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 don’t 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 won’t 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.