Seasonal variations

Because the Earth's axis is tilted (23.5°), there is uneven heating between the northern and southern hemispheres, at any one point in time. When the southern hemisphere is tilted towards the Sun, it is summer in Australia (and winter in Europe). Conversely when the southern hemisphere is tilted away from the Sun, it is winter in Australia.

Therefore there is an apparent annual migration of maximum radiation between the two tropics (Tropic of Cancer at 23.5°N and Tropic of Capricorn at 23.5°S).

This results in an apparent latitudinal movement of air pressure systems. When the maximum insolation hits the Tropic of Capricorn (ie. in an southern hemisphere summer), a low pressure cell is generated at that latitude (23.5°S). However, when the maximum insolation is over the Tropic of Cancer, the low pressure cell moves northwards altering the global patterns across the Earth's surface. This is covered in more detail in the section on Global Atmospheric Circulation.

DIAGRAM SHOWING THE shift of the Hadley cell