ets. They contain dense dust-laden air in less dense clear air. They begin as narrow jet-like formations, suddenly expanding downwind both vertically and horizontally. Dust plumes leave recognizable deposits with distinctive soils and several plumes may form in a dust storm.
Pye (1987) said that the frequency of dust storms generally tends to show a weak inverse relationship with mean annual precipitation. There is a markedly higher frequency of such storms in areas that receive 100-200 mm of rain. In those areas that can be characterized as hyper-arid or as receiving < 100 mm of rain, dust storm frequency tends to decline. This may be because infrequent stream runoff limits the dust supply. Alternatively, the strong winds associated with fronts and cyclonic disturbances are rare in such areas and this may also prevent dust storms from occurring.
The attached chart, depicting the annual frequency of dust storms in China, is taken from Pye (1987, p. 83). This figure illustrates the relationship between mean annual rainfall expressed in millimeters (mm) and the frequency of dust storms. In those regions in which dust storms tend to be most frequent, a distinct lack of significant annual rainfall can be observed.
It has also been suggested by Pye (1987) that the higher frequency of dust storms taking place in desert marginal areas may be an artifact of greater fluvial activity, a greater supply of dust, and the more common occurrence of strong winds. This change may also reflect the effects of new cultivation of desert-marginal soils. Pye (1987), as well as Clements (1963) have suggested that the contemporary pattern of dust storm activity is heavily influenced by agricultural activity which has expanded in many regions of the world.
A second chart, also presented by Pye (1987, p. 84), illustrates the annual frequency of dust storms in southwest Asia. The highest concentration of such storms รป more than 30...