Precipitation

Precipitation is the technical word for any kind of water falling from the sky: rain, snow, sleet, hail, drizzle and a few rarer types. It is the third stage of the water cycle: the moment when water that has spent days drifting through the atmosphere finally falls back to Earth. Different types of precipitation form depending on the temperature inside the clouds and on the way down to the ground.

  • Main types5Rain, snow, sleet, hail, drizzle
  • Raindrop size0.5 to 5 mmBigger drops break apart as they fall
  • Snowflake sizeUp to 5 cmLargest ever: 38 cm (Montana, 1887)
  • Biggest hailstone20 cm wideVivian, South Dakota, 2010
  • Wettest placeMawsynram, IndiaAverage 11,800 mm of rain per year
  • DriestAtacama Desert, ChileSome areas have no recorded rain

How rain forms

Inside a cloud, tiny water droplets and ice crystals bump into each other and stick together. As more droplets join, the combined droplet grows bigger and heavier. When a droplet becomes too heavy for the rising air in the cloud to hold it up, it falls towards the ground as a raindrop. A single raindrop typically contains the water of about a million cloud droplets combined.

Raindrops are not actually shaped like teardrops. Small drops are nearly spherical. Larger drops are flattened by air resistance into a shape like a hamburger bun (round on top, flat below). Above about 5 mm, drops become unstable and break apart, which is why heavy rain produces lots of medium-sized drops rather than a few huge ones.

The different types

  • Rain: liquid water drops larger than 0.5 mm.
  • Drizzle: very small drops (under 0.5 mm), so light they almost float.
  • Snow: water vapour freezes directly into ice crystals inside clouds and falls as snowflakes when the air below is cold enough.
  • Sleet: rain that freezes back into small ice pellets as it falls through a layer of cold air below the cloud.
  • Hail: ice balls that form in thunderstorms when raindrops are carried up and down through freezing layers, adding layer after layer of ice.
  • Freezing rain: rain that stays liquid until it touches a freezing surface, then freezes instantly. Dangerous because it covers everything in a sheet of ice.
  • Graupel: small pellets of soft icy snow.

How snowflakes form

Snowflakes form when water vapour in cold clouds turns directly into ice crystals (a process called deposition). Each crystal starts on a tiny speck of dust or pollen. As more water vapour deposits onto the crystal, it grows the famous six-sided shape that all snowflakes share. The exact pattern depends on the temperature and humidity at every moment of the snowflake's growth.

It is often said that no two snowflakes are alike. Considering the trillions of snowflakes that form in a single snowstorm, this is mathematically impossible to test, but the variety of shapes is astonishing. Each snowflake takes a slightly different path through the cloud, encountering different conditions, so each ends up with its own unique pattern.

Fact The largest single hailstone ever recorded weighed almost 1 kg and was 20 cm wide, the size of a small grapefruit. It fell in Vivian, South Dakota, USA, during a fierce thunderstorm on 23 July 2010. Hailstones that big form in supercell thunderstorms where powerful updrafts hold the growing ice balls in the freezing zone for long enough to build up layer after layer of ice.

The wettest and driest places

Rainfall is not spread evenly across the planet.

  • The wettest places on Earth get over 10,000 mm of rain a year (Mawsynram and Cherrapunji in India, both in the foothills of the Himalayas).
  • The driest places get almost none. Parts of the Atacama Desert in Chile have gone over 400 years without measurable rain in some recorded periods.
  • The UK averages about 1,200 mm a year, with western and northern areas wetter and the south-east drier.
  • The Sahara averages under 100 mm a year.
Did you know? Sometimes it rains things that are not just water. In rare events called animal rain, fish, frogs and even small birds have been recorded falling from the sky after powerful waterspouts (water-based tornadoes) suck them up from a lake or sea, carry them through the clouds, and drop them somewhere else when the spout weakens. Animal rain has been reported in dozens of countries over hundreds of years.
Deeper dive: why your rainwater carries dust from the Sahara

Almost every raindrop that falls on Britain (and indeed almost any rain anywhere) is not actually pure water. Every raindrop forms around a tiny particle called a condensation nucleus. Without these particles, water vapour cannot easily condense and rain cannot form.

The nuclei can come from many sources: dust blown from deserts, salt from sea spray, soot from forest fires, pollen from plants, ash from volcanoes, even pollution particles from factories and vehicles. Once a raindrop has formed around the nucleus, it may also pick up many more particles as it falls through the air.

One of the most surprising sources is the Sahara Desert. Powerful winds regularly blow huge clouds of fine red dust off the Sahara and across the Atlantic Ocean. Some of this dust falls into the ocean; some makes it as far as the Caribbean and even South America. Sahara dust regularly forms part of the rain that falls on western Europe, including Britain. In particularly dusty events, parked cars in southern England can be coated with a thin layer of orange dust after rain.

Even more remarkably, the Amazon rainforest depends on Saharan dust for some of its mineral nutrients. Approximately 27 million tonnes of dust blow off the Sahara each year and reach the Amazon, providing essential phosphorus and other nutrients that the rainforest soil itself lacks. The two ecosystems, separated by an ocean, are quietly linked by atmospheric dust.

For the next stage, see run-off and groundwater. For where the water came from, see condensation.