Evaporation

Water found in the soil is not always pulled downward into the rock below. The hydrologic cycle continues as plants use their roots to pull water from the soil. Once inside a plant, the moisture, now called sap, travels through the plant's trunk or stem and out to its leaves. Tiny holes on the undersurface of leaves allow moisture to escape into the air. The process is known as transpiration. Water also evaporates from leaf surfaces. During the processes of evaporation and transpiration, a full-grown tree can release as much as 40,000 gallons of water a year-enough to fill a very large swimming pool!

Water also evaporates from oceans, lakes, rivers, and the land's surface, and becomes water vapor in the atmosphere. Heat energy from the sun is used to break the bonds between water molecules, causing evaporation. As molecules are heated, they begin moving rapidly. Scientists say that rapidly moving molecules are "excited." The rapid movement of excited water molecules is strong enough to break their bonds so they can change into vapor.
Water does not evaporate at the same rate everywhere. Warm air absorbs more moisture than cold air. For each 18° F (8° C) increase in air temperature, the air can hold two times more water. Also, warm water evaporates more quickly than cold water. The exact point at which air becomes saturated with water vapor varies according to the temperature.