Rain

Rain is liquid water falling from the sky. It is the most common type of precipitation on Earth and the main way fresh water is delivered to the land. Rain forms when tiny water droplets in clouds grow heavy enough to fall, either by combining with other droplets or by growing around tiny ice crystals. Different parts of the world get very different amounts of rain, from over 11 metres a year in tropical rainforests to almost none at all in some deserts.

• Smallest raindropApproximately 0.5 mmAnything smaller is drizzle
• Largest raindropApproximately 5 mmBigger ones break apart while falling
• Speed of falling rain8 to 10 m/sAbout 30 km/h at terminal velocity
• Wettest placeMawsynram, India11,800 mm of rain per year
• Driest placeAtacama Desert, ChileSome areas have no recorded rain
• UK average rainfallAround 1,150 mm/yearWetter in west and north

How raindrops form

Clouds are full of tiny water droplets, each only around 0.02 mm wide (about as wide as a human hair). These are far too small to fall as rain; they just float around in the air. For rain to form, the droplets need to grow much bigger.

This happens in two main ways.

• Collision and coalescence: droplets bump into each other and stick together, gradually growing bigger. Most common in warm tropical clouds.
• The ice crystal process (also called the Bergeron process): in cold clouds where ice and water co-exist, water vapour preferentially condenses onto ice crystals, growing them quickly. The ice crystals melt as they fall through warmer air and arrive as raindrops. Most rain in temperate places like Britain forms this way.

Why raindrops are not teardrop-shaped

Despite how they are usually drawn, raindrops are not teardrop-shaped. Small drops (under 1 mm) are almost perfectly spherical, held into shape by surface tension. Larger drops (1 to 4 mm) are flattened by the air resistance they push through as they fall, ending up shaped like a hamburger bun (round on top, flat on the bottom). Above about 5 mm, drops become unstable and break apart, which is why even the heaviest rain produces lots of medium-sized drops rather than a few huge ones.

How fast raindrops fall

Raindrops accelerate as they fall, but they do not keep getting faster forever. Air resistance grows with speed until it balances gravity, at which point the drop falls at a constant speed called the terminal velocity. For raindrops this is between 8 and 10 metres per second (around 30 km/h), depending on drop size. Small drops fall slower; larger drops fall faster. Hailstones and large hail can fall much faster, sometimes over 100 km/h.

Where it rains most and least

Rainfall is wildly uneven across the planet.

• Wettest places: Mawsynram and Cherrapunji in north-east India, with over 11,000 mm of rain per year. Both sit at the foot of the Himalayas, where monsoon clouds rise over the mountains and dump their water.
• Tropical rainforests: Amazon, Congo and Indonesian rainforests typically receive 2,000 to 4,000 mm of rain per year.
• Temperate regions (most of Europe, eastern USA, Japan): typically 500 to 1,500 mm per year.
• Deserts: typically under 250 mm per year. The Sahara averages around 100 mm.
• The Atacama Desert in Chile: the driest non-polar place on Earth. Some weather stations have never recorded rain in their entire history.
• Antarctica: technically the driest continent, despite being covered in ice. Most precipitation falls as snow, very slowly.

Fact The wettest place on Earth, Mawsynram in north-east India, gets an average of 11,800 millimetres of rain per year. That is nearly 12 metres of rain: enough to drown a 3-storey building. The wettest month ever recorded was July 1861 at nearby Cherrapunji, which received 9,300 mm in a single month. Most of this rain falls during the summer monsoon, when wet winds from the Indian Ocean meet the steep wall of the Himalayas.

The water cycle

Rain is just one stage of the planet's endless water cycle. Water evaporates from the oceans, rises as water vapour, condenses into clouds, falls as rain, then flows back to the oceans through rivers or seeps into the ground as groundwater. The cycle never stops. Every drop of rain that falls today has been part of countless previous cycles over the past 4.5 billion years.

Did you know? The world's biggest single rainfall event happened on Reunion Island in the Indian Ocean on 7 to 8 January 1966, when Tropical Cyclone Denise dropped approximately 1,825 mm of rain in 24 hours: more than the entire average annual rainfall of London. Tropical cyclones can produce enormous rainfalls because they continuously suck up warm wet air from the ocean and dump it as rain.

Deeper dive: why the British complain about rain so much

British weather has a worldwide reputation for being rainy, but Britain is not actually one of the wettest places on Earth. London gets about 600 mm of rain per year, less than Sydney (1,200 mm), Singapore (2,300 mm), or New York City (1,250 mm). What makes British rain stand out is not how much falls, but how often.

The reason is that Britain sits in the path of the Atlantic westerlies: prevailing winds that blow from west to east across the ocean, picking up huge amounts of moisture along the way. Low-pressure weather systems (depressions) regularly sweep in off the Atlantic, bringing fronts of cloud and light rain. These fronts arrive every few days throughout the year, which means it can rain on roughly 150 days a year in Britain, more than almost anywhere else of similar size in the temperate world.

So Britain gets many days with a small amount of rain, rather than fewer days with heavier rain. This is why British rain feels constant: it really is. A typical British day might bring a few light showers and then clear up, but the cycle repeats again the next day, and the day after that.

Within Britain, the west is much wetter than the east. The Lake District, Snowdonia and the western Scottish Highlands get over 2,000 mm of rain per year, while parts of East Anglia get under 500 mm. The pattern matches what you would expect: rain falls heavily on the western mountains, leaving the east in a rain shadow. Cambridge is actually one of the driest cities in the UK.

For more, see clouds, precipitation and the water cycle.