Why are there deserts around 30 degrees latitude?

The descending branch of the Hadley cell sinks near 30 degrees latitude, warming and drying the air, which suppresses clouds and rainfall and gives rise to the great subtropical desert belts.

What drives the prevailing westerly winds of the midlatitudes?

Baroclinic eddies, the traveling highs and lows of midlatitude weather, transport momentum that maintains the surface westerlies and the eddy-driven jet stream, so the westerlies are sustained by weather systems themselves.

The General Circulation

Averaged over weather and seasons, the atmosphere settles into a recurring global pattern of overturning cells, trade winds, and jet streams that redistributes heat from the tropics toward the poles.

Definition

The general circulation is the statistically averaged large-scale flow of the global atmosphere, encompassing the mean meridional cells, the prevailing surface winds, and the upper-level jets that together transport energy and angular momentum across latitudes.

Scope

This topic covers the time- and zonally-averaged structure of the global atmosphere, including the Hadley, Ferrel, and polar cells, the trade winds and midlatitude westerlies, the subtropical and polar-front jet streams, and the role of eddies in transporting heat and momentum.

Core questions

What gives rise to the trade winds, westerlies, and polar easterlies?
How do the Hadley, Ferrel, and polar cells overturn the atmosphere?
How do mean cells and transient eddies share the job of transporting heat poleward?
What sets the position and strength of the jet streams?

Key theories

Hadley circulation: Intense solar heating near the equator drives rising motion and a thermally direct overturning cell whose poleward upper branch, deflected by rotation, produces the subtropical jet and the trade winds at the surface.
Eddy-driven circulation of the midlatitudes: In the extratropics, baroclinic eddies rather than a simple overturning cell accomplish most of the poleward heat and momentum transport, maintaining the surface westerlies and the eddy-driven polar-front jet.

Mechanisms

Uneven solar heating warms the tropics more than the poles, and the atmosphere responds by transporting energy poleward. In the tropics this occurs through the thermally direct Hadley cells, whose upper branches conserve angular momentum to form the subtropical jets and whose descending branches create the subtropical deserts. In the midlatitudes, baroclinic eddies do most of the transport, sustaining the surface westerlies, the Ferrel cell, and the polar-front jet, while weak polar cells complete the pattern.

Clinical relevance

The general circulation sets the planet's climate zones, the locations of deserts and rain belts, the prevailing winds that shape commerce and aviation, and the storm tracks along which weather systems travel, making it the backdrop against which all regional weather and climate are understood.

History

George Hadley proposed a single tropical overturning cell in 1735 to explain the trade winds; William Ferrel added a midlatitude cell in the nineteenth century, and twentieth-century observational and theoretical work, including Lorenz's synthesis and Starr's analysis of eddy fluxes, established the modern, eddy-aware picture of the general circulation.

Key figures

George Hadley
William Ferrel
Edward Lorenz
Victor Starr

Seminal works

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vallis2017

Frequently asked questions

Why are there deserts around 30 degrees latitude?: The descending branch of the Hadley cell sinks near 30 degrees latitude, warming and drying the air, which suppresses clouds and rainfall and gives rise to the great subtropical desert belts.
What drives the prevailing westerly winds of the midlatitudes?: Baroclinic eddies, the traveling highs and lows of midlatitude weather, transport momentum that maintains the surface westerlies and the eddy-driven jet stream, so the westerlies are sustained by weather systems themselves.