Atmospheric Circulation Lab

One machine, every continent

Put a world map of biomes next to a world map of latitude and something obvious jumps out. Rainforest sits in a band along the equator, in South America, in Africa, in south-east Asia. Hot desert sits in two bands at roughly thirty degrees north and south: the Sahara and the Arabian, the Kalahari, the Atacama, the Australian deserts. Different continents, different rocks, different histories, same latitudes. Something global is deciding this, and it is the way air moves. The sun does not heat the Earth evenly. Near the equator its rays arrive almost vertically and are concentrated on a small area; near the poles the same beam is spread across a much larger area and has more atmosphere to pass through. So the equator is heated hard, the poles barely at all, and the atmosphere spends its entire existence trying to even that out by moving heat polewards. It does so in three loops in each hemisphere, named after the people who worked them out: the Hadley cell, described by George Hadley in 1735, the Ferrel cell, added by William Ferrel in 1856, and the polar cell. That machine is worth understanding properly, because almost everything else in this part of the specification falls out of it. Where air rises you get low pressure, cloud and rain. Where air sinks you get high pressure, clear skies and drought. And the places where warm ocean, low pressure and enough spin coincide are exactly the places tropical storms are born. Build the model first; then spend it.

Five terms that make the model work

The first two are the engine, the third and fourth are what you feel at the surface, and the fifth is the one that explains where hurricanes can and cannot exist.

Order the loop nearest the equator

These six stages describe one complete circuit of the Hadley cell. Put them in order, starting at the equator.

• Intense insolation heats the surface and the air above it
• That warm air rises, creating low pressure at the surface beneath it
• As it climbs it cools, its moisture condenses, and heavy rain falls
• Now dry, it spreads polewards high in the atmosphere
• At around thirty degrees it cools enough to sink, creating high pressure and clear skies
• It flows back towards the equator across the surface, deflected into the trade winds

Why is thirty degrees dry?

The great hot deserts sit at around thirty degrees north and south on every continent. Which answer explains that?

• That is where the poleward-moving upper air of the Hadley cell descends. It has already lost its moisture as rain over the equator, and as it sinks it is compressed and warms, so its capacity to hold water vapour increases rather than decreases. Air that is warming can evaporate moisture from the surface but cannot condense any out, so you get persistent high pressure, cloudless skies and almost no rainfall, which is the definition of a desert climate.
• Those latitudes receive the most intense sunshine on Earth, which evaporates all the water.
• Continents happen to be widest at those latitudes, so moisture cannot reach the interior.
• The trade winds blow all the rain clouds away from those regions.

The three cells, and what each puts on the map

Read each column as a complete unit: where the air moves, what pressure that creates at the surface, and which climate the world actually has there as a result. The last row of each is the payoff, because that is what the exam question is really about.

Label the circulation

The cross-section runs from the equator on the left to the pole on the right, with the latitude in degrees marked along the base. Tap a label, then tap the pin it belongs to. Two of the labels do not belong on this diagram.

Match each latitude to what happens there

Spend the model, do not just describe it

The examiner report on this key element makes the same complaint every year. Candidates learn the diagram, reproduce it faithfully, and stop. A question that asks you to explain the distribution of tropical storms, or why a particular region is arid, is not asking you to recite three cells. It is asking you to use them. The habit to build is a two-part sentence: what the air is doing, and therefore what the weather is. Not "there is low pressure at the equator" but "air is rising at the equator, so it cools, condenses and gives daily rainfall". Not "there is high pressure at thirty degrees" but "air is sinking at thirty degrees and warming as it does, so cloud cannot form and the land beneath is desert". Every mark in this topic lives in the word "so". Then apply it. Tropical storms need several conditions at once, and the circulation model supplies most of them: a warm enough ocean, deep enough to keep feeding the storm; converging surface air to start it lifting, which the trade winds provide where they meet; low pressure aloft so the rising air can escape rather than capping the system; and enough deflection from the rotation of the Earth to set it spinning. Because that last condition fails near the equator, there is a band right along it where the other three are satisfied and storms still never form. Being able to say why that gap exists is one of the surest signs of a top answer, because it shows you are reasoning from the model rather than reciting a map. Finally, watch the command word. "Describe the global atmospheric circulation model" wants the structure with no reasons. "Explain how it affects climate" wants the causal chain. "Assess" wants a judgement at the end.

