ABSTRACT
The dry lands of the world cover a large area of Earth’s surface where moisture levels are limiting. In the tropics they extend from the savanna or seasonal forest zone to areas of extreme aridity in the desert cores. In polar regions and parts of continental interiors there are areas that qualify as dry in terms of their mean annual precipitation and the seasonality of water availability. Low temperatures reduce evaporation and so water levels are usually sufficient for some plant growth during the growing season. Clearly there are degrees of dryness that can be used to subdivide this large area. Climatologists have devised indices based on the inputs of precipitation relative to evaporation outputs in order to quantify the degree of dryness. For the purposes of this chapter we will retain the broad definition and consider dry lands as those areas of the world where there is a significant moisture deficit (Figure 27.1)
CLIMATE
The core areas of the dry environments are the subtropical high-pressure systems that also act as the meteorological boundary between the tropical and temperate latitudes. The dominant air movement at the surface is away from the highs, with the flow being sustained by sinking air from higher levels as part of the Hadley cell of the tropics. Because the air is subsiding it tends to be warming and drying. An inversion of temperature usually develops near the surface (Figure 7.12) and so the core areas of the highs are generally cloud-free and deficient in rain. Where the highs remain fairly constant in position we find the main desert areas of the world – the Sahara, the Kalahari and the Great Australian Desert.
