The Hadley cell and the ITCZ

Overview

Why air rises at the Inter Tropical Convergence Zone, why the trade winds blow as they do, and the chain of cause and effect from the rising limb to the equatorial climate and the desert belt.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is the Hadley cell?

   Model answer

   A thermally direct atmospheric circulation cell occupying each hemisphere between the equator and roughly 30 degrees of latitude. Air rises at the Inter Tropical Convergence Zone close to the equator, moves polewards at high altitude, sinks at about 20-30 degrees north and south, and returns to the equator at the surface as the trade winds. Because rising air produces rainfall and sinking air suppresses it, this single cell determines where the wet and dry tropical belts lie.

2. 2

   Why does air rise at the ITCZ in the first place?

   Model answer

   The sun is close to overhead near the equator all year, so insolation is intense and is concentrated over a small ground area. The heated ground warms the air above it by conduction and radiation, that air expands, becomes less dense than its surroundings and rises by free convection. Its removal from the surface leaves an area of low atmospheric pressure, which draws in more air from north and south.

3. 3

   What exactly is the Inter Tropical Convergence Zone?

   Model answer

   The low-pressure belt where the north-east trade winds of the northern hemisphere and the south-east trade winds of the southern hemisphere converge and are forced to rise. It is a zone rather than a line, typically a few hundred kilometres wide, marked on satellite images by an almost continuous band of towering cumulonimbus cloud.

4. 4

   Why are the surface trade winds north-easterly in the northern hemisphere and south-easterly in the southern hemisphere rather than blowing straight towards the equator?

   Model answer

   Air returning at the surface from the subtropical high pressure towards the equatorial low is deflected by the Coriolis effect, caused by the Earth's rotation. Deflection is to the right of the direction of travel in the northern hemisphere, turning a southward flow into a north-easterly wind, and to the left in the southern hemisphere, turning a northward flow into a south-easterly wind.

5. 5

   Take the rising limb of the Hadley cell and follow the chain of cause and effect through to the climate of tropical forest.

   Model answer

   Intense overhead insolation heats the surface, so air rises by convection at the ITCZ. As it rises it expands and cools adiabatically. Once cooled to dew point the water vapour it carries condenses, releasing latent heat that drives the parcel higher still, so cloud builds to great depth as cumulonimbus. The result is heavy convectional rainfall, usually as afternoon and evening downpours with thunder. Because the ITCZ sits over the equatorial zone for most or all of the year, rainfall is high and there is no real dry season, which with constantly high temperatures gives the hot, wet, aseasonal climate that supports tropical forest.

6. 6

   Now follow the descending limb through to the desert belt.

   Model answer

   Air that rose at the ITCZ spreads polewards at high altitude, radiates heat to space and becomes denser, so by about 20-30 degrees north and south it subsides. As it descends it is compressed and warms adiabatically. Warming raises the temperature of the air and therefore its capacity to hold water vapour, so relative humidity falls, condensation cannot occur and cloud cannot form. Subsidence also creates a temperature inversion that caps any convection from below. At the surface the air piles up as the subtropical high pressure belt, giving stable, cloudless, rain-free conditions, which is why the world's great hot deserts lie in two belts centred near those latitudes.

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