Differential heating of the Earth

The over-riding force that initiates global atmospheric circulation is due to the unequal amount of INcoming SOLar radiATION or "insolation" that reaches the Earth's surface.

The diagram below illustrates how intercepted insolation varies across the Earth's surface, where there is a global surplus over the tropics and a global deficit over the poles.

Click here for a map showing the average solar radiation on a horizontal surface at ground level.

From the diagram above, it can be seen that for a given segment of insolation, the area that is covered in the tropics is much smaller than at the poles. In other words, the same amount of energy that hits the Earth's surface at the poles is much weaker and more dissapated than at the equator. You will also notice that the distance that the segment of insolation passes through the atmosphere at the poles is greater than that at the equator. This will result in more of the insolation being reflected by the atmosphere (due to cloud cover, particulate matter in the atmosphere etc.) at the poles.