The structure of a tropical cyclone

Overview

Overall dimensions of a tropical cyclone, eye of a tropical cyclone and spiral rainbands.

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 are the overall dimensions of a tropical cyclone?

   Model answer

   A mature tropical cyclone is typically 300 to 800 kilometres across, although the very largest can exceed 1000 kilometres, and its circulation extends upwards through the whole depth of the troposphere to the tropopause at about 12 to 15 kilometres. It is therefore far wider than it is deep, more like a vast spinning disc than a column. It moves forward slowly, at around 15 to 30 kilometres per hour, so a large storm can take the best part of a day to pass over one place.

2. 2

   Describe the eye of a tropical cyclone.

   Model answer

   The eye is the calm central core, typically 20 to 50 kilometres across, around which the whole storm rotates. Inside it air is gently subsiding rather than rising, so it is compressed and warmed and any cloud evaporates, leaving clear or almost clear skies with the sun or stars visible. Winds are light or calm, temperatures are the highest in the storm and the atmospheric pressure is at its lowest, commonly 950 millibars and in the most intense storms below 900. Its arrival gives a sudden and deceptive lull.

3. 3

   Describe the eyewall and explain why it is the most dangerous part of a tropical cyclone.

   Model answer

   The eyewall is a dense ring of towering cumulonimbus clouds, reaching the tropopause, that encircles the eye. This is where the inward-spiralling air finally rises, so it contains the strongest updraughts, the heaviest rainfall of the whole system and the fastest winds, which can exceed 250 kilometres per hour in a major storm. The pressure gradient is steepest here, immediately outside the very low pressure of the eye, which is precisely why the winds reach their peak. Any place that passes beneath the eyewall receives the storm's full destructive force.

4. 4

   What are spiral rainbands?

   Model answer

   Spiral rainbands are curving bands of cumulonimbus cloud, tens of kilometres wide and hundreds long, that wrap inwards towards the eyewall and mark the paths along which moist air is spiralling into the storm. They produce alternating episodes of torrential rain and squally gusts separated by calmer, drier gaps, so a place on the outer edge of a cyclone experiences the weather coming in waves rather than steadily. They are the outermost part of the storm to arrive and the last to leave.

5. 5

   What happens to the air at the top of a tropical cyclone?

   Model answer

   Air that has risen in the eyewall and rainbands reaches the tropopause, where it can rise no further, and spreads outwards as an outflow. Because the Coriolis effect deflects it as it moves away from the centre, this outflow rotates in the opposite sense to the inflow below, so in the Northern Hemisphere the surface winds spiral in anticlockwise while the high cloud spirals out clockwise. The outflow forms a vast canopy of cirrus that is the first sign of the storm on a satellite image, and by exporting air from the top of the column it allows surface pressure to keep falling.

6. 6

   Describe how atmospheric pressure varies from the edge of a tropical cyclone to its centre.

   Model answer

   Pressure is close to normal, around 1010 millibars, at the outer edge, and falls slowly as the rainbands arrive. The fall becomes extremely rapid across the eyewall, dropping tens of millibars over just a few tens of kilometres, and reaches its minimum in the eye, commonly 950 millibars and below 900 in the most intense storms. It then rises just as steeply on the far side as the opposite eyewall passes, before easing back towards normal. A graph of pressure against distance would look like a deep, steep-sided funnel.

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