Precipitation is the name we give any moisture that falls from the air to the ground and includes snow, sleet, hail, and drizzle as well as rain. (Fog, mist and haze are not precipitation - see separate pages. The total amount of precipitation in one year is 5,000 million million tonnes (tons). Although the oceans hold 97% of Earth's water, air is never completely dry. Even in the mid-Sahara there is water vapour in the air. It contains varying levels of water vapour - the most important gas as far as weather is concerned. The amount of water held in the atmosphere at any time is sufficient to produce about 2.5 cm (1 in) of rain over the surface of the Earth. Humidity is the measure of the amount of water vapour in the air. It is measured as a percentage called relative humidity. The higher the percentage, the more humid it is. When it is 100%, the air is saturated. There is no evaporation. Wet surfaces do not dry and neither will your washing. The average raindrop has a million times more water than the average cloud droplet and the fastest raindrops can fall at about 9m/second, compared to snow which only falls at ½ to 1 m /second.

The water cycle operates because air absorbs and releases water vapour. Water vapour enters the air by evaporation from wet surfaces such as the oceans (more than two thirds of the Earth's surface), but also from rivers, lakes and falling rain and snow. It also arises from evapotranspiration from vegetation and in small amounts from the combustion of fossil fuels and from the depths of the Earth in volcanic gases. When air rises it cools and can release the water as precipitation - rain, snow, hail, dew, frost. The water travels back to the sea via rocks, lakes, rivers, plants.