Deserts- Distribution and Characteristics

Aridity Index:

  • Water balance = relationship between mean annual precipitaion (P) and potential evapotranspiration (PET). If P>PET = water surplus, vice versa = water deficit.
  • In deserts, P<PET (water deficit). The size of the deficit (difference between P and PET) gives the aridity index
  • A low value = aridity is high – a value below 0.2 = desert. Desert margins (semi-arid) = 0.2-0.5


Where are most deserts found?

  • Around 30 degrees N&S of the equator: Global atmospheric circulation explanation)
    • Air moves in circular patterns between the equator called Hadley cells.
    • In a Hadley cell, air rises at the equator. The air cools as it rises, and moisture condenses and falls as rain, leaving the air dry.
    • The dry air descends around 30 degrees north and south of the equator.
    • In areas where the air descends a zone of high pressure is created.
    • Winds blow outwards from high pressure areas – so no moisture is brought in by the wind.
    • This means that the area has very low precipitation E.g Sahara
  • In the middle of continents: Continentality
    • Moist wind from the sea moves inland and the moisture held is dropped as precipitation.
    • So when the wind reaches the centre of the large continent it’s carrying very little moisture,
    • g Turkestan desert in it’s central part of Asia.
  • Next to mountain ranges: Rainshadow effect explanation
    • Tall mountain ranges force winds upwards (up the windward side of the mountain)
    • As air rises it cools and is less able to hold water.
    • Any moisture held is dropped as precipitation over the mountains, so the wind that moves inland (down leeward side) has very little moisture
    • For example, the Atacama Desert in South America because of the Andes mountains.
  • Near cold ocean currents: In some places, cold ocean currents run along the coast.
    • Wind is cooled as it travels over the cold water and its ability to hold moisture is reduced.
    • Moisture stored in the atmosphere is released as rain over the ocean before reaching land.
    • So, when the wind reaches the land there’s very little moisture left, so very little rain falls.
    • For example, Namib desert in Africa exists because of the Benguela Current


Why is there little vegetation in Deserts?

  • Lack of water > difficult for things to grow > low biomass
  • Small shrubs, grasses and cacti species
  • More vegetation, increases the further you go from the desert, because there’s more water