What is a Hadley cell? How does it work?


What is a Hadley cell? How does it work?

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The Hadley cell is a large, semi-permanent vortex in the Earth’s atmosphere. It’s one of several major atmospheric cells that drive Earth’s climate at the highest scales.

They’re caused by differences in heating across the Earth. In general, the equator gets the most heat, while areas away from the equator get less. This makes air near the equator tend to rise more than air rises elsewhere, because it’s warmer than the air elsewhere. That creates a band of rising air at the equator, which creates lower air pressure at the ground.

Since there’s lower pressure at the ground, air blows in from the subtropics towards the tropics. In other words, the equator has a region of *convergence* – surrounding winds are blowing towards it. This creates a semipermanent band of thunderstorms near the equator called the [intertropical convergence zone](https://en.wikipedia.org/wiki/Intertropical_Convergence_Zone) or ITCZ.

Once the air rises, though, it creates *high* pressure aloft, because it has added extra air there. That causes winds aloft to blow *away* from the equator. (As a general rule, areas of low-level convergence are also areas of high-level divergence, because the air has to go somewhere.) Those winds blow towards the subtropics. That air then cools and sinks in the subtropics, forming a complete loop of subtropics low-level -> tropics low-level -> tropics high level -> subtropics high level -> subtropics low-level and so on. This loop is the Hadley cell, and the sinking dry air is the reason many subtropical areas have large deserts (like the Sahara and Namib in Africa, or the Sonoran in North America).

But the same process repeats at higher latitudes, too. The sinking air in the subtropics spreads out both towards *and* away from the equator. When it spreads away from the equator, it then rises again in another band in the temperate latitudes, forming another cell (the “mid-latitude cell”) that goes low-level subtropics -> low-level temperate -> high level temperate -> high level subtropics -> low-level subtropics. And finally, a third cell (the “polar cell”) exists between the temperate latitudes and the polar regions.

Ultimately what this gets you is, roughly, alternating patterns of wet and dry and of low and high pressure as you move away from the equator:

* Tropics = wet, low pressure
* Subtropics = dry, high pressure
* Temperate = wet, low, pressure
* Polar regions = dry, high pressure