Cloud

A cloud is a visible collection of water droplets or ice crystals suspended in the air. Clouds form when moist air cools to its dew point, causing water vapor to condense into tiny droplets. These droplets accumulate around microscopic particles in the atmosphere—dust, salt, or pollution—known as condensation nuclei.

Clouds are a crucial part of Earth's water cycle. They transport water from oceans and land through evaporation and return it to the surface as rain, snow, or sleet. Clouds also affect the planet's heat balance: some reflect incoming sunlight back to space, while others trap heat radiating from Earth's surface.

Cloud formation

Air rises when heated by the ground, pushed up mountains, or when two air masses of different temperatures meet. As air rises, atmospheric pressure and temperature decrease, and the air expands and cools. When it reaches the dew point temperature, water vapor begins to condense into tiny droplets. Billions of these droplets collect together to form visible clouds.

Types and altitudes

Meteorology classifies clouds by their form and altitude. High-altitude clouds (such as cirrus) consist mainly of ice crystals and are light and thin. Mid-level clouds (such as altocumulus and altostratus) appear as layers or patches. Low-altitude clouds (such as stratus and cumulus) are closest to the ground and sometimes touch the surface. Towering clouds at high altitudes, particularly cumulonimbus, are associated with thunderstorms, lightning, and heavy rain.

Precipitation and clouds

Not all clouds produce rain. Cloud droplets are extremely small and remain suspended in air. Precipitation occurs only when droplets accumulate and become heavy enough to fall, or when ice crystals in upper clouds grow and descend before melting. Thick, moisture-laden clouds produce rain, while thin, light clouds often only add moisture to the air.