The Earth is not flat; different places receive different levels of insolation, leading to differential heating of the Earth's spherical surface. Due to the difference in the insolation received at the equator and that received at the poles, the additional energy received at the equator will rise and move towards the poles in order to balance the energy of insolation throughout the Earth's atmosphere. Likewise, cold air at the poles will sink and move towards the equator to complete the loop and satisfy continuity constraints on the flow of air in the atmosphere.

In a non-rotating environment, this movement is straightforward; a simple cycle of air from the cold poles to the warm equator and back would form. This movement is complicated in practice by the rotation of the Earth. As the air at the equator moves toward the poles, its distance from the Earth's axis of rotation decreases and the conservation of its rotational momentum will cause the northward-moving air to curve to the right according to the Coriolis force. Likewise, the air moving back towards the equator will also curve right. This circular motion of air is known as a Hadley cell.

The Trade Winds

On Earth, this Coriolis-forced wind movement results in the phenomenon known as the trade winds. Above about the 30th parallel, the conservation of momentum causes a breakdown in the regime of the Hadley cell, and air sinks. We may observe this in practice by examining the average value of ω, the vertical movement at the air, as averaged over all longitudes.

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