What Is Precipitation?
QUICK ANSWER
Precipitation is water in any form falling from clouds to Earth's surface. The main types are rain (liquid drops), snow (ice crystals), sleet (frozen rain), hail (layered ice), and freezing rain. Precipitation happens when cloud droplets grow large enough to overcome air resistance and fall.
Precipitation is one of the most familiar parts of the water cycle, the process by which water returns from the atmosphere to Earth's surface. The many forms precipitation can take, from gentle rain to driving hail, reflect different conditions in the clouds and atmosphere where it forms. Understanding precipitation explains everything from gentle showers to dramatic snowstorms.
What forms can precipitation take?
Precipitation comes in several forms depending on temperature conditions. Rain is liquid water droplets falling from clouds; the most common form globally. Snow consists of ice crystals that form directly in cold clouds. Sleet is rain that freezes into ice pellets while falling through a cold air layer. Hail is layered ice that grows in thunderstorm updrafts. Freezing rain falls as liquid but freezes on contact with cold surfaces. Light forms include drizzle (very small drops) and snow flurries. Each form requires specific atmospheric conditions for its formation.
How does precipitation form?
Precipitation forms when cloud droplets or ice crystals grow large enough to overcome air resistance and gravity. The process happens through two main mechanisms. Collision-coalescence: small water droplets collide and merge into larger drops in warm clouds. The Bergeron process: ice crystals grow at the expense of surrounding supercooled water droplets in cold clouds. Both processes can produce raindrops several millimeters in diameter, large enough to fall as visible precipitation. Cloud droplets that don't grow large enough remain suspended as cloud.
What determines the type of precipitation?
The type of precipitation depends mostly on atmospheric temperature profiles. If temperatures stay above freezing from cloud to ground, precipitation falls as rain. If clouds are below freezing and the air column stays below freezing to the ground, snow falls. Sleet forms when rain falls through a cold layer near the ground (snow falls into a warm layer where it melts, then refreezes in a cold layer below). Hail requires powerful updrafts to keep ice particles aloft in thunderstorms. Freezing rain forms when rain falls through a thin cold layer near the surface.
How is precipitation measured?
Precipitation is measured by depth of accumulation. Rain gauges collect rain in a graduated cylinder for direct measurement, typically reported in inches or millimeters. Snow depth measurements add a separate calculation for snow water equivalent (how much liquid water the snow contains). Radar measures precipitation rate remotely from above by detecting raindrops in clouds. Satellites estimate global precipitation from space using various sensing techniques. Annual precipitation varies enormously by region: the driest deserts get less than 1 inch per year while the wettest rainforests get over 400 inches.
Precipitation is water falling from clouds in forms including rain, snow, sleet, hail, and freezing rain. It forms when cloud droplets or ice crystals grow large enough to fall, through collision-coalescence or the Bergeron process. The type depends on atmospheric temperature profiles between cloud and ground. Precipitation is measured directly with rain gauges, snow gauges, and remotely with radar and satellites.
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