Environmental Engineering Reference
In-Depth Information
Chapter
27
Dryland environments
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.
If we look at a map of surface pressure, the high-pressure centres that we would
expect over the desert areas may be absent, especially in summer (Figure 27.2). Indeed,
there is often a weak low-pressure area. These lows are the result of intense heating of the
ground surface during the cloudless days, reducing air density and so surface pressure.
Temperatures may rise to above 40° C in summer. As they are a product of surface
heating, they tend to be fairly shallow and are replaced by relatively high pressure at
higher levels of the atmosphere. Such thermal lows are frequent over the Sahara, the Thar
desert in India and even occur over Iberia in summer.
From what has been said, we would expect the climate of these zones to be
characterized by little rain and extremes of temperature. The data for Atbara, Sudan
(Figure 27.3), confirm this view. In midsummer the mean maximum temperature is over
40° C but in winter the mean minimum temperature is only 14° C and ground frost can
occasionally occur. The very dry atmosphere helps by allowing long-wave radiation from
the ground to escape to space with little counter-radiation from water vapour or clouds.
