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!
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.