Astrophysics (Index)About

Hadley cell

(circulation pattern of up at the equator, poleward, down, and back)

A Hadley cell is an atmospheric circulation pattern of upward motion at the equator a few kilometers, then toward the poles to about 30 degrees latitude (on Earth) then down to the surface and back to the equator. It was identified by George Hadley in the 18th century and explains the easterly trade winds at the base of the cell, because the flow toward the equator (e.g., north-to-south in the northern hemisphere) at the surface is deflected by the Coriolis force (this deflection being called the Coriolis effect). The term Hadley circulation refers to the general circulation pattern including Hadley cells.

The Earth also has polar cells aka polar vortices, air rising closer to 60 degrees latitude, within the troposphere (to no more than 8 km), moving poleward and descending close to the pole. The Earth also has Ferrel cells aka zones of mixing, circulation patterns joining the Hadley cells with the polar cells: some of the air rising as per the polar cell splitting from it, moving toward the equator and descending with the current of the Hadley cell, then returning toward the pole to rejoin the polar cell. Ferrel cells were first theorized by William Ferrel in the 19th century. Analogously associated with them are the winds known as the westerlies.

These are useful elements in models of weather on other planets and moons. Their existence and extent varies based on the size of the body and its frequency of rotation. The faster the rotation, the more pronounced the Coriolis effect, and the nearer the equator that air moving poleward ends up moving essentially "eastward" (specifically, toward the direction of the planet's rotation, a superrotating wind), so for a large body with a short rotation period, the cell extends only a small fraction of the planet's equator-to-pole extent from the equator and another cell (e.g., the Earth's Ferrel cell) forms poleward from it. A large planet with a fast rotation may have five or more cells from equator to pole, and a planet with no rotation or a small planet with little rotation may have just a single cell. The number of cells has an effect on climate and potentially, habitability: a single cell structure mixes atmosphere from equator to pole, leading to less temperature variation across the planet. Titan's circulation is basically just a Hadley cell extending to its polar region. Jupiter and Saturn, which rotate faster than Earth, are divided north/south into more of these sorts of cells, which visibly show as their east-west stripes, known as belts or zones.

Further reading:

Referenced by pages:
intertropical convergence zone (ITCZ)
one dimensional climate model
polar cyclone
superrotating wind
zonal flow