The poet Henry Wadsworth Longfellow once expressed the idea that "into each life some rain must fall." For some, this excerpt from "The Rainy Day" might imply that rainfall, the water falling to the earth in drops from darkened clouds, may have a negative value. For others a rainfall might be interpreted with a good feeling that the rain means a renewal of life will follow, for without rain crops will not grow, and livestock will be unable to be sustained. Rain, therefore, is a necessity for all life.
Rainfall really means precipitation--generally all the water that falls or forms on an area, including rain, snow (it takes ten or twelve inches of snow to equal one inch of rain), sleet, hail, dew, and frost from the moisture in the atmosphere.
The ascent of moist air, which is the main cause of the formation of clouds, is the sole cause of important amounts of precipitation. After the formation of clouds, continued rapid cooling produces precipitation as it relates to the humidity and cooling of the rising air.
The precipitation may be deemed convectional rain, which results when the upward movement of warmer air engages the surrounding cooler air aloft; orographic rain, produced when rising ground deflects or uplifts wind-driven moisture laden air; or cyclonic rain, caused by the encounters of air currents of different temperatures, such as occurs in the cyclonic storms of varied air mass movements in the middle latitudes. The lines of disparities between the air masses are known as fronts on a weather map.
Rainfall totals are measured in inches or millimeters (.03937 inch). Rain is always falling somewhere on the earth, but there is a huge difference in the amounts that people of certain geographic places experience. On