The structure of the atmosphere and the three-cell global circulation model; how pressure belts and surface winds create the global distribution of arid and humid zones; and how circulation links to extreme weather such as drought.

What this dot point is asking

This is OCR GCSE Geography B (J384) Component 1, Our Natural World, the opening enquiry of Global Hazards: "How does the global circulation of the atmosphere cause extreme weather?" OCR expects you to describe the structure of the atmosphere, explain the three-cell global circulation model (the Hadley, Ferrel and Polar cells), link the rising and sinking limbs of those cells to belts of low and high pressure, and use this to explain the global distribution of humid and arid zones and the conditions that produce extreme weather such as drought. This dot point is the foundation for tropical storms, which form within this same circulation.

The structure of the atmosphere and uneven heating

The atmosphere is a thin layer of gases held to the Earth by gravity; almost all weather happens in the lowest layer, the troposphere. The engine of the whole system is the uneven heating of the Earth by the Sun. The Sun's rays strike the Equator almost vertically, concentrating energy on a small area, so the tropics are hot. Near the poles the same energy is spread over a much larger area and passes through more atmosphere, so the poles are cold. This temperature difference between the Equator and the poles is what drives the global circulation: the atmosphere is constantly trying to move heat from the warm tropics towards the cold poles.

The three-cell model

In each hemisphere the air moves in three giant loops called cells.

The Hadley cell is the most important for global weather. At the Equator, intense heating makes warm, moist air rise. As it rises it cools, water vapour condenses, and heavy rain falls, which is why the tropical rainforests lie along the Equator. This rising air creates a belt of low pressure. High in the atmosphere the air spreads polewards, cools, and at about 30 degrees north and south it sinks back to the surface. Sinking air warms and dries, so it brings clear skies and little rain. This belt of high pressure is where the world's great hot deserts lie, including the Sahara, the Arabian Desert and the Australian outback.

The Ferrel and Polar cells complete the pattern at higher latitudes, with rising air and low pressure at about 60 degrees (giving the changeable weather of the UK) and sinking air at the poles.

Pressure belts, winds and the Coriolis effect

Air at the surface always flows from high pressure to low pressure, and these movements are the prevailing winds. Because the Earth spins, moving air is deflected: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect. It turns the simple north-south flow into the curved global wind belts: the trade winds blow from the subtropical highs towards the Equator, and the westerlies blow from the subtropical highs towards 60 degrees. The trade winds of both hemispheres meet near the Equator in a zone of converging air, rising motion and storms.

Linking circulation to extreme weather

The circulation explains where extreme weather is likely.

• Heavy rainfall and storms cluster near the Equator and along the converging trade winds, where rising air is the rule.
• Drought is most common under the descending limb of the Hadley cell and within persistent high-pressure systems, where sinking air suppresses cloud and rain.

Drought is not caused by circulation alone. Changes in ocean temperatures, especially the El Nino warming of the Pacific, can shift rainfall patterns worldwide, and human pressures such as deforestation and over-abstraction of rivers can deepen a natural dry spell into a crisis.

Try this

Q1. Describe the global distribution of areas of low pressure. [3 marks]

• Cue. A belt at the Equator (rising air) and a zone at about 60 degrees north and south.

Q2. Suggest how a change in the global atmospheric circulation could increase the risk of drought in the Sahel. [4 marks]

• Cue. The tropical rain belt fails to migrate far enough north, so the seasonal rains do not arrive, and sinking air keeps skies clear.