General Circulation

Meteorology · Chapter 20 · DGCA CPL and ATPL

The planet-sized wind machine: why the tropics are rainy, the deserts sit at 30°, the trade winds blow so steadily and mid-latitude weather drifts from west to east.

Remember

Tropics: radiation surplus; poles: deficit; winds and ocean currents carry heat poleward (general circulation). Synoptic meteorology studies the moving systems.
Hadley: rise at ITCZ, sink at 30° (subtropical high, deserts), trades back. Ferrel: surface westerlies 30-60°, rise at polar front. Polar: sinking at pole, polar easterlies.
Belts: equatorial low (doldrums), horse latitudes 30-40° (calm, high), westerlies 35-60° (Roaring Forties), polar front, polar high.
Aloft: zonal westerlies except easterlies near the equator. Mid-latitude systems move W→E, tropical E→W, recurving at the transition.
Rossby (planetary) waves = meandering jet; standing waves = time-averaged, anchored by mountains, continents, oceans.

Why does the air move round the globe at all?

A planet-sized engine

Four cards with real satellite images. Swipe or tap Next.

Hot belly, cold head Near the equator each square metre gets far more sunshine than near the poles. Averaged over a year the whole Earth balances what comes in and what goes out, but by latitude it does not: a surplus in the tropics, a deficit at the poles. This AIRS map of temperature shows the warm tropics (red) and the cold poles (blue and purple).
Winds and currents spread it out To stop the tropics getting ever hotter and the poles ever colder, the atmosphere and oceans move heat (and momentum) poleward. Averaged over a long time, the winds that do this form the general circulation of the atmosphere. Day-to-day systems (highs, lows, cyclonic circulations) move within it; studying them is synoptic meteorology.
Why deserts sit near 30° Air that rose at the equator sinks again around 30° N and S, under the subtropical highs. Sinking air warms and dries and stops cloud forming. That is why the great deserts, the Sahara, Arabia, the Thar, Australia and the Kalahari, lie near 30°.
Which way will it move? A low forms over the Bay of Bengal in the tropical easterlies, and another forms over Europe in the westerlies.

Takeaway: Unequal heating drives the circulation: the tropics gain more energy than they lose, the poles lose more than they gain, and the winds carry the difference.

Why must the atmosphere move heat around?

Surplus and deficit

Energy coming in and going out over a year, by latitude. Tap Next.

Incoming sunlight The sun heats the equator far more, for each square metre, than the poles. Near the equator the sun is high and its rays hit the ground almost straight on. Near the poles the sun is low, so the same beam is spread over a much larger area and also passes through more air. That is why the yellow curve peaks at the equator.
Outgoing heat The Earth loses heat to space as long-wave radiation, from every latitude and much more evenly. Over the whole globe and the whole year, what comes in and what goes out nearly balance: the energy balance.
Surplus and deficit But it does not balance at each latitude. The tropics get more than they lose: a surplus. The polar regions lose more than they get: a deficit. The tropics have a radiation surplus.
So the air moves Winds and ocean currents carry the extra heat towards the poles and even it out. These are the transport processes. The long-term average of all those winds is the general circulation. The study of the moving highs, lows and other systems within it, and their weather, is synoptic meteorology. Studying the moving weather systems is synoptic meteorology.

Memory hook: Surplus in the tropics, deficit at the poles, so winds and currents carry heat poleward.

Takeaway: Over the whole Earth and a whole year, the sunlight coming in (short-wave) and the heat going out (long-wave) balance. But the tropics take in more than they lose (a surplus) and the polar regions lose more than they take in (a deficit). Winds and ocean currents carry the extra heat towards the poles. That long-term pattern of moving air is the general circulation.