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Global winds are winds that blow across the entire planet. They are known by different names, but some of them are polar westerlies, prevailing easterlies, trading breezes, and the equator winds. The process is often referred to as atmospheric circulation and is the mechanism by which thermal energy is distributed across the Earth. The wind belts here on Earth are confined to three organized belts: the Hadley, Ferrel, and Polar cells.
The Hadley cell is well understood and are often called the trade winds. It is a closed circulation loop, beginning at the equator with the lifting of warm, moist air by equatorial low pressure zones into the tropopause and then moved towards the poles. Somewhere around the 30°N/S latitude the air descends in a high pressure zone. Some of that descending air travels along the equator, closes the Hadley cell and creates the trade winds. The Hadley is said to occupy the equator, but it actually occupies the thermal equator(follows the Sun’s zenith). The thermal equator has a semi-annual north/south migration.
The Polar cell is a simple system. Although it contains air that is cool and dry compared to equatorial air, it is still warm and moist enough to undergo convection and drive a thermal loop. Warm air rises at lower latitudes and moves towards the north and south poles. When the air reaches the polar areas, it descends as a cold, dry high pressure area, moving away from the pole along Earth’s surface, but twists westward because of the Coriolis effect and creates the polar easterlies. The flow of this air creates the Rossby waves that are important in forming the jet stream. The jet stream travels between the tropopause and the Ferrel cell. The Polar cell acts as a heat sink and balances the Earth’s energy equation with the Hadley cell. The Hadley cell and the Polar cell are thermally direct(they exist as a direct consequence of surface temperatures), so they override the effects of weather in their domain. The volume of energy the Hadley cell transports, and the Polar cell heat sink, ensure that the effects of unstable weather phenomena are not experienced across the whole system and are rarely able to form.
The Ferrel cell is a secondary circulation feature that only exists at the whim of the Hadley and Polar cells. It is a result of the eddy circulations (high and low pressures) of the mid-latitudes. To the south it overrides the Hadley cell and in the north it overrides the Polar cell. The Westerlies can be found beneath the Ferrel cell. The Ferrel cell is not a true loop, so the Westerlies are at the mercy of passing weather systems. A low moving polewards or a high moving equator wards maintains or even accelerates a westerly flow; the local passage of a cold front may change that in a matter of minutes, and frequently does. A strong high moving polewards may bring easterly winds for days.
While there are other elements that contribute to the global winds on our planet, these three cells are dominant.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.