Complete the circulation facts

Air rises at the equator because insolation there is intense, which creates _____ pressure at the surface and the daily rainfall that supports the rainforests. That air travels polewards high up and sinks again at around _____ degrees, where it warms as it descends, cloud cannot form, and the great hot deserts lie. Surface air returning from there towards the equator is deflected by the _____ effect into the trade winds. Further north, warm westerlies meet cold polar air at around sixty degrees along the polar _____, which is why the UK receives one depression after another. Tropical storms need a sea surface of roughly _____ degrees Celsius or warmer, but none forms right on the equator, because there the deflection that makes a storm spin falls to zero.

Spot the three accurate circulation claims

Select the THREE statements that are true.

• No tropical storm forms within roughly five degrees of the equator, because the deflection caused by the rotation of the Earth falls to zero there and nothing can set the system spinning, even though the ocean is warm enough.
• Antarctica sits under permanent high pressure from descending air, and receives so little precipitation that it counts as a desert by the same measure used for the Sahara.
• The belt of rising air at the equator shifts north and south through the year as the overhead sun moves, which is why many tropical places have a distinct wet season rather than steady rain.
• Air sinking at thirty degrees cools as it descends, which is why no rain falls there.
• The equator is dry because the sun there is so intense that all the moisture evaporates.
• There are two circulation cells in each hemisphere.

A 6-mark explanation, marked

Explain how global atmospheric circulation affects the distribution of the world's hot deserts and rainforests. [6 marks] The equator receives the most intense insolation because the sun is close to overhead there, so the surface and the air above it are heated strongly and that air rises, producing a belt of low pressure. As the air climbs it cools, the water vapour it carries condenses, and heavy rain falls on most days, which is why the tropical rainforests lie in a band along the equator in South America, Africa and south-east Asia. Having lost its moisture, the air then travels polewards high in the atmosphere as part of the Hadley cell, and at around thirty degrees north and south it cools enough to descend. As it sinks it is compressed and warms, and because warming air can hold more water vapour rather than less, condensation cannot occur and cloud does not form. This produces a belt of persistent high pressure with clear skies and almost no rainfall, which is exactly where the Sahara, the Arabian, the Kalahari and the Australian deserts are found. The distribution of both biomes is therefore a direct consequence of the same circulation: rainforest where the air of the Hadley cell rises, desert where it returns to the surface. What earned the marks: a causal chain in which every step follows from the last, rather than a description of the model; the crucial point that sinking air warms and so cannot produce rain, which is where most answers go wrong; named regions for both biomes; and a closing sentence that ties the two distributions back to the single mechanism that produced them.

Find the two sentences that use the model

Five sentences from a student's answer. Three of them only describe the model. Tap the TWO that actually use it to explain something.

• Air descends at around thirty degrees and warms as it does so, which prevents condensation and leaves the surface beneath it arid.
• There are three circulation cells in each hemisphere.
• Because the deflecting force from the Earth's rotation is absent at the equator itself, storms cannot begin to rotate there and the belt remains free of them despite the warm ocean.
• The Hadley cell was named after George Hadley and the Ferrel cell after William Ferrel.
• The trade winds blow towards the equator.

Build the sinking air sentence

Assemble the sentence that most answers get wrong.

Read four places from the model

Each stage describes a location without naming it. Work out what the circulation is doing there.

• A lowland region almost on the equator. Mornings are bright, towering clouds build through the afternoon, and there is a heavy downpour most days of the year.
• A region at about twenty-five degrees north. Skies are clear for months on end, rainfall is under 250 mm a year, and the daily temperature range is enormous.
• An island at about fifty-five degrees north on the eastern edge of an ocean. The weather changes every few days, with one band of rain after another arriving from the west.
• An ocean area at three degrees north. The water is well over 27 degrees Celsius and deep, and surface air is converging. Yet no tropical storm has ever been recorded forming here.

Use the model to explain the storms

Explain how global atmospheric circulation helps to account for the global distribution of tropical storms. [9 marks]

• Set out briefly where tropical storms actually occur, in terms of latitude and ocean basins, so the distribution you are explaining is clear
• Use the circulation to supply the conditions: the warm ocean of the tropics, converging surface air where the trade winds meet, and rising air in the low pressure belt
• Write in two-part sentences: what the air is doing, and therefore what follows from it
• Explain the gap along the equator itself, where the deflection from the rotation of the Earth falls away and storms cannot begin to spin
• Close by saying what the model can and cannot account for, since sea temperature and the season also decide where and when storms form