Atmospheric Circulation

The movement of air is called atmospheric circulation- Solar radiation hits the equator at 90 degrees, causinig intense heating. At the poles radiation hits the surface at a lower angle, meaning the radiation has to heat a larger surface area, making it colder. 

Heat from the Equator is moved by 1) Ocean currents 2) The Hadley, Ferrel and Polar cells 

High pressure-> Cold, dense air sinks towards the ground. this means few clouds form, and there is little rainfallas there is no moisture in the air to condense.

eg) Arid areas- Deserts- average under 250mm of rain annually. Found between the Hadley and Ferrel cellls. Both cells are returning air to the surface, crestinig a zone of high pressure. As the air warms up it dries out, so no moisture in the air can condense to form clouds.

Low pressure-> Solar radiation heats the ground, and this heats the air above it. this air expands, and rises. As this air rises, it cools and condenses to form clouds.

eg) the ITCZ (Inter-Tropical Convergence Zone) This is a permenant area of low pressure in the Tropics where two Hadley cells converge. High temperatures cause the tropical air to rise rapidly forming low pressure. Condensation of water vapour forms Cumulonimbus clouds (thunderclouds) so rainfall is heavy and regular.