Why do the warm tropical air sink when they reach the poles?

Answer 1

Think of the air at the Equator as a column. If you heat it, the air in the column expands and rises, cooling as it does so. As the column gets taller, at altitude you have relatively higher pressure, and the air flows out towards a lower pressure area. When the air flows out from the top of the column, it leaves lower pressure at the surface.

The opposite happens at the poles where the air is cold and dense. The column is shorter and heavier than the column at the Equator. The pressure is lower at altitude and so the high pressure air from the Equator flows there. Adding air to the top of the polar column raises the surface pressure and the air flows from there to a lower pressure area at the equator.

So you get a circulation, out from the Equator to the poles at high level, and in towards the Equator at lower level. The air that left the Equator was warm when it was on the surface at the Equator, but by the time it has risen 40,000-50,000 feet and traveled several thousand miles at high altitude, it is no longer warm.

The rotation of the earth and the roughness of the terrain affect the flow greatly, and the air at the surface tends to turn to the right causing rotating areas of high and low pressure.

Answer 2

Warm tropical air tends to rise near the equator due to its lower density. As it moves towards higher latitudes, it cools and becomes denser, eventually sinking near the poles as part of the atmospheric circulation patterns known as the Hadley and Ferrel cells. This sinking motion is balanced by rising air at the tropics, creating a continuous circulation pattern in the atmosphere.