How do Clouds Form?

Regardless of the type of cloud, the basic principles behind how they form are the same. Air containing water vapor rises, the water vapor condenses out of the air, and a visible body of water vapor, or cloud, forms. There are several types of clouds, but the basic classifications are stratus, cirrus, and cumulus.

The first step of a cloud’s formation is evaporation, transpiration, and the heating of air. Evaporation and transpiration, collectively called evapotranspiration, are vital parts of the hydrologic cycle. Evaporation occurs when a body of water, like a lake or ocean, is heated by the sun so that the water of the surface sublimates, or turns into a vapor. During transpiration, plants “sweat” water through their leaves and stems as part of a cooling process. This water then turns to vapor and mixes with air. Similarly, water from the soil may also turn into vapor when exposed to heat and air.

As a consequence of the ground and water surfaces becoming warm, the air near them is also heated. Because hot air is less dense than cool air, the heated air begins to rise into the atmosphere. As the vapor parcel rises, the atmospheric pressure drops and the parcel begins to expand. This expansion causes the vapor to cool, and the cooling causes the water to condense, or clump together, in the air. This is because cool air is not as able to hold as much water vapor as warm air is.

To summarize, the heat causes the air parcel to rise, the drop in pressure at increasing altitudes causes expansion of the air, the expansion of air causes cooling, and the cooling causes the condensation of water out of the air in the form of liquid droplets. The temperature at which condensation begins is called the dewpoint. The dewpoint occurs when the air is saturated, or holding as much water vapor as it can, given its temperature and pressure. More water continues to condense as the air rises, until the air parcel reaches the same temperature as the surrounding atmosphere, or its equilibrium temperature. If the water vapor condenses into enough liquid water to become visible, it is called a cloud.

The shapes and types of clouds largely depend on how long the parcel of vapor rises after it reaches dewpoint, or in other words, the distance it rises between dewpoint temperature and equilibrium temperature. An air parcel that continues to rise for a long time after reaching dewpoint will create a tall, fluffy mass, such as a cumulus cloud. This type is puffy and has defined edges. An air parcel that reaches equilibrium at nearly the same time it reaches dewpoint will create a flatter, layered mass, usually a stratus cloud. They form low to the ground and look like a gray blanket.

On a particularly hot day, it may take the air parcel a very long time to cool down, causing a high formation of clouds. Cirrus clouds, otherwise known as “mare’s tails,” are wispy or feathery masses that sit so high in the atmosphere that their droplets crystallize. There may be combinations of cloud classifications, such as cirrostratus or cirrocumulus. A nimbus is a cloud which produces precipitation.