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Now another difficulty remains in that the question should be answered: how do such
large particles originate in a cloud In the first place this may be due t when ,$
id'
there is more or less strong turbulence in the cloud. However, as a rule this process leads
to no more than the relatively small droplets that occur in a drizzle.
To come to real precipitation, i.e. real rain or real snow, nature has two important
processes at its disposal. It has already been mentioned that condensation nuclei of differ- '"
s t dimensions can be present in the atmosphere and that the larger condensation nuclei
generally lead to the occurrence of larger drops. Here one finds a first possibility for large
particles to be originated in a cloud. These large particles fall, with regard to the smaller
ones, coagulation takes place and precipitation drops are formed. This process mainly
el
0 "C -T
2. Water vapour pressure as a function of temperature.
e = vapour pressure;
E = maximum vapour pressure for temperatures above freezing-point;
E, = maximum vapour pressure for vapour in contact with supercooled water;
Ei = maximum vapour pressure for vapour in contact with ice;
T = temperature.