365 Days of Climate Awareness 38: Global Wind Circulation Cells

The basic convective air cycle is for incoming solar radiation to warm the planet (earth and sea), creating a low-pressure, high-temperature zone of rising air. Cooler, higher-pressure air moves in laterally to replace it, while the warm air rises to an elevation where it has cooled adiabatically to match its surroundings, and then spreads out laterally. It cools further, being far above the planet surface, and then begins to descend. This creates, roughly, a turbine-style convective cycle.

The topography of the continents disrupts this cycle to a large extent, but especially over the ocean, these cycles organize themselves into cells based on latitude. THere are three such broad zones in each hemisphere. The equatorial cells, which extend to roughly 30° N/S, are called Hadley cells. The next band, which operates in tandem with the Hadley cells, is called the Ferrel cell band. The downward current of cooler air around 60° N/S is their strongest feature. Over the poles are nearly constant cold, high-pressure domes with air flowing outward along the planet surface. The wind warms as it moves south, and then circulates upward and poleward again. These are known as the polar cells.

On the planet surface, the winds associated with these cells have common names. The surface winds of the tropical Hadley cells are called the Trade Winds, because they blow very steadily and were of major importance to sail-based commerce. In the northern hemisphere, where most trade has historically been concentrated, north of the Trade Winds are the Horse Latitudes, characterized by high pressure, clear skies and less frequent winds. Further to the pole--north and south--are the westerlies. And closer to the poles blow the Polar Easterlies, as a result of the (nearly) perpetual high pressure zones there.

These are the global wind patterns set up by the unequal heating of the planet by the sun. You will have noticed that the winds bend on the map. This is due to the rotation of the earth, an influence we name the Coriolis effect. And for future reference: the winds drive a large portion of the ocean's surface circulation, and it is in the ocean that the majority of heat transfer around the world happens.

Tomorrow: the Coriolis effect.

Be well!