Definition · Plain-language
Condensation
Condensation is the process by which water vapour, a gas, cools and changes into liquid water.
The step most authors miss
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Gas turning into liquid
Condensation is the phase change in which a gas becomes a liquid. For water, it means invisible water vapour in the air turning into visible liquid droplets. It happens when warm, moisture-laden air is cooled — cooling reduces how much vapour the air can hold, so the excess condenses out. This is the exact reverse of evaporation, which uses energy to turn liquid into gas; condensation releases that energy as the vapour gives up its heat. The everyday droplets on a cold drinks can or a bathroom mirror are condensation in action.
How clouds, fog and dew form
Condensation is responsible for several familiar sights. Clouds form when air rises, cools to its saturation point, and water vapour condenses onto tiny floating particles called condensation nuclei, producing countless cloud droplets. Fog and mist are essentially clouds at ground level, formed when air near the surface cools enough to condense. Dew appears when a clear, calm night cools surfaces such as grass below the temperature at which the surrounding air becomes saturated, so vapour condenses directly onto them. All three are the same process at different heights.
Its place in the water cycle
Within the water cycle, condensation is the bridge between evaporation and precipitation. Water evaporates from oceans, lakes and land, rising as vapour into the atmosphere. As that vapour reaches higher, cooler altitudes, it condenses into the droplets and ice crystals that make up clouds. If those droplets grow large enough, they fall as precipitation, returning water to the surface. Without condensation there would be no clouds and no rain — it is the step that gathers scattered vapour back into water that the sky can release.
Key facts
At a glance
- Definition: the change of a gas (water vapour) into a liquid
- Opposite of: evaporation
- Cause: warm, moist air cooling below its saturation point
- Needs: condensation nuclei — tiny particles for droplets to form on
- Everyday signs: clouds, fog, mist, dew and droplets on cold surfaces
- Role: forms cloud, linking evaporation to precipitation
Common misconceptions
What people often get wrong
Often heard: Condensation creates new water out of nowhere.
Actually: No new water is made. Condensation simply changes existing water vapour, already present in the air as a gas, back into visible liquid droplets.
Often heard: Condensation and precipitation are the same thing.
Actually: Condensation turns vapour into the tiny droplets that form clouds. Precipitation is the separate, later stage where those droplets grow heavy enough to fall as rain or snow.
Often heard: Condensation only happens high in the sky.
Actually: It happens wherever moist air cools enough. Dew on grass, fog at ground level and droplets on a cold window are all condensation occurring at or near the surface.
Going deeper
