Geoscience Reference
In-Depth Information
26
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 26.1
).
the surface (
Figure 7.14
)
and so the core areas of the highs
are generally cloud-free and deficient in rain. Where these
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 26.2
).
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 the Iberian peninsula 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, on the
river Nile, north of Khartoum, Sudan (
Figure 26.3
),
confirm this view. In midsummer the mean maximum
temperature is over 40
C but in winter the mean
minimum temperature is only 14
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
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. Even at Bahrain (26
N), on the Arabian Gulf,
night-time temperatures are cool in winter, though frost
