Dominick Bonamassa, age 10, of Yorkville, New York, for his question:
Why are some raindrops bigger than others?
Whopping raindrops splash down from a summer thunderstorm and smaller raindrops fall softly from a skyful of gentle gray clouds. A thick fog may drizzle a shower of fine moisture. Scientists, like you, are very interested in why raindrops come in assorted sizes.
The science of weather is meteorology and at present we need to know lots more about it. Every hour one billion tons of rain or snow, sleet or hail, are dumped from the clouds onto the earth's land and sea. So the problem is a big one. It is also very, very complicated. The processes involve the changing to and fro of gaseous water vapor into liquid water or solid ice. This, you may think, should be a simple matter of changing the temperature to freeze liquid water into solid ice or boiling it into gaseous vapor. But conditions in the immense atmosphere do not follow these down to earth regulations.
High in the clouds, droplets of moisture may remain liquid when super cooled to far below the normal freezing point of water. When it comes to the formation of sizeable raindrops, these misty droplets pose another, problem. Under a microscope, they look somewhat like the tiny scattered stars in the sky. In a lazy cloud, it may take them two days and a night to drift a mile. Chances of meeting and merging with their widely scattered neighbors are very slim.
We know now that other ingredients are needed to form raindrops from misty cloud moisture. The tiny droplets tend to gel around solid microscopic cores called nuclei. These rain making nuclei may be fragments of salt or smoke or tiny crystals of ice. And breezes in the cloud must toss the misty droplets around so that they are more likely to meet and merge around the nuclei. A gentle breeze in a gentle cloud may help the misty droplets to form a shower of smallish raindrops, just big enough to fall softly to the earth.
Sometimes the weathery atmosphere stirs up blustering turbulence on a grand scale. The winds in a thunderhead blow up and down and whirl every which way, some¬ times at speeds of 150 miles per hour. In this wild hurly burly, misty droplets form swarms of raindrops and small raindrops are bashed together to form bigger ones. Raindrops must be heavy enough to shower down through the air and small drops can fall when the lower air is calm. But at the base of a raging thunderhead there are powerful updrafts of rising air. When raindrops start to fall they are whisked back up and around the cloud, often again and again. Lots of them collide and form bigger and still bigger raindrops. At last they grow heavy enough to fall through the breezy updrafts and the whopping raindrops of a thunderhead splash down to the ground.
Tropical rains tend to be fiercer than ours in the temperate zone. There the clouds range from very hot to icy cold. The winds in thunderheads and other turbu¬lent storms tend to be wilder. These conditions build up larger raindrops to come pelting down. In most of our temperate clouds the breezes and temperature ranges are gentler. Small and medium raindrops are heavy enough to shower down through the air. But one of our thunderstorms builds whopping raindrops because its turbulence is as wild as the usual rain clouds of the tropics.