Causes of Aridity

Global Atmospheric Circulation

Continentality

Relief

Cold Ocean Currents

Rain Shadow

Most of the world's hot and arid environments occur between 20 - 35 degrees N &S of the equator. These are zones of persistent dry descending air and stable high pressure systems.

Hadley Cells

• Starts at the Doldrums, which is an area of intense low pressure found at the equator where the intense heating of the earth's surface by the sun forces air to rise through the Troposphere.
• This area is is known as the Inter Tropical Convergence Zone (ITCZ).
• As this air rises by convection, it cools and condenses forming a belt of cumulonimbus and rain.
• Then, some of this air migrates towards the poles whilst in the upper Troposphere. This equalises out the temperature and insolation differences of our globe.
• The air cools relative to the air around as it migrates north and south. It becomes denser so sinks to the Earth's surface at around 20-30 degrees N&S of the equator, creating a band of high pressure. It warms and compresses so residual moisture evaporates and skies are clear (subtropical anticyclones). Air diverges at ground level and migrates towards the low pressure area at the equator.

Ferrel Cells

• Some of the air from the deserts continues towards the poles.
• When this air reaches 60 degrees N & S, it reaches cold polar air.
• This causes the warmer, less dense tropical air to rise through the atmosphere again, creating an area of low surface pressure. It is this zone where we find unstable conditions and mid-latitude weather systems that characterise British weather.

Polar Cells

• The rest of the air migrates to the pole at high levels, where it cools and sinks, creating high pressure in the Polar regions and completing a weak polar cell. 

As air from the Hadley and Ferrel cells converge at high levels in the sub-tropics, it is forced down towards the Earth's surface creating an area of high pressure. The sinking air is warmed by compression, giving it an increasing capacity to hold moisture so cloud formation is unlikely. Any moisture will evaporate. Absence of cloud cover = heat to build during day & lost during night.

Many arid areas are within large land masses in dry continental interiors. If an air mass moves from the sea over a continent, it will rain near the coast. Places inland will be dry because the air passes over a dry continent and does not pick up moisture.

Several deserts that lie along the west coast of continents have formed partly due to cold ocean surface currents. The cold currents are part of a circulation, taking cold water from the poles towards the equator. The cold water of the current cools any air that moves across the surface of the sea. Moisture is condensed offshore into fog & mist, which then travel inland to be burnt off by strong tropical sunlight. Because the onshore winds have been cooled, they have a low moisture-carrying capacity so precipitation of any sort is unlikely.

Examples:

• Peru/Humboldt current - Atacama Desert
• Benguela current - Namib Desert

Some very dry areas lie in the rain shadow of mountains. Moist air that has been brought inland by prevailing winds is forced to rise over mountains, leading to condensation and precipitation on the windward side.

As the air descends on the leeward side, it warms up with a consequent drop in relative humidity.

In the southern hemisphere, the prevailing southeast trade winds meet the Andes. As it descends the western slopes, the air becomes warm and dry, leading to arid conditions.