cumulonimbus clouds.
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