Rain falls from clouds filled with large water drops and ice crystals. The thick clouds block out the sunlight.
The technical name for rain is precipitation, which also includes snow, sleet and hail.
Drizzle is 0.2-0.5 mm drops falling from nimbostratus clouds. Rain from nimbostratus is 1-2 mm drops. Drops from thunderclouds can be 5 mm. Snow is ice crystals. Sleet is a mix of rain or snow, or partly melted snow.
Rain starts when water drops or ice crystals inside clouds grow too large for the air to support them.
Cloud drops grow when moist air is swept upwards and cools, causing lots of drops to condense. This happens when pockets of warm, rising air form thunderclouds – at weather fronts or when air is forced up over hills.
In the tropics raindrops grow in clouds by colliding with each other. In cool places, they also grow on ice crystals.
The rainiest place is Mt Waialeale in Hawaii, where it rains 350 days a year.
The wettest place is Tutunendo in Colombia, which gets 11,770 mm of rain every year. (London gets about 70 mm.)
La Reunion in the Indian Ocean received 1870 mm of rain in one day in 1952.
Guadeloupe in the West Indies received 38.1 mm of rain in one minute in 1970.