Various rain-making experiments and researches have brought about many interesting facts. Droplets produced by the condensation process are indeed very small in size, averaging less than 10 micrometers in diameter.
To get a still better idea of their size, compare them with a human hair which is about 75 micrometers in diameter. According to Taylor, the condensation process can produce droplets up to 50-200 microns, but generally it does not produce them much over 10 microns.
Drizzle particles, which have an upper limit of 500 microns, seem to float in the air. Thus, because of their small size, the could droplets fall at a very slow rate.
If a cloud were at an altitude of 1000 meters, then an average cloud drop would take approximately 48 hours to reach the earth. In fact, such a tiny droplet of water fails to reach the ground, because it is likely to evaporate before it fell a few meters below the cloud base.
Moreover, since clouds are made of a large number of these droplets all competing for available water, their continued growth through the process of condensation is extremely slow.
Rain drops, on the other hand, have diameters ranging from about 200 microns up to 7000 microns. Drops larger than this upper limit have a fall velocity larger than 10 m/sec.
At such high velocities, the drop breaks into a few smaller drops, such as rain drops and drizzle. It is obvious that there is a limit to the growth of the size of cloud drops or rain drops in the atmosphere.
If we make a comparison between cloud droplets and rain drops, it becomes clear that millions of tiny cloud droplets are required to make one single rain drop. To get a really good sized rain drop, say 3 millimeters in diameter, it would take 27 millions of the 10 micron-droplets.
A rain drop large enough to reach the ground without evaporating contains roughly a million times water of a cloud droplet. For precipitation to occur, these cloud droplets must somehow join together to grow to sizes which can no longer be kept in suspension by the air.
The exact process or the processes which cause the cloud droplets to join together to form large rain drops capable of falling to the earth as precipitation are not fully understood. However, two mechanisms have been proposed to explain these processes, namely, Bergeron Process and Collision-Coalescence Process.