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Natural Hazards - Global Atmospheric Circulation

The Earth’s atmosphere is in constant motion and is driven by the energy we receive from the sun.  The air moving around the globe does so because we get more energy in tropical areas and less at the poles.  Air movements or winds help to balance this out.  They do so according to the model below. 


A model of atmospheric circulation

Most insolation arrives between the 2 Tropics. This causes air to rise from the surface UP through the atmosphere in thermals at the Inter Tropical Convergence Zone (ITCZ).  This creates huge cumulonimbus clouds as the air cools, Tropical storms with low pressure occur here. As the air heads North and South it cools and then sinks back down to the surface at approximately 30°N & S giving HIGH pressure. This goes back to the Equator as the TRADE winds or tropical easterlies. Two further cells exist further North and South.  These cells are called the Hadley, Ferrel and Polar cells, giving 6 in total (3 in either hemisphere). 
• Where air is sinking in the model this gives high pressure.  These areas coincide with many of the earth’s deserts and dry areas as the air is sinking so little condensation occurs as the air warms. 
• Where air is rising in the model this gives low pressure.  These areas coincide with wetter areas with lush vegetation as air cools as it rises, allowing water vapour to condense to droplets allowing more rain. 
• WINDS occur because air molecules move from areas of high pressure to areas of low pressure.
The model is disrupted by mountain ranges and differences between the land and sea.



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