Precipitation starts when there are drops heavy enough to have a terminal velocity downwards which is greater than the cloud's updraught. The 'terminal velocity' is the speed a body falling under gravity eventually attains in still air, and it depends on the size of a drop, being faster for a larger one (Table 9.1). Therefore larger drops are needed for rainfall to occur where the updraught is greater, as in a convective cloud (Table 8.1). In the case of normal cloud droplets, the terminal velocity is practically zero because of their size, and rainfall depends on their amalgamation into raindrops perhaps a million times as big.
The amalgamation is not easy; it is much harder to form raindrops than to create cloud droplets. The latter arise as soon as the Relative Humidity reaches 100 per cent (i.e. when the air is cooled to the dewpoint temperature— Section 6.2), the droplets growing on cloud condensation nuclei (CCN) which are amply available in even the cleanest air (Note 8.D). On the other hand, amalgamation to form raindrops does not occur spontaneously, because collisions between droplets are infrequent as the spacing between them is typically 500 times their diameter (Section 8.2 and Table 9.1). An alternative to collision as a way of forming raindrops is condensation from the atmosphere onto individual cloud droplets, but this takes hours. The time needed means that such precipitation comes only from clouds that contain sustained (not turbulent) uplift (Section 8.1).
Different processes of droplet amalgamation occur within warm clouds (whose tops are at temperatures above freezing), and cold clouds, which extend higher than the freezing level. That level depends on the season and weather, but is typically 2 km at midlatitudes and 4 km between the Tropics (Section 1.8).
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