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and evaporation rates will also have an important influence on the movement of sand
grains, as the presence of active coastal dunes in relatively wet areas with strong
winds and abundant sand attests. The source-bordering dunes that form downwind
of sandy channels in semi-arid areas provide another exception to the general rule
that dunes cease to be mobile once the rainfall exceeds about 150 mm. As we shall
see in more detail in Chapter 8 , there are three main prerequisites for the formation
of source-bordering dunes. First is a regular, usually seasonal replenishment of river
channel sands or of sandy beaches by long-shore drift in deep lakes. Second is a strong
seasonal unidirectional wind, and third is a lack of riparian or lake-margin vegetation.
The first prerequisite, a regular renewal of the sand supply from seasonally active
rivers, precludes a fully arid climate.
Bagnold's classic observations on the Libyan Desert and his detailed experimental
work (Bagnold, 1941 ) demonstrated that the volume of desert sand transported bywind
increases exponentially with wind velocity above a certain threshold value, a finding
confirmed by later workers (Pye and Tsoar, 1990 ;Cookeetal., 1993 ; Lancaster, 1995 ;
Warren, 2013 ). Where sand supply and wind velocities are not limiting factors, dune
mobility will increase as vegetation cover decreases. Rainfall and evapotranspiration
are the primary controls over plant cover in arid areas. There is therefore a close
relationship between the amount of rainfall and the average outer limit of active dunes
in such deserts as the Thar or the Sahara. The belts of fixed and vegetated dunes along
what are now the semi-arid margins of these two deserts have been mapped in detail
from air photographs and satellite imagery. Assuming that the relationship between
rainfall and dune mobility held good in the recent past, then the presence of these fixed
dunes indicates that the effective range of the Sahara once extended 400 to 600 km
further south and that of the Thar Desert some 350 km further south-east. Chapter 8
enlarges on these general propositions.
2.5 Conclusion
Four main factors are responsible for aridity. The first factor is latitude. The hot
tropical deserts are located in latitudes characterised by dry subsiding air. The reason
for this is linked to the global atmospheric circulation system in which solar heating
in equatorial latitudes causes air to rise and move towards the poles. The moist air
aloft cools and sheds much of its water vapour as rain that falls over the equator
and the adjacent seasonally wet tropics. As the air aloft continues its path towards
the poles, it becomes cooler and denser and starts to subside between latitudes 25
°
and 30
north and south, creating zones of high pressure known as anticyclones. As
it subsides, the air becomes warmer and its relative humidity decreases, so it has a
desiccating effect on the land below. The second factor causing aridity is distance
inland, an effect known as 'continentality'. Except in equatorial latitudes, the greater
the distance from the coast, the lower the rainfall. The third factor is the presence
°
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