Cyclones need ocean heat, moisture and an organised circulation. Their rain can continue inland after the strongest winds weaken.
A tropical cyclone is an organised, rotating low-pressure system powered by heat and moisture from a warm ocean. It operates at the regional, or synoptic, scale. Formation is favoured by sea-surface temperatures around 26.5-27 degrees C or higher over a sufficiently deep warm layer. A thin warm surface can cool quickly as winds stir colder water upwards.
The syllabus focuses on storms with sustained winds of at least 119 km/h: hurricane or typhoon strength. They are called hurricanes in the Atlantic and eastern North Pacific, typhoons in the western North Pacific, and cyclones in the Indian Ocean and Australian region. Weather agencies also use tropical cyclone as a broader term that includes weaker depressions and storms; classifications vary by basin.
Moist, unstable air supports deep convection. A pre-existing disturbance and converging surface winds help organise rising air. Low vertical wind shear, meaning a small change of wind speed or direction with height, keeps the circulation and thunderstorms aligned. Sufficient Coriolis effect is also needed, so formation is uncommon within about 5 degrees of the equator; these are typical conditions, not an absolute latitude barrier.
Warm seawater supplies vapour by evaporation. As moist air rises and condenses, latent heat warms the storm's core. Upper-level outflow removes air aloft while surface air spirals inward. This maintains low pressure and renewed convection. The strongest ascent and rain often occur in the eyewall and spiral rainbands; the eye usually has descending air.
Slow-moving storms can deliver prolonged extreme rainfall. Mountains can intensify rain by forcing moist air upwards. Landfall usually weakens the ocean heat supply, but a weakening storm can still cause serious inland flooding. Storm surge is coastal seawater flooding, a different mechanism from rivers overflowing after rain.
Step by step
Supply heat and moisture
A deep warm ocean layer fuels evaporation and convection.
Organise the storm
Convergence, rotation and low shear maintain an aligned circulation.
Concentrate rainfall
Repeated rainbands, slow movement and relief can raise flood-producing totals.
Worked example: Two equally warm ocean locations
In an illustrative comparison, both seas are 28 degrees C. One has strong wind shear that separates the thunderstorms from the surface circulation. The other has low shear and a rotating disturbance. The second is more favourable, but development is still not guaranteed.
Watch out for this
A cyclone that weakens over land can no longer cause flooding.
Wind intensity and rainfall hazard are not identical. Slow movement, abundant moisture and terrain can sustain damaging rain.
Check your understanding
Which change most directly disrupts an organising tropical cyclone?
- A deeper layer of warm water.
- Strong vertical wind shear separates its circulation and convection.
- Moist air condenses in rising thunderstorms.