RAINFALL is produced when damp air rises, and cools as it expands. The ascent may be due to (a) simple convection due to surface heating, (b) the effect of upward sloping ground in caus ing the air motion to follow the slope, or (c) the action at a front, where a warm current is forced to ascend over a wedge of cold air. Most rain in middle latitudes is due to the third of these causes, but the amount of rainfall is increased where the air is forced to rise over ground. Heavy rain due to the forced ascent of damp air over high ground is most strongly exemplified by the rain of the south-west monsoon of India, and that of the western slopes of the Rocky mountains. The rainfall on the lee slopes of the mountains is very much less than that on the wind ward slopes. Similar, but less marked effects are shown by lower hills, such as the Pennine Range in Northern England.
Rainfall is slight in the central regions of the sub-tropical anticyclones, which are therefore the desert regions of the earth. In parts of the deserts no appreciable rain has ever been observed. Over most of Europe the annual rainfall exceeds 20 inches, while over most of Asia, excluding India, Tibet, and China, the annual rainfall is less than 20 inches, being less than Io inches in a long tongue extending from Arabia across to N.E. Mongolia. The central regions of Australia, a part of S.W. Africa, and a small area of Arizona, U.S.A., also have less than HD inches of rain in the year. Portions of the West coast of Africa, between the equator and io° N., a strip of the West coast of India, parts
of Assam, and a coastal strip of Burma all have more than Ioo inches of rain in the year.
The following are some of the heaviest rainfalls recorded within 24 hours: Berlin, Apr. 14, 1902, 6.6in. ; Alexandria, Dec. 9, 1888, 9.6in.; Bruton, England, June 28, 1917, 9.7in., Cherrapunji, June 14, 1876, 41.4in., Baguio, Philippines, July 14-15, 1911, 46.7in., Fiji, Aug. 8, 1906, 37.6in.
In still shorter periods, rainfall at heavier rates has been ob served. Campo, U.S.A., Aug. 12, 1891, 11.7in. in 8o minutes. Rumania, July 7, 1889, 8.2in. in 20 minutes. Portobello, Panama, Nov. 29, 1911, 2.5in. in 3 minutes. Norwich, England, July 14, 1917, o•48in. in 2 minutes. Kew Observatory, o. in. in I minute.
The wettest part of the globe is to be found in Assam and Lower Burma. At Cherrapunji the annual rainfall averages 428 inches, while at Mavijuram, in Assam, 27o inches of rain fell in the month of August, 1841.
The average rainfall or rainfall plus snow melted to water of each month, and of the year, is given in the following table for some of the chief cities of the world, together with the average number of days in the year on which snow or measurable rain fell, where such data are available.
Further data for other places will be found in any standard textbook on Climatology, such as Kendrew's Climates of the Con tinents, Oxford 1936.