Tropical cyclones – formation, structure and storm surge
GeographyWeather & ClimateAges 15–16
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Sign in to playSet the sea-surface temperature, the latitude where a disturbance starts, the vertical wind shear and the humidity of the middle troposphere to see whether a storm forms, how strong it gets on the Saffir–Simpson scale and which track it follows. A cross-section shows the eye, eyewall and rain bands; at landfall the central pressure, wind speed and shape of the coast set the height of the storm surge. A responses panel compares warnings, evacuation, sea walls and mangroves in six real cyclone basins.
Lesson: Tropical cyclones (hurricanes, typhoons): conditions for formation, structure, tracks, storm surge and responses
What it shows
A tropical cyclone forms only over warm water of about 26.5 °C or more, at least about 5° from the equator so that the Coriolis effect can start the spin, where the winds change little with height and the middle troposphere is moist. Warm, moist air spirals in, rises in the eyewall and releases latent heat, which drives the winds. Over cool water or land this fuel is cut off and the storm weakens. At the coast, onshore winds pile water up over a shallow shelf and the low pressure lifts the sea, producing a storm surge.
How to use
Move the Sea temperature, Start latitude, Wind shear and Mid-level humidity sliders, then press Run. Follow the track on the map, the intensity graph and the cross-section through the storm. Choose a Basin, a Coast, Coastal defence and Landfall at high tide to change the storm surge. Tick Forecasts and warnings or Evacuation plan and shelters and compare the people left in the flooded zone. New storm restarts; each storm adds a row to the results table.
Parameters you can change
- Sea-surface temperature where the storm starts 24–31 °C
- Latitude where the disturbance starts (distance from the equator) 0–30 degrees
- Vertical wind shear (difference between upper and lower winds) 0–25 m/s
- Relative humidity of the middle troposphere 30–90 %
- Cyclone basin North Atlantic, Eastern North Pacific, Western North Pacific, North Indian Ocean, South-West Indian Ocean, Australian region
- Type of coast at landfall Wide, shallow continental shelf; low coastal plain, Narrow, steep shelf; high coast, Funnel-shaped bay on a shallow shelf
- Coastal defence None, Sea wall 3 m high, Mangrove belt 2 km wide
- Landfall at high tide
- Forecasts and storm warnings
- Evacuation plan and cyclone shelters
Questions to explore
- Why do almost no tropical cyclones form within 5° of the equator, even though the sea there is warm?
- Why is the storm surge of a category 4 storm much higher on a shallow coast or in a funnel-shaped bay than on a steep coast?
- Why does a cyclone weaken quickly after landfall or when it moves over cooler water?
