Weather and climate, Year 10: atmospheric circulation and tropical storms

It lowers it (a belt of low pressure)

HintRising air presses less heavily on the ground beneath it.

The whyRising air means less weight of air pressing on the surface. As the air rises it cools, water vapour condenses, and cloud and rain form. The UK lies close to this belt, which is one reason its weather is so often unsettled.

Strongly heated air rises, cools and its water vapour condenses

HintThe Sun is most nearly overhead there. What does that do to the ground and the layer above it?

The whyThe Sun's rays are most concentrated at the Equator, so the surface heats the air above it. Warm air is less dense and rises, which lowers the pressure at the surface. As it rises it cools and forms towering clouds and daily downpours.

Ferrel; Polar

HintOne is named after a nineteenth-century American scientist; the other is named after where it sits.

The whyEach cell is a loop of rising and sinking air. Air rises at the Equator and at about 60°, and sinks at about 30° and at the poles. Together the cells carry heat away from the tropics.

Sinking, so clouds cannot form

HintIt is the opposite of what happens over the rainforests.

The whyAir that rose at the Equator moves away at height, cools and descends at about 30°. Descending air is compressed and warms, so any cloud evaporates. The result is a belt of high pressure with clear skies and almost no rain.

The trade winds

HintSailing ships carrying goods across the oceans relied on them.

The whyAir moves along the surface from high pressure to low pressure. The Earth's rotation bends its path, so these winds arrive from the north-east in the northern hemisphere and from the south-east in the southern hemisphere. They meet near the Equator, where the air rises.

Enough spin from the Earth's rotation (the Coriolis effect)

HintThink about what turns a cluster of thunderstorms into a single whirling system.

The whyA tropical storm has to rotate, and the turning effect supplied by the spinning planet is zero on the Equator and grows towards the poles. Storms therefore form a little way north or south of it, with only the rarest exceptions.

5; 30

HintToo near the middle there is no spin; too far away the sea is too cool.

The whyInside this band the sea surface reaches at least 27 °C and the Earth's rotation is strong enough to make the rising air spiral. Storms form mainly in late summer and autumn, when the sea is at its warmest.

Westwards

HintIn the Atlantic, storms born near Africa head for the Caribbean.

The whyThe trade winds blow from the east, so a new storm drifts with them, and slightly away from the Equator. Many storms later reach the mid-latitudes, where the prevailing westerlies turn them back towards the east. This is how the global circulation steers them.

It releases heat, which makes the air rise even faster

HintEvaporating a liquid takes energy in. What happens when the change runs the other way?

The whyThe energy released, called latent heat, warms the surrounding air. Warmer air is lighter, so it rises faster and pressure at the sea surface falls further. More moist air is then sucked in from all sides, and the cycle repeats.

The share of storms that are very intense (stronger winds, heavier rain)

HintNot how many, but how powerful.

The whyWarmer seas give each storm more energy, and warmer air holds more moisture. So a larger proportion are expected to reach the top categories, with stronger winds and heavier rain. The total number worldwide is projected to stay about the same or fall.

Seas further from the Equator will become warm enough to sustain them

HintWhich ingredient is a warming climate spreading over a wider area?

The whyA tropical storm needs sea water of at least 27 °C. As oceans warm, that threshold is reached further north and south. Observations already show storms in the western North Pacific reaching their peak strength further north than they used to.

The eyewall of a tropical storm

HintIt stands like the side of a deep well around the calm middle.

The whyAir spirals in towards the centre at the surface and then rushes upwards in this ring. Rapid uplift builds very tall clouds, giving torrential rain and the storm's most violent winds. The worst damage happens as it passes overhead.

Sinking

HintIt moves the opposite way from the air in the clouds all around.

The whyAround the eye, air rises rapidly. In the eye itself air descends, warms and dries, so cloud cannot form. Winds there are light and the sky may even be blue, although the air pressure is the lowest in the whole storm.

The far side of the eyewall is about to arrive, with violent winds from the opposite direction

HintPicture a ring doughnut sliding across the town. Where is the town now?

The whyThe calm lasts only as long as the eye takes to cross, from a few minutes to an hour or two. Then the strongest winds return almost at once, blowing the other way, and hit buildings already weakened. Warnings tell people to stay sheltered until the whole storm has gone.

Anticlockwise

HintIt is the reverse of the way a clock's hands turn. South of the Equator the spin is reversed again.

The whyThe Earth's rotation deflects air flowing in towards the low-pressure centre, so it spirals instead of moving straight. North of the Equator the spiral turns one way; south of the Equator it turns the other. At the top of the storm the air flows outwards in the opposite direction.

rainbands

HintA compound word: what falls from the cloud, joined to a word for a strip or stripe.

The whyThese strips can stretch for hundreds of kilometres from the centre. Between them the weather eases for a while, then the next strip brings another burst of downpour and gusty wind. They give a tropical storm its spiral look in satellite images.