Barbara Martin, age 10, of Alurora, Colo. for her question:

 How does rain form?

All living things need a steady supply of water. Old Mother Nature supplies these needs with Operation Water Cycle. Rainfall is but one stage In this planet‑wide operation. Every day millions of tons of water are lifted from the face of the world, carried miles and miles and then droplets as rain, snow or hail. Some of this water feeds the plants and animals. Much of it runs down to join the oceans. The operation is called a cycle because it repeats and repeats. There is always a supply of water being lifted into the atmosphere, carried and sprinkled elsewhere.

The water rises aloft in the form of vapor. The sun evaporates it from the seas, rivers, lakes, swamps, little puddles and the laundry on the line.  Millions of tons of water become vapor every day. The vapor is a gas which mingles with the air. The air can hold only so much vapor and cool air can hold less than warm air.

Warm air tends to expand and rise, taking its vapor aloft with it. However, the expansion causes the warm air to cool. Now it can hold less vapor. Some of the vapor is squeezed into droplets of liquid moisture. These droplets form the clouds, light enough to float above the earth.

The misty clouds are blown by the winds. till the time, however, they are slowly falling. When conditions are right, the misty cloud stuff gets into rain drops large enough to tumble down to the earth from which it came.

The weight of falling rain gives you an idea of the vast forces involved in the operation. An inch of rainfall in one hour rates as a downpour. The weight of this one inch of rainfall over one acre is 113 tons. An. inch of rainfall over one square mile equals 72,320 tons. The weight of one inch of rainfall, or ten inches of light snow. Over the entire state of Colorado is about one and a half billion tons. All that water was lifted several miles above the ground and blown many, many miles before it fell.

Cloud droplets are small and widely separated. To become raindrops, millions of them must get together. This can only happen if there are solid fragments in the misty cloud, for the tiny droplets need this solid nucleus around which to cling. As a rule there ire plenty of these nuclei­ in the air. They may be fragments of smoke or dust, crystals of ice or salt from the ocean spray.

Raindrops form as the misty droplets gel around solid fragments. Soon the cloud moisture is too heavy to float in the air. It starts to fall as rain. For a while the rain may be held aloft by a warm, rising updraft. When this happens, the raindrops grow in size and numbers. At last the drops are large and heavy enough to fall. In falling they are broken to bits, so the largest drops that reach the ground are less than a quarter of an inch.