Present-day desert environments - Climatic Change in Deserts - page 15

Figure 2.1. Schematic cross section of the global atmospheric circulation, showing
location of the Hadley cells.

the heavy convectional downpours so characteristic of the wet and seasonally wet
tropics.
Meanwhile, the air aloft becomes colder and denser and moves further away from
the equator, until it finally begins to subside ( Figure 2.1 ). The tropical latitudes centred
on about 20-30 north and south are zones dominated by atmospheric subsidence.

Depleted of much of its excess water vapour, the air over the two tropics is dry during
the winter months but may become moist for a fewmonths during the seasonal passage
of the sun overhead and the attendant displacement of moist tropical air masses, known
as the movement of the Intertropical Convergence Zone ,orITCZ( Figure 2.2 ). The
further the distance from the equator, the shorter the tropical wet season becomes, until
it ceases altogether and the deserts take over from the tropical savannas. Hot tropical
deserts like the Sahara and Arabia are in latitudes where the air aloft is dry and sub-
siding and the atmospheric pressure is high for much of the year. The surface winds in
deserts are therefore generally directed outwards, towards areas of lower atmospheric
pressure, so there is minimal inflow of moisture from surface winds. As the air over
the deserts subsides, it is compressed and becomes warmer, so that its capacity to
absorb additional water vapour is increased. The result is that the relative humidity
of desert air is usually very low and only reaches dew point (100 per cent relative
humidity) when the night temperatures fall sufficiently for desert dew to precipitate

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