The definition of precipitation is any form of water - liquid or solid - falling from the sky. It includes rain, sleet, snow, hail and drizzle plus a few less common occurrences such as ice pellets, diamond dust and freezing rain.
Rain is drops of liquid water falling from the sky. In order for the raindrops to become heavy enough to fall, droplets of water in the cloud collide together with other droplets and other particles in the air - like soot and dust - to become larger. Once the drops become too heavy to stay in the cloud, we get rain.
There are three main types of rainfall - frontal rain, orographic rain and convective rain.
Size: Raindrops can be up to 6 mm in diameter, but anything less than 0.5 mm in diameter is classed as drizzle.
Drizzle consists of very small droplets of water falling from low level stratus clouds.
In the UK we're very familiar with drizzle. As we have a mild temperate climate we don't tend to get the extreme temperatures that can cause heavy rain, but our prevailing winds from the Atlantic pick up lots of moisture, and therefore clouds, as they travel over the sea.
Size: Drizzle droplets are less than 0.5 mm in diameter - larger than the droplets in clouds, but smaller than raindrops.
Freezing rain is rain droplets which fall in supercooled liquid form, but freeze on impact with the ground or another object to form clear ice - also known as glaze. Supercooling occurs in clouds where droplets remain in a liquid from in temperatures below the normal freezing point. In order for the supercooled droplets to freeze on impact, the ground temperature is normally close to or below 0 °C.
Size: Freezing rain can fall as rain droplets or drizzle.
Hail is solid precipitation in the form of balls or pieces of ice known as hailstones. Hail only forms in cumulonimbus clouds - more commonly known as thunder clouds.
In thunderclouds, drops of water are continuously taken up and down though the cloud. When they go to the top of the cloud, it is very cold and they freeze. As the updraughts in thunderclouds are very big, they can keep these hailstones for a long time, so they get larger and larger by becoming coated with more and more ice. Then, when they get really big, the updraughts in the cloud cannot hold them up anymore and they fall to earth, and by this time they are big balls of ice, and so don't have time to melt before they reach the ground. Hail can only be formed in this way, unlike snow which can be formed in fronts, and orographically too, just like rain.
Size: Hailstones can vary in size from 5 mm to 150 mm in diameter, however most hailstones are smaller than 25 mm.
Ice pellets are snowflakes which have started to melt, and then re-frozen as they fall through colder air. The result is a grainy snow pellet encased in ice. Ice pellets are generally smaller than hailstones and bounce when they hit the ground.
Snow is tiny ice crystals stuck together to become snowflakes. If enough ice crystals stick together, they'll become heavy enough to fall to the ground.
Visit our snow and snow forecasting pages for more information on snowfall.
Sleet has no internationally agreed definition but is reported in meteorological observations as "rain and snow mixed". Sleet, which is sometimes known as ice pellets, is basically snow which has begun the melting process before it reaches the ground.
Snow grains are very small white and opaque grains of ice, less than 1 mm in diameter.
Size: Less than 1 mm in diameter.
Diamond dust, sometimes just called ice crystals, consists of extremely small ice crystals, usually formed at low levels at temperatures below -30 °C. The name diamond dust comes from the sparkling effect created when light reflects on the ice crystals in the air.
Last updated: 20 February 2013
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