چکیده:
The term ‘normal’ was first introduced into meteorology in 1840 by the Prussian physicist Heinrich Dove in his monograph on temperature variations around the world (Landsberg, 1955). For Dove, the normal climate of a location was to be described through the averaging of long series of regular meteorological observations. This practice slowly gained acceptance through the later nineteenth century, but it was not until 1935, at their meeting in Warsaw, that the International Meteorological Organisation (IMO) formally agreed that the 30-year period 1901-1930 should be used worldwide for computing climatic normals (IMO, 1937). This ‘normal’ was superseded in the 1960s by the adoption of 1931-1960 as the standard WMO normal, in turn replaced in the 1990s by the period 1961-1990.