4.2.1: Evaporation

From Water to Vapor: The Role of Heat in Evaporation

Evaporation occurs when water transitions from a liquid state into a gas. This process depends on three primary factors: a supply of energy, the presence of liquid water, and a vertical moisture gradient. To transform one gram of liquid water into water vapor, approximately 600 calories of heat energy must be added. This specific energy is known as the latent heat of vaporization. This heat is consumed to break the hydrogen bonds holding the molecules together, and it remains stored within the molecules as they move into the air. This stored energy is eventually released back into the environment as sensible heat when the vapor undergoes condensation to become a liquid again. This release of sensible heat is a significant factor in warming the atmosphere.

Figure \(\PageIndex{1}\): Evaporation shows how solar energy drives evaporation and stores latent heat in water vapor, transferring energy into the atmosphere.
(CC BY-SA 4.0; Sohini Dutt via AI Image Generated by Gemini).

 Figure \(\PageIndex{1}\) explains evaporation as a phase change driven by solar energy and shows how latent heat is transferred from liquid water into water vapor.

On the left side, the Sun directs solar radiation toward a water surface. Labels indicate that available energy from solar heat warms the surface. This shows that evaporation requires an energy input.

At the bottom center, a body of liquid water is shown as the source of evaporation. A label identifies this as available water, meaning that liquid water must be present for the process to occur.

In the center, arrows rise upward from the water surface into the air. These arrows are labeled evaporation and latent heat absorbed. This indicates that water molecules take in energy and change from liquid water to water vapor.

At the top center, the diagram shows water vapor as a gas with molecules spaced farther apart and moving more freely than in the liquid state. A label explains that the absorbed energy becomes stored in the water vapor as latent heat.

Overall, the diagram shows that solar energy warms liquid water, evaporation changes the water into vapor, and the absorbed latent heat is carried upward in the atmosphere by the water vapor.

The availability of water is a self-evident but essential requirement. In many desert environments, the lack of precipitation is partially due to the absence of surface water available for evaporation. Globally, the vast majority of atmospheric moisture originates from the world's oceans, specifically those located between 60 degrees north and south latitude. The highest rates of evaporation are found in tropical and subtropical regions where solar radiation is most intense.

Finally, evaporation requires a vertical moisture gradient, meaning the air directly at the water's surface must be more humid than the air situated above it. As long as the overlying air is not fully saturated, evaporation will continue. While not strictly a requirement for the phase change itself, wind significantly assists this process. By moving moisture-laden air away from the surface, wind helps maintain the necessary gradient for continued evaporation.