2. The trade winds are forced to rise because of the heat that they have gained from the warm tropical oceans

3. Area of low pressure

4. This air is rapidly cooled, often producing towering cumulonimbus clouds (NB: these strong upward currents form the 'powerhouse of global circulation')

5. As the air cools, it ceases to rise and it begins to move away from the equator

6. Increased density as a result of the air cooling, together with the Coriolis force, causes the air to sink (causing a high pressure area as it pushes downwards on the earth's surface)

7. Subsiding air causes the descending limb of the 'Hadley Cell' - usually found at about 30° N of the equator

8. Area of high pressure

9. Mirror image of the Northern Hemisphere occurs in Southern Hemisphere

10. On reaching the earth' surface the cell is completed

11. Some of the air returns to the equator as the NE trade winds

12. Remaining air is diverted pole-wards, forming the warm SW winds

13. Warm air of the South Westerlies meets the cold air at the Polar Front (about 60°N)

14. Warm air rises above cold air creating the rising limbs of the Polar and Ferrel Cells

15. Polar Front Jet Stream occurs aloft at this point (PFJS)

16. Resulting unstable conditions produce cyclonic rainfall associated with mid-latitude depressions

17. Some of this rising air returns to tropics, while some travels towards to the poles

18. Stable area of high pressure