Hadley Cell

The Hadly cell decribes how warm air rises near the equator, then travels towards the pole at high altitude. As it cools it sinks and then warms again as it travels towards the equator. These are the largest global circulation cells as they extend from the equator to between 30 and 40 degrees North and South.

This model of the earth’s atmospheric circulation was proposed by George Hadley in 1735 to explain trade winds blowing westward and towards the equator. The missing element in his model was the impact of the earths rotation or the Coriolis effect, which also deflects air sideways complicating the simple north-south circulation theory.

Later the Ferrel cell or Mid Cell was proposed to account for mid latitude westerly winds as statistically averaged circulation. Air converges at low altitudes and ascends along the boundaries of cool polar air and warm subtropical air around between 60 and 70 degrees north and south. This is often near the latitude of the UK, hence the varying weather patterns of that region. The circulation of this cell is impacted by the return flow of high altitude air around the tropics, where it joins sinking air from the Hadley cell. These cells move in the opposite direction of that of the Hadley and Polar cells, the smallest and weakest cells extending 60 to 70 degrees north and south, this air sinks over the highest latitudes and flows out towards the lower latitudes at the surface.

Atmospheric circulation around the Earth is impacted by all three cells (and jet streams) along with the Corriolis effect.

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