Merri Goss, age 11, of Spokane, Washington, for her question:
How does a summer cloud become a rain cloud?
Every cloud in the sky is sure to be changeable. A fluffy white summer cloud changes its shape from minute to minute. It may be speeding or dawdling, rising or falling. It may change into a glowering thunderhead. When this happens, there are dramatic changes inside the cloud.
Most clouds hang a mile to five miles above our heads and we have a grounds eye view of them. From these distances, we can observe many outward changes. But we cannot observe the misty material in detail or the weathery conditions that exist inside even the gentlest clouds. In fact, even expert researchers have puzzled and probed for years to solve some of these cloudy mysteries. And meteorologists still are unsure of some of the details that occur when a fluffy white cumulus cloud changes into a threatening thunderhead.
The weathery drama begins with a mass of misty droplets of water. These droplets of suspended liquid foam the basic material in all clouds. They are so small that meteorologists measure their diameters in microns. The micron unit is based on the meter of the metric system, which is a little longer than a yard. There are a million microns in a meter, or about 25,000 of these slim slivers in an inch. The average cloud droplet is a glob of liquid about five microns wide. However, in clouds, the misty population cam3vary in size from less than one to more than 200 microns.
These droplets are suspended in the air somewhat like assorted polka dots, with plenty of space between them. Above t'he.clouds, the beaming sun shines down upon the earth. As sunbeams pierce through the misty material, the cloud droplets play'_:tricks upon them. The fine droplets are too small to cast shadows. They are, of course, made of liquid water and the sunbeams darting among them reflect from their shiny surfaces. This causes a cloud to look like an airy pile of frothy, white foam.
The droplets in such a cumulus cloud range from six to 70 microns. The number suspended in each cubic centimeter of air averages from 250 to 400. As this misty material changes into a rain cloud, countless small droplets merge together in bigger droplets. Their number decreases, but many of them double in size. In a thunderhead., the biggest droplets reach 200 microns, though there are no more than 200 of them to the average cubic centimeter.
Millions of droplets in a rain cloud are big enough to stop the sunbeams from passing through and they cast their tiny shadows among the misty material. What's more, weather conditions in such a cloud tend to be extreme. Temperatures and pressures may be changing in different corners of the cloud. The growing droplets may be whirled up, down and around by blustering drafts. Their billions of tiny shadows tend to merge and blur and, seen from the ground, the stormy clouds look like a dark, heavy curtain of almost solid material.
The amount of misty moisture in the gauzy cloud is immense. The total water in a sizable thunderhead may weigh from 50,000 to 300,000 tons. Droplets of 200 microns drizzle down and drops measuring up to 1/5 of an inch pelt the earth with rain. If a cloud covers a square mile with one inch of rainfall, it has shed over 72,000 tons of water. Five billion tons of water must fall from the cloudy skies in order to cover the state of Washington with one inch of rainfall.