The air over deserts contains little water vapor. That over and near oceans, lakes, and rivers contains much. Water vapor in the air may form clouds, and water may fall from clouds as rain, snow, sleet, or hail.
Almost all objects upon the earth’s surface contain some water. All living things must have water. Water in the form of an invisible vapor is constantly being added to the air from innumerable sources by evaporation.
The amount of water that goes into the air from the leaves and other surfaces of growing plants is enormous. For example, corn plants give off an average of 15 tons of water in producing one bushel of ripened corn. Thus a corn crop of 50 bushels to the acre, during its entire growing period, gives off an average of 750 tons of water per acre.