Precipitation is defined as any of all of the forms of water particles, whether liquid or solid, that falls from the atmosphere and reach the ground. The forms of precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail, and ice pellets.

Countries’ precipitation ranges from 100 mm/yr in arid, desert-like climates to over 3,400 mm/yr in tropical and highly mountainous terrains.

About 40% of the precipitation that falls on land comes from ocean-derived vapour. The remaining 60% comes from land-based sources.

The monsoon, tropical cyclones and mid-latitude frontal and convective storm systems are important mechanisms controlling precipitation, while orographic lifting is another.

Towards the poles and with increasing altitude, a greater proportion of the precipitation occurs as snow. The annual snowfall over the earth is estimated to be about 1.7×1013 tons, covering an area that varies from year to year between 100 and 126 million km2.

Snowfall can contribute a large percentage of a region’s total precipitation in temperate and cold climate regions. For example, in the western US, Canada and Europe, 40% to 75% of regional precipitation can occur as snow.

Small annual precipitation totals (200 mm and less) occur in the subtropics, the Polar Regions and in areas furthest from the oceans. There are also rain shadows in the lee of mountains, such as in the valleys east of the Sierra Nevada in the western United States where totals are small.

Shiklomanov estimates the total precipitation on the land surface to be 119,000 km3 per year with other estimates ranging from 107,000 to 119,000 km3.

Global circulation models of the atmosphere suggest that the increased carbon dioxide and other greenhouses gases are likely to cause changes to the global climate. More precipitation can be expected from 30° North and 30° South because of increased evapotranspiration. In contrast, many tropical and subtropical regions are expected to receive lower and more erratic precipitation in the future. Climate change is also likely to lead to increased magnitude and frequency of precipitation-related disasters – floods, droughts, mudslides, typhoons and cyclones.

Information from:
the Arctic Climatology and Meteorology glossary
the 1st United Nations World Water Development Report: ‘Water for People, Water for Life’
the 2nd UN World Water Development Report: ‘Water, a shared responsibility’

Source: UNESCO Water Portal, March 2007