Precipitation, commonly referred to as rainfall, is a measure of the quantity of atmospheric water in the form of either rain, hail, or snow that reaches the ground. The average annual precipitation over the whole of the United States is thirty-six inches per, year. It should be understood, however, that all precipitation is not measured equally. For example, a foot of snow does not equal a foot of precipitation. According to the general formula for computing the precipitation of snowfall, ten inches of snow equals one inch of precipitation. In upper New York, for example, where there is typically a large amount of snowfall every year, a hundred inches of snow in one year would be recorded as only ten inches of precipitation, On the other hand, rain is rain. Forty inches of rain would be recorded as forty inches of precipitation. The total annual precipitation for an area with forty inches of rain and one hundred inches of snow would be recorded as fifty inches of precipitation.

The amount of precipitation that an area receives is a combined result of several factors, including location, altitude, proximity to the sea, and the direction of prevailing winds. Most of the precipitation in the United States is brought originally by prevailing winds from the Pacific Ocean, the Gulf of Mexico, the Atlantic Ocean, and the Great Lakes. Because these prevailing winds generally come from the west, the Pacific Coast receives more annual precipitation than the Atlantic Coast. Along the Pacific Coast itself, however, altitude causes some diversity in rainfall. The mountain ranges of the United States, especially the Rocky Mountain Range and the Appalachian Mountain Range, influence the amount of precipitation in their areas. East of the Rocky Mountains, the annual precipitation is substantially less than that of west of the Rocky mountains. The precipitation north of the Appalachian Mountains is about 40 percent less than that of south of the Appalachian Mountains. As air currents from the oceans move over land, the air must rise to pass over the mountains. The air cools, and the water that is held in the clouds falls as rain or snow on the ascending side of the mountains. The air, therefore, is much drier on the other side of the mountains.