Evaporation is the process of a liquid becoming vaporized. In other words, a change in phase in the atmosphere occurs when substances change from a liquid to a gaseous, or vapor, form. Because we are talking about atmospheric processes that drive the weather, we will refer to the evaporation of water although other liquids can evaporate into the air. Also note that solids can evaporate, or be transformed into a gas, but in meteorology, this is generally referred to as sublimation.
Evaporation in the atmosphere is a crucial step in the water cycle. Water on Earth's surface will evaporate into the atmosphere as energy is absorbed by liquid water. Water molecules that exist in the liquid phase are free-flowing and in no particular fixed position. Once energy is added to water by heat from the sun, the bonds between the water molecules gain kinetic energy, or energy in motion. Once the gas, called water vapor or humidity, reaches the atmosphere, various types of clouds can form.