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2017-01-26

Precipitation is derived from water vapour in the air, and it includes all forms of moisture falling on the earth's surface. Condensed water vapour accumulates in clouds, and precipitation occurs when the constituent ice crystals or water droplets grow too large to resist gravitational attraction to earth. The atmospheric moisture lost through precipitation is replenished by transpiration from vegetation, and by evaporation from the soil and from water bodies. The oceans, which cover 71 per cent of the earth's surface, are the primary source, though large fresh water bodies may be important locally, particularly in more southern latitudes. The map shows the annual precipitation (in millimetres) based on the 30-year period 1941 – 1970. The map was prepared from measurements at stations in the national precipitation network. In 1974 this network consisted of about 2700 stations, however, many have been in existence for only short periods. For statistical analysis a long series of data is required. To begin the map, those stations having a complete 25-year record for the standard normal period 1941-1970 were selected. The average yearly precipitation value for each station was…

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