Atmospheric circulation: cells, wind belts and the westerlies

Uneven heating drives the winds

• Near the equator the Sun is high all year, so each square metre receives more energy than it radiates away; near the poles the reverse is true.
• Without a way to move heat, the tropics would keep warming and the poles cooling. The atmosphere and ocean carry the surplus polewards, and in the atmosphere this happens through convection: warm, less dense air rises, cool, dense air sinks.
• Rising air leaves low pressure at the surface and sinking air makes high pressure, and surface winds blow from high pressure towards low pressure.
• On a non-spinning Earth one huge cell in each hemisphere might do the job. Earth's rotation, through the Coriolis effect, deflects the moving air so much that the circulation breaks into three cells in each hemisphere.

The Hadley cell

• At the equator strong heating makes moist air rise in towering thunderstorms. The rising air leaves low pressure at the surface along the intertropical convergence zone (ITCZ), with heavy rain and light, variable winds: the doldrums.
• The air spreads polewards near the tropopause, cools by radiating heat to space and is deflected by the Coriolis effect, so it piles up and sinks at about 30° N and 30° S.
• Sinking air is compressed and warms, so clouds evaporate: the subtropical high-pressure belt brings clear skies, which is why the great deserts, including central Australia's, lie near 30°.
• From the subtropical highs, surface air flows back towards the equatorial low, turned to the left by the Coriolis effect in the Southern Hemisphere: the south-east trade winds.

Try it: a convection cell

The polar and Ferrel cells

• Over Antarctica, very cold, dense air sinks and builds high pressure. It flows out towards about 60° S, turned to the left to become the polar easterlies.
• At about 60° S the cold polar air meets milder air flowing from the subtropics along the polar front. The milder air is forced up, giving the subpolar low-pressure belt, with frequent storms.
• Between 30° and 60° the Ferrel cell is driven by the two cells either side, like a gear: surface air flows polewards from the subtropical highs, and the Coriolis effect turns it towards the east, making the westerlies.
• The Ferrel cell is the least tidy of the three. In reality the mid-latitudes are full of moving highs, lows and fronts, which do much of the heat transport there.

The Coriolis effect and air around highs and lows

• Air moving across the spinning Earth is deflected to the left in the Southern Hemisphere and to the right in the Northern. The size of the effect is set by the Coriolis parameter, f = 2Ω sin φ, where Ω = 7.29 × 10⁻⁵ rad s⁻¹ is Earth's rotation rate and φ the latitude.
• f is zero at the equator, which is why tropical cyclones never form right on it, and largest at the poles; the sideways acceleration on moving air is f times its speed.
• Air starts to flow from high to low pressure, but the Coriolis effect keeps turning it until, above the friction of the ground, it blows almost parallel to the isobars.
• So in the Southern Hemisphere air circulates anticlockwise around a high and clockwise around a low. Near the surface, friction slows the wind, weakening the Coriolis effect, so the air spirals out of highs and into lows across the isobars.

The wind belts of the Southern Hemisphere

• Winds are named for the direction they blow from. Each surface belt runs from a high-pressure belt towards a low-pressure belt, turned left by the Coriolis effect.
• The belts in the Northern Hemisphere mirror these: north-east trades, south-westerlies and north-easterly polar winds.

Try it: name the wind belt

New Zealand in the westerlies

• New Zealand lies between about 34° S and 47° S, across the boundary of the subtropical highs and the westerlies, with the strong westerlies of the Roaring Forties to the south.
• High-pressure systems drift east across the country roughly every week, with troughs and cold fronts between them; a front usually brings a change from warm north-westerlies to cold south-westerlies.
• The Southern Alps and the North Island ranges stand across the westerly flow. Moist air from the Tasman Sea is forced up, cools and drops heavy orographic rain on the west.
• Having lost its moisture, the air descends the eastern side, warming as it is compressed: the hot, dry, gusty föhn wind Canterbury calls a nor'wester. The east lies in a rain shadow.

The belts shift with the seasons

• The whole pattern follows the Sun: in the southern summer the ITCZ and the subtropical high-pressure belt move south, and in winter they move north.
• In summer the subtropical highs often sit over or near northern New Zealand, giving long settled spells, while the westerlies are further south.
• In winter and spring the highs lie further north and the westerlies reach further up the country, so the weather is windier and more changeable, and the West Coast gets more rain.
• The Southern Annular Mode adds to this: when the westerlies contract towards Antarctica (positive phase) New Zealand tends to have lighter winds and settled weather; when they expand north (negative phase), it is windier, with more westerly rain.

What Achieved, Merit and Excellence look like

• Excellence links heating, convection, pressure, the Coriolis effect and the weather into one explanation.