The process that converts a liquid to a gas at the surface is evaporation.
Evaporation is a form of vaporization that occurs at a liquid’s exposed surface, where some molecules gain enough energy to escape into the gas phase. It can happen below the liquid’s boiling point because the molecules do not all have the same energy. The fastest-moving molecules are the most likely to leave.
Evaporation differs from boiling. Boiling occurs throughout a liquid when its vapor pressure matches the surrounding pressure, producing vapor bubbles. Evaporation is confined mainly to the surface. Sublimation is the direct change from a solid to a gas, while condensation is the reverse of vaporization.
Evaporation also causes cooling. As higher-energy molecules escape, the average kinetic energy of the remaining liquid falls. This explains why perspiration cools skin and why water left in a shallow dish disappears faster than water in a deep container. Temperature, exposed surface area, airflow, and humidity all affect the rate: warm, dry, moving air generally speeds evaporation.