Types of Precipitation

Precipitation is water, liquid or frozen, that falls from a cloud and reaches the ground. Weather services around the world classify it using definitions set by the World Meteorological Organization (WMO) in its International Cloud Atlas.

What type reaches you depends on two things: how the drops or ice particles grew inside the cloud, and the temperature of the air they fall through on the way down. The same cloud can give snow on a hilltop and rain in the valley below.

Drizzle is made of very fine drops, usually less than 0.5 millimetres across, falling close together. It usually comes from a low, grey layer of stratus cloud and is so light that the drops seem to drift on the slightest breeze. Rain drops are larger, and their size varies with how heavy the rain is.

Raindrops are not shaped like teardrops. Small drops under about 1 mm are almost round. Drops of 2 to 3 mm are flattened underneath by the air rushing past them, and drops bigger than about 4.5 mm stretch into a parachute shape and soon break apart into smaller drops.

Snow is ice crystals that fall from a cloud, either one at a time or stuck together. When the air is warmer than about –5 °C, crystals tend to clump into large, fluffy snowflakes. Snow reaches the ground as snow when the air stays at or below 0 °C from the cloud base all the way down.

Graupel, also called snow pellets, forms when an ice crystal collects supercooled cloud droplets that freeze onto it instantly. The result is a soft, white pellet up to about 5 mm across that breaks easily when it lands.

Hail forms only in thunderstorms. Strong rising air carries raindrops up into freezing parts of the storm, where they freeze. The hailstones then grow as more supercooled drops freeze onto them. They fall when the updraft can no longer hold up their weight. Hailstones are generally 5 to 50 mm across and occasionally much larger. Damaging hailstorms happen in many parts of the world, including China, India, Russia, northern Italy and the central United States.

Most precipitation that forms in winter clouds starts as snow, because the tops of the clouds are cold enough to make ice crystals. Then the layers of air below decide what lands:

1. Cold all the way down: it stays as snow.
2. A shallow warm layer, then a deep freezing layer: snowflakes partly melt and then refreeze into ice pellets, small see-through balls of ice that bounce.
3. A deep warm layer, then a thin freezing layer at the ground: the snow melts completely, and the raindrops are supercooled but have no time to refreeze in the air. They freeze the instant they touch the ground, trees or power lines. This is freezing rain, and the clear ice it leaves is called glaze.
4. Warm all the way down: it falls as rain.

Sleet has no internationally agreed definition. In the UK, the Met Office uses it for rain and snow falling together, or snow that has partly melted on the way down. In the United States, the National Weather Service uses sleet to mean ice pellets: frozen or mostly frozen raindrops, or refrozen partly melted snowflakes, that bounce when they land.

So if a British forecast and an American forecast both say 'sleet', they are describing different things. The WMO avoids the confusion by using the term ice pellets. When you read about sleet, check which country the writer is from.