global atmospheric circulation

• The sun heats the earths surface unevenly - insolation is greater at the equato that the polars
• the differences in temperature can cause differences in air pressure
• wind blow from the areas of high pressure to the areas of low pressure, transferring heat away from the equator
• winds are part of global atmospheric circulation loops (called cells). These loops have warm rising air which creates a low pressure blet, and cool falling air which creates a high pressure belt
• There are three cells in each hemisphere - the Hadley, Ferrel and Polar cells
• At the equator the sun warms the earth, which transfers heat to the air above, causing it to rise. This creates a low pressure belt. As the air rises, it cools and condenses forming clouds and rain
• the cool, dry air moves out to 30 degrees north and south of the equator
• at 30 degrees north and south of the equator, the cool air sinks, creating a high pressure belt with cloudless skies and very low rainfall
• The cool air reaches the ground surface and moves as surface winds either back to the equator or towards the poles
• Surface winds blowing towards the equator are called trade winds
• Trade winds blow from the SE in the Southern Hemisphere are from the NE in the Northern Hemisphere. At the equator, the trade winds meet and are heated by the sun. This causes them to rise and form clouds
• Surface winds blowing towards the poles are called westerlies. They blow from the NW in the southern hemisphere and from the SW in the northern hemisphere
• At 60 degrees north and south of the equator, the warmer surface winds meet cooler air from the poles. The warmer air is less dense than the cold air so it is forced to rise, creating low pressure and frontal rain (rain that forms where the warm and cold air masses meet)
• Some of the air moves back towards the equator, and the rest moves towards the poles
• At the poles the cool air sinks, creating high pressure. The high pressure air is drawn back towards the equator as surface winds
• Ocean currents are large scale movements of water that transfer heat energy from warmer to cooler regions.
• Surface currents are caused by winds and help transfer heat away from the equator

  e.g. the Gulf Stream brings warm water from the Caribbean and keeps Western Europe warmer than it would otherwise be.
• There are also deep ocean currents driven be differences in water density.
• when water freezes at the polars, surrounding waters gets slatier, increasing its density.
• As the water gets denser, it sinks, carousing warmer water to flow in at the surface - creating a current.
• This warmer water is cooled and sinks, continuing the cycle
• This cycle of cooling and sinking more water is a big loop round the earth - this is known as the atmospheric circulation
• The pressure belts caused by global atmospheric circulation cause variation in climate
• Arid (dry)

  sinking air from the Hadley and Ferrol cells meeting cause high pressure and prevents rainfall. Rainfall is very low for all or most of the year. Temperatures are hot and warm
• Polar

  Sinking air from the polar cells creates and area of high pressure at the poles. temperatures are low all year round and there’s very little rainfall
• Tropical

  Rising air from the two Hadley cells meeting causes low pressure and lots of rainfall. Temperatures are hot all the time and rainfall is high