This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. T. Neil Davis is a seismologist at the institute. Drop size in rain seems to be directly related to the upward speed of the air from which the rain falls. Clouds tend to dissipate and no longer release rain when the cloud layer decreases in altitude.
Clouds form and produce rain as they rise in altitude.
Gentle, drizzle-like rains are common along the coast and in interior Alaska and Yukon Territory when air masses are being moved from lowland to upland areas. Falling at a speed of about 6 kmph, those tiny drops are still far larger than the 0.02 mm sized particles in a non-precipitating cloud. Large raindrops evidently grow mainly through collision of smaller drops when there is sufficient updraft and turbulence to keep the drops circulating in the cloud and in various directions so that they will collide. The very largest raindrops, those up to 4 mm in diameter, fall at a speed of about 30 kmph. However, updrafts in thunderclouds can be twice this fast, so even these very large raindrops can be supported within the cloud.