4 Humidity, Clouds, and Precipitation

Water moves between surface and atmosphere

Water continually moves between Earth’s surface and the atmosphere. Evaporation adds water vapor to the air, condensation forms liquid droplets or ice crystals, and precipitation returns water to the surface. Together, these processes connect humidity, clouds, and rain or snow in the water cycle.

Water vapor and humidity

Water vapor is water in its invisible gaseous state. It enters the atmosphere mainly through evaporation from oceans, lakes, soil, and other wet surfaces. Plants also release water vapor through transpiration.

Humidity describes the water vapor in the air. Relative humidity is the amount of water vapor present compared with the amount needed for saturation at the same temperature, expressed as a percentage. Warmer air can contain more water vapor before reaching saturation than cooler air. As a result, relative humidity can change when temperature changes, even if the amount of water vapor stays the same.

The dew point is the temperature air must cool to, at its current pressure and water-vapor content, to become saturated. A higher dew point generally indicates more water vapor in the air. When air cools to its dew point, condensation can begin.

Evaporation and condensation

Evaporation changes liquid water into water vapor. At a water surface, some molecules gain enough energy to escape into the air. Sunlight supplies much of the energy that drives evaporation, although evaporation can occur at temperatures below boiling.

Condensation changes water vapor into liquid water. For example, droplets on the outside of a cold glass form when nearby air cools and water vapor condenses on the glass. In the atmosphere, condensation usually begins on tiny airborne particles, such as dust, sea salt, or smoke, called condensation nuclei. When water vapor changes directly into ice, the process is called deposition.

How clouds form

Clouds form when moist air cools enough to reach saturation. Rising air is a common cause of cooling: as air rises into lower pressure, it expands and cools. If it cools to its dew point, water vapor condenses onto condensation nuclei, forming countless tiny liquid droplets. In cold conditions, clouds may contain ice crystals, and some clouds contain both ice and liquid water. Clouds are visible because their droplets and crystals scatter light; water vapor itself is invisible.

For example, air rising over a mountain can expand and cool as it climbs. If it reaches its dew point, a cloud may form on the mountain’s windward side. The cloud can disappear as the air descends, warms, and its droplets evaporate.

How precipitation forms

Cloud droplets and ice crystals are very small and can remain suspended in the air. To produce precipitation, they must grow large enough to fall.

In relatively warm clouds, droplets collide and merge into larger drops through collision and coalescence. In colder clouds, ice crystals can grow as water vapor deposits onto them and as crystals collide. When particles become heavy enough to fall, they leave the cloud as precipitation.

Precipitation includes rain and drizzle, as well as snow, sleet, freezing rain, and hail. Which form reaches the ground depends in part on the temperatures within the cloud and in the layers of air below it. Not every cloud produces precipitation: droplets or crystals may remain suspended, evaporate, or melt before reaching the surface.

Connecting the water-cycle processes

The sequence links processes at the surface and in the atmosphere: evaporation and plant transpiration add water vapor to the air; cooling can bring moist air to saturation; condensation forms cloud droplets, while deposition can form ice; and droplets or crystals may grow until they fall as precipitation. Whether precipitation reaches the ground, and in what form, depends on particle growth and temperatures in the cloud and the air below it.