At midday the sun is directly over the equator, but as it moves towards the poles, its rays become slanted. The more perpendicular the rays of the sun, the more hear the sun gives off. The slanted sunlight must warm up a larger area, where the heat loss is greater. On the equator, the air is thus warmer than at the poles.

The effect of the sea must also be considered. At the equator the sea is warmer and evaporation is significant. The heated air expands and rises. Higher air cools down, and travels northward and southward, towards the poles.

At sea level, air and water create all sorts of currents. The sea currents from the equator are warm. Those coming from the North and South Poles are cold. To summarise: the sun's slanting rays and the coldness of the water prevent the Poles from warming up. The North Pole is largely made of ice; even in summer ice breakers are needed to sail there. As for the South Pole, it consists of a large expanse of land covered with a thick layer of ice, up to a kilometre (half a mile) thick.