Rainfall, snowfall, sleet, and hail are collectively known as precipitation, a word derived from Latin, meaning to fall headlong. The word rainfall is also used in the general sense to mean precipitation.
Where does most of the moisture that falls from the clouds as rain come from? Water evaporates from the ground surface, from vegetation, from all open bodies of water, such as lakes and rivers, and, of course, from the ocean. Large amounts of water are discharged to the atmosphere by the process of transpiration. For example, an acre of corn gives off to the air approximately 11,000 to 150,000 liters of water each day. A large oak tree gives off approximately 150,000 liters per year. This water is first taken up by the roots from the soil, moves up the trunk as sap, and emerges from the plant through thousands of small holes on the under side of every leaf.
Transpiration from plants is one of the important sources of water vapor in the air and often produces more vapor than does evaporation from land surface, lakes, and streams. However, the most important source of moisture in the air is evaporation from the oceans, particularly those parts of the ocean that lie in the warm parts of the earth.
For this reason, the rain that falls on cities in the central United States is probably largely composed of particles of water that were evaporated from the ocean near the equator or from the Gulf of Mexico. Only a relatively small part was evaporated or transpired from rivers, lakes and plants in the vicinity of the rainfall. Winds in the upper air carry moisture long distances from the oceans where evaporation is great.
The heat required to change the water from liquid to vapor is the familiar process known as evaporation. The air carries away the heat with the vapor, and the heat is given up when the vapor condenses to form clouds. Thus the earth's atmosphere is a vast heat engine powered by the sun. Through the energy provided by the sun, water evaporates from the land and ocean, is carried as vapor in the air, falls somewhere as rain or snow, and returns to the ocean or to the land again to go through the same process
Although this universal truth was forgotten in the Dark Ages, the ancients may have had some appreciation of this grand cycle.According to the Bible, "All the rivers run into the sea, yet the sea is never full; unto the place from whence the rivers come thither they return again."
As the water circulates over the earth through this grand cycle, usable water is accessible only while it is on the land surface, or in the ground. p. 5. [Source]
