The world's deserts show great diversity in terms of both their landscapes and their geomorphological processes (Goudie, 2002). Climate is one major control of their character. Thus aridity determines the extent to which different types of salt can accumulate, but above all it determines the nature of the vegetation cover, which in turn controls the rate of operation of slope, fluvial and aeolian processes. For example, dunes will not for the most part move if is there is a substantial vegetation cover, nor will dust storms be generated. Deserts such as the Atacama, Libyan and Namib are hyper-arid, whereas those of the Thar, Kalahari and Australia are considerably moister. Some deserts are high energy wind environments, while others are not, and this helps to explain variations in dune forms, and the presence or absence of wind erosion features such as yardangs. Some have unidirectional wind regimes, whereas others are more variable.