Evaporation happens when a liquid substance becomes a gas. When water is heated, it evaporates. The molecules move and vibrate so quickly that they escape into the atmosphere as molecules of water vapor.

Evaporation is a very important part of the water cycle. Heat from the sun, or solar energy, powers the evaporation process. It soaks up moisture from soil in a garden, as well as the biggest oceans and lakes. The water level will decrease as it is exposed to the heat of the sun.

Although the level of a lake, pool, or glass of water will decrease due to evaporation, the escaped water molecules dont disappear. They stay in the atmosphere, affecting humidity, or the amount of moisture in the air. Areas with high temperatures and large bodies of water, such as tropical islands and swamps, are usually very humid for this reason. Water is evaporating, but staying in the air as a vapor.

Once water evaporates, it also helps form clouds. The clouds then release the moisture as rain or snow. The liquid water falls to Earth, waiting to be evaporated. The cycle starts all over again.

Many factors affect how evaporation happens. If the air is already clogged, or saturated, with other substances, there wont be enough room in the air for liquid to evaporate quickly. When the humidity is 100 percent, the air is saturated with water. No more water can evaporate.

Air pressure also affects evaporation. If air pressure is high on the surface of a body of water, then the water will not evaporate easily. The pressure pushing down on the water makes it difficult for water to escape into the atmosphere as vapor. Storms are often high-pressure systems that prevent evaporation.

Temperature, of course, affects how quickly evaporation happens. Boiling-hot water will evaporate quickly as steam.

Evaporation is the opposite of condensation, the process of water vapor turning into liquid water.

The Process of Evaporation
Boiling water evaporates into thin air.

Rate of Evaporation
The National Weather Service in the United States measures the rate of evaporation at different locations every year. Scientists there found that the rate of evaporation can be below 76 centimeters (30 inches) per year at the low end, to 305 centimeters (120 inches) per year on the high end.

air pressure
Noun

force pressed on an object by air or atmosphere.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

Noun

process by which water vapor becomes liquid.

decrease
Verb

to lower.

disappear
Verb

to vanish or leave without a trace.

earth
Noun

soil or dirt.

Noun

process by which liquid water becomes water vapor.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

high-pressure system
Noun

weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.

Noun

amount of water vapor in the air.

liquid
Noun

state of matter with no fixed shape and molecules that remain loosely bound with each other.

molecule
Noun

smallest physical unit of a substance, consisting of two or more atoms linked together.

saturate
Verb

to fill one substance with as much of another substance as it can take.

snow
Noun

precipitation made of ice crystals.

Noun

radiation from the sun.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

Noun

body of flowing water.

Noun

land permanently saturated with water and sometimes covered with it.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

tropical
Adjective

existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

vapor
Noun

visible liquid suspended in the air, such as fog.

water
Noun

chemical compound that is necessary for all forms of life.

Noun

movement of water between atmosphere, land, and ocean.