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Causes of Aridity

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Aridity
Aridity
Causes of Aridity (1) – Atmospheric Pressure (on graph)
Causes of Aridity (2) – Cold Ocean Currents:
Several deserts lying along western coasts of continents owe their formation to cold ocean surface currents. Air blowing towards the shore, chilled by contact with cold water, produces a layer of fog. This heavy fog drifts onto land. Although humidity is high, the atmospheric pressure that normally causes rainfall is not present. A coastal desert may be almost totally rainless, yet damp with fog
The Atacama Desert, on the pacific shore of Chile, is a coastal desert. Some areas of the Atacama often covered by fog but the region can go decades without rainfall. In fact, the Atacama Desert is the driest on earth. Some weather stations have never recorded a drop of rain.
Causes of Aridity (3) - Rain Shadow Effect:
Dry Air Dry Air

3. The leeward side
3. The leeward side
2.The windward side
2.The windward side
Rain from expansion and cooling
Rain from expansion and cooling 1. Approaching Air mass 2. Approaching Air mass
High Mountain Ranges may lead to the formation of desert areas by intercepting precipitation from moist, prevailing winds
Leeward: facing away from prevailing winds
Leeward: facing away from prevailing winds

In these circumstances, windward slopes of the mountain ranges receive more rainfall than leeward slopes which will be in the rain shadow. In extreme cases, the leeward slopes may experience arid or semi-arid climate conditions. 1. Approaching Air mass – moist air brought inland by prevailing winds over a body of water 2. The windward side – as moist air is forced to rise over mountains by prevailing winds it cools and expands. Condensation occurs on the windward side of the mountains, since the colder air cannot hold as much water vapour as the warm water. The result is cloud formation and precipitation where the mountain range is high, most of the water will be “expended” as precipitation on the windward slopes. 3. The Leeward Side: one the air mass has crossed the summit; it descends on the leeward slopes. As it descends, the air is compressed and warmed and the clouds evaporate. As a result, the leeward slopes receive little moisture, creating an arid or semi-arid environment. For example, Arica, located in the Atacama Desert and in the leeward side of the Andes Mountains, receives av. 0.5mm a year
Causes of Aridity- Continentality
Many arid areas occur within the largest land masses of the wold – in dry continental areas. As an air mass moves from an ocean over a continent, it will lose moisture as precipitation, but will not pick up much moisture because little evaporation takes place over the land. Consequently, places with a coastal location will have more rainfall than places inland. Areas in centre of continents have little rainfall because air has dried out whilst moving over land.
Deserts found in the interior of large continents are called continental isolation deserts
Gobi Desert is so dry because part of it is over 2000km away from nearest ocean.
Causes of Aridity (5) – Winds (offshore)
Many hot arid areas are located where the prevailing wind blows from the land to the sea, thus carrying little moisture. This can be seen in areas such as the Sahara Desert. Despite the proximity of the Atlantic Ocean, the prevailing winds are the northeast trades, and they blow across North Africa towards The Atlantic Ocean. By the time they reach the Sahara Desert, there is little moisture available or precipitation.
Possible 15 mark:
Assess the importance of different factors causing desert and arid regions
Possible 15 mark:
Assess the importance of different factors causing desert and arid regions Southeast trades
Southeast trades
Northeast trades
Northeast trades
Hot deserts
Hot deserts
Hot deserts
Hot deserts
Forests
Forests
Cold Deserts
Cold Deserts
Forests
Forests
Cold Deserts
Cold Deserts

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