In this morning's lecture on weather, we're looking at global circulation patterns. In the northern hemisphere there are three large-scale wind systems known as the Hadley cell, Ferrel cell and Polar cell, and these same weather systems are mirrored in the southern hemisphere. Within each of these cells, air flows up through the entire troposphere, which, as you know, is the part of the atmosphere stretching from the surface of the Earth upwards around 15 km. The warm, moist air produced in the tropical regions near the equator is pushed towards the polar latitudes by the three cells. Taking the Hadley cells to begin with, each one spans around 30 degrees north and south from the equator. Then we have the Ferrel cells, which are both north and south of the Hadley cells at approximately 60 and 70 degrees in each hemisphere. You can blame Ferrel cells for the changeable weather we get here in the UK. Finally, there are the Polar cells, which, as you would expect, extend from the Ferrel cells as far as the north and south poles and are the weakest of the three types of large-scale wind circulations. Going back to that unsettled weather the UK experiences, this is a result of that hot, wet air from the tropics that I mentioned earlier, getting pushed north by the Ferrel cells' surface winds. Unstable weather occurs where this wind hits the Polar cells—just around the UK's latitude.