Condensation is the process where a gas, such as water vapor, changes its state into a liquid. This transformation is the opposite of evaporation and is a fundamental part of the Earth’s water cycle. For this phase change to occur, the gaseous substance must release thermal energy, which is why condensation is considered an exothermic process. We encounter countless examples of this phenomenon every day.
How Water Vapor Becomes Liquid
The transition of water vapor into liquid water is driven by a reduction in the kinetic energy of the gas molecules. As air containing water vapor cools, the rapidly moving water molecules slow down and lose energy. This decrease in movement allows the attractive forces between the molecules to pull them closer together.
Air holds a specific maximum amount of water vapor at any given temperature, known as the saturation point. When the air temperature drops to the point where it can no longer hold all the vapor, this temperature is called the dew point. Once the temperature falls below the dew point, the excess water vapor changes into liquid droplets.
For liquid formation to be visible, the water molecules typically need a surface to collect upon, known as a nucleation site. In the atmosphere, these sites are microscopic particles like dust, pollen, or sea salt. When the process occurs on a solid surface, such as a window, the surface itself acts as the site for the water molecules to coalesce.
Condensation in the Home
A common example of condensation is the “sweat” that forms on the outside of a cold beverage glass on a warm day. The surface of the glass, cooled by the icy drink inside, chills the layer of air directly surrounding it. As this air cools below its dew point, the water vapor changes into liquid droplets on the exterior of the glass.
The fogging of a bathroom mirror during a hot shower is another example. The shower releases a large amount of hot, moisture-saturated air into the small space. When this warm, humid air contacts the mirror, which is at a much lower temperature, the rapid cooling causes the vapor to condense into a thin film of water.
Condensation frequently appears on window panes, especially during winter months. Warm, moist indoor air meets the cold glass surface, which has been chilled by the outside temperature. This temperature difference causes the vapor to condense on the inner glass surface.
Condensation in Nature
Condensation is responsible for many natural weather phenomena, beginning with the formation of dew. On clear nights, objects near the ground, like blades of grass or car roofs, cool rapidly through radiation. When the temperature of these surfaces drops below the dew point of the surrounding air, the water vapor condenses directly onto them as liquid droplets.
Fog and mist are clouds that form at ground level when a layer of air cools sufficiently to reach its dew point. This cooling causes the water vapor to condense around microscopic airborne particles, forming tiny suspended water droplets that reduce visibility. The difference between fog and mist is primarily a matter of density, with fog being thicker and reducing visibility more significantly.
Clouds themselves are massive collections of condensed water droplets or ice crystals suspended high in the atmosphere. As warm, moist air rises, it expands and cools adiabatically—without losing heat to the outside. This cooling causes the water vapor to condense onto atmospheric particles, forming clouds.