The world pattern of the distribution of precipitation is undoubtedly complex. Latitude, temperature, available moisture, atmospheric disturbances, landform barriers, frontal activity, air mass movement, and differential heating are some of the factors involved in causing precipitation.
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Since these climatic factors are found in more than one combination over different parts of the earth, the resultant pattern of precipitation distribution is far from simple.
However, some generalizations may be made from the world map of precipitation. Before analyzing the world distribution of mean annual precipitation, a brief discussion of the principal factors affecting the distribution of precipitation is necessary.