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. It is possible to observe this in practice by first studying the potential temperature, θ, averaged over all longitudes.

This plot, which depicts the average potential temperature in the atmosphere in January demonstrates the warmer temperature observed near the equator, and colder temperatures observed near the poles. The center of the warmer temperatures appears to the south of the equator, due to the southern position of the sun in January. Likewise, cooler temperatures are observed over the North Pole rather than the South Pole. What is particularly notable about this plot is that the potential temperature is actually cooler in the stratosphere over the tropics than it is over the poles, indicating that the higher altitudes are warmed over the poles, where the lower altitudes are warmed near the surface. The warm temperature over the tropics translates to rising motion, which may be determined from the average value of ω, the vertical movement of the air, as averaged over all longitudes.

Here, air may be observed to rise close to the equator and sink near 25 degrees north latitude. Since this average is the January average, during winter in the northern hemisphere, the strongest movement is observed in the northern hemisphere, where the temperature gradient is strongest. Similarly, the sinking motion is observed at a lower latitude due to the axis pointing away from the sun. The meridianal movement of the air may also be plotted to demonstrate that this rising and sinking is connected in a single cell.

The Hadley cell is visibly completed by the stronger southerly movement of about 3 m/s near the top of the rising column at about the 150mb level, and stronger northerly movement near the surface. The latter movement is the meridianal component of the trade winds.

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