WHY RAIN SOMETIMES AND SNOW OTHERS?
When the temperature is right, precipitation will come down from the sky in the form of snow rather than rain. Meteorologists, as weather scientists are called, are always trying to increase their knowledge about how precipitation occurs. "Tomorrow it will snow,"the weather report may say. Four factors have helped the weather scientist makehis prediction: first, the humidity of an approaching low pressure air mass; next, the air mass temperature; then the nearness of another approaching, cold air mass; finally, the kind of dust particles in the air masses.
Recently another factor has been added to help determine approaching weather, and that is the position and motion of high altitude jet streams of air.
Some form of precipitation will occur when a warm, moist air mass meets a cold, dry and dense mass. The meeting boundary of these two masses is called a front. The cold air will push along the earth's surface under the warm mass. Thus, the humid, warm mass starts to rise. When this happens it expands and starts to cool. This rising and cooling makes the warm, humid mass unstable. It can no longer hold the contained water vapor as a gas. Instead this vapor condenses either into raindrops or ice crystals. This condensation appears as nimbus or cumulonimbus clouds.
The raindrops or ice crystals are so dense that they begin at once to fall through the up drifting currents of the cloud. If they meet air at or below freezing, the condensed particles reach earth as snowflakes or sleet. If the lower air is above freezing, the particles reach the earth as rain.
Dust particles of certain types are important in starting precipitation for a rising, warm humid cloud often will not start condensing into drops or snowflakes when in pure, dust free air. If dust particles are present, they form precipitation nuclei that enable the cloud's vapor to condense upon them, acting as starters for the drops. In 1946, first tests were made to provide an artificial precipitation nuclei to cause rain and snow. Planes flew over nimbus clouds and "seeded" them. Rain making experiments continue to be promising.