Deserts


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Factors that help create a desert

Why is the desert so dry

Cold current

The current comes from the poles and is cold so it creates mist and does not carry or evaporate water so the area a few kilometres from the sea is dry.

Rain shadow

Two things can create the rain shadow: Mountains and continental location.

Deserts such as the Gobi desert are created as it is so far away from the sea that the water canít make it that far and so it falls before arriving to the desert.

Mountains such as the Jura are higher than the surrounding land and so the rain has two choices: to fall or to go up the slope and so the mountain gets all the moisture and the people the other side gets less rain.

High pressure

High pressure is when cold air sinks down and so canít evaporate the water that means that no clouds are formed meaning that there wonít be any rainfall.

 

Water action in the desert

180,000 years ago the Sahara was much wetter than it is today. At that time rivers ran across the land carving out deep valley. Today there are usually dry and are known as Wadis.

If a desert storm lasts 30 minutes draw a possible storm hydrograph for the wadi to show the water output.

When there is a sudden rainstorm floodwaters flow down Wadis. They are short lived but carry sediments in large quantities. This is deposited as alluvial pans at the foot of steep slopes. Several fans join together to form a Bajada.

Aquifer

An aquifer is an underground collection of water. It is a water holding layer of rock where rain water collects. A well can be sunk in order to get to the water table and then pumped up to the surface. Hollows in the sand may be due to deflation. I.e.: the blowing action of the wind. Their downward limit is marked by the water table e.g. Alfayom depression on Egypt, which produce a wide range of crops.

 

Types of sand dunes

The fine particles of sand which blow in the deserts usually collect in certain regions and when they blow they various sand dune shapes.

Barchans

Barchans may form around an obstacle. They are crescent shaped and face down wind. They migrate and may advance a few meters a year. E.g. Sahara

Seif dunes

Seif dunes are usually a few hundred meters high. They are long narrow dunes parallel in the prevailing wind. E.g. Thar desert (India)

Parabolic:

These dunes are shaped with their arch pointing away from the wind. The middle section has moved forward with the wind.

Star Shaped dunes:

These are undulating with an uneven shape. They are formed where winds of equal strength and duration blow from all directions.

Tranverse sand dunes are oriented at a right angle to the wind and are long, as you find under your feet at a sandy beach. Theys are formed by steady winds such as the trade winds. Vegation may be found to stabilise the dunes with the wind.


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