Global atmospheric circulation

Global atmospheric circulation

1. The most important influence on worldwide variations in climate is latitude

   • Because of the curved surface of the earth the equator receive much higher insolation than polar latitudes
     • Insolation - the amount of solar radiation or sunlight an area receives over a specified period of time
     • At the arctic circle, solar energy strikes the ground at a low angle and is spread over a large area - the ground is heated only gently
     • At the equator solar energy strikes the ground at almost a right angle so the ground is heated strongly

2. In global atmospheric circulation there are six convection cells in total - the Hadley cell, the Ferrel cell and the polar cell operate in the northern and southern hemispheres

   • A convection cell is a self-contained area in a to I'd in which upward motion of warmer fluid is balanced by downward motion of a cooler fluid

3. Hadley cell

   • Due to differences in insolation
   • Air at the equator is heated strongly becoming less dens so it rises to a higher altitude
     • After rising the air spreads out and begins to flow towards the north and south poles
       • At about 30 degrees north and south the lower insolation results in colder denser air and so the air now sinks towards ground level and flows back the the equator
   • Along the equator there are tropical climate zones - rainforests occur here because as the air rises, the water vapour condenses causing heavy rainfall
   • Creates an equatorial zone (area of low pressure) as as the hot air rises, there is less pressure on the earth
     • Low pressure belt at the equator is also known as the equatorial low
   • At 30 degrees north and south there is high pressure known as the subtropical high as air is sinking towards the ground
     • The high pressure results in hot, dry desert conditions (arid climate zones)

4. Polar cell

   • Also due to differences in insolation
   • From 60 north and south of the equator
   • Air at the poles is cool as it receives very little insolation - less concentrated as it is spread over a large area
     • At around 60 degrees the polar air is warmed and so rises and returns poleward, carrying heat energy
   • Creates high pressure at the poles (polar high) as the air is cool and so doesn't rise until it is heated when it travels closer to the equator

5. Ferrell cell

   • Caused by friction due to the neighbouring cells
     • The movement of the Hadley cell and the polar cell next to the Ferrell cell causes the air to be dragged along too
   • Active from 30 degrees north and south to 60 degrees
   • At 60 degrees there is the mid-latitude low as the air in the Ferrel cell rises as the air in the polar cell also rises
   • At the sub-tropical high air in the Ferrel cell is sinking alongside that of the Hadley cell

6. Jet streams

   • Strong high-altitude currents of air
   • Flow towards the east as the earth spins and interact with the convection cells
     • Coriolis effect - the effect of the spin of the earth on weather systems
   • The wind is skewed as the earth is tilted so the wind travels on the earth in zigzags
   • The weather is also dragged by jet streams

7. How do global pressure and surface winds influence precipitation?

   • Rainfall is high and constant throughout the year near the equator
     • As hot air rises it cools and water vapour is converted into droplets of conventional rain
   • Air rises around the equator and triggers burst of torrential rain
     • Sometimes the ITCZ grows a wave of low pressure which extends further than usual - tropical storms develop along these waves
     • The low pressure zone around the equator is called the intertropical convergence zone
   • Rainfall is often higher in coastal areas in Western Europe due to the movement of the jet stream over the Atlantic
     • Depressions follow the jet stream often bringing stormy conditions o the UK's west coast
   • Rainfall is often low around the tropics
     • Dry air descends there as part of he Hadley cell, resulting in arid conditions
   • Precipitation is also very low in polar regions and falls mostly as snow. The cold air has a limited ability to hold water vapour