AQA AS GEOGRAPHY DESERT ENVIRONMENTS NOTES

1. Location and characteristics of hot deserts and their margins  There are 3 global belts of high rainfall and four of low rainfall. The high rainfall belts are the three regions of global convergence- the inter tropical convergence zone (ITCZ) and the two polar fronts.  The four belts of low rainfall are the regions of divergence- the two belts of subtropical highs centered on latitudes 30N and 30S, and the two polar regions.  The effect of the global air-circulation system is most clearly demonstrated by the distribution of deserts. Location Hot deserts and their margins are located within and just outside the tropic of cancer and capricorn. In the southern hemisphere they occur on the Western side of the continents.  Major deserts are found in: The most extensive deserts, the Sahara, Kalahari, Great Australian, are associated with the two circumglobal levels of divergence, where dry air descends on the downward flowing limbs of the Hadley cells, centered between latitudes 20 degrees and 30 degrees.   Deserts such as the Gobi form because wind that travels a very long distance over land, especially land that rises up to high plateaus eventually contains so little water vapour that hardly any is left for precipitation. A third kind of desert is found where a mountain range creates a barrier to the flow of moist air, causing orographic lifting and heavy rains, on the windward side, along with a zone of low precipitation called a rain shadow on the downward side.   Climate  There are big temperature changes diurnally- between day and night in deserts.  During the day, especially in summer, there are high levels of insolation as the incoming solar radiation is not implemented by cloud cover.  At night, the reverse happens when clear skies allow long wave radiation to escape and ground temperatures fall rapidly.  Deserts close to the sea have a lower diurnal range, owing to the presence of cold of shore currents.   Rainfall in deserts is low and extremely unreliable. The lower the annual rainfall, the less reliable it is.  When rain does fall, produces rapid surface runoff which together with low infiltration and high evaporation minimises its effectiveness for vegetation.  Deserts are also subject to extreme rainfall events during which more than the average rainfall can fall in a few hours.   The descending air of deserts is warned by compression. It is warmed further by insolation, which is stronger in these latitudes not far from the equator.  Insolation is particularly strong during the summer period, when the sun is overhead. However, the lack of cloud at night time allows heat to radiate very quickly and temperatures fall considerably.   This cooling can lead to mist and dew forming, in many desert areas, dew is one of the major forms of precipitation.   Convection currents often build up because of the heating of the surface but are rarely strong enough to overcome the downward pressure of the descending air. When they do, they can rise high enough to cause enough cooling for…