Rainfall in a given locality is governed by two sets of factors: (1) Hydrodynamic factors, such as topography and orographic conditions, which, together with atmospheric circulation, general and local, determine the occurrence and frequency of ascending air‐masses; (2) moisture‐supply. Either of these factors may be adequate to produce rain but if the other is deficient, little or no rain occurs. Air‐masses rich in moisture pass in summer from the Pacific Ocean over the coastal plains of California, producing little or no rain until forced above the condensation‐level by the coastal mountain ranges.
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Robert Horton (1943) studied this question.