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Tropical storms — known as hurricanes in the Atlantic and eastern Pacific, typhoons in the western Pacific, and cyclones in the Indian Ocean and South Pacific — are among the most powerful and destructive atmospheric hazards. They affect approximately 90 million people per year and cause average annual damages exceeding $26 billion globally (Munich Re, 2020). This lesson examines their formation, structure, impacts and management, with detailed case studies as required by the AQA specification.
Tropical storms require a very specific set of conditions to form. All six must be present:
| Condition | Detail |
|---|---|
| Sea surface temperature (SST) > 26.5 degrees C | Warm water provides the energy source through evaporation and subsequent latent heat release when water vapour condenses. The warm water must extend to at least 50–70 m depth (a shallow warm layer is insufficient) |
| Latitude > 5 degrees from the equator | The Coriolis effect must be strong enough to initiate the spinning motion. At the equator (0 degrees), Coriolis force is zero, so storms cannot form there |
| Low wind shear | The difference in wind speed and direction between the lower and upper troposphere must be small. High wind shear disrupts the vertical structure of the storm and prevents organisation |
| Atmospheric instability | Warm, moist air near the surface must be able to rise rapidly through the troposphere. Instability is enhanced when air at altitude is cool relative to the rising surface air |
| Pre-existing disturbance | A cluster of thunderstorms (often associated with easterly waves off the African coast) provides the initial organisation. About 60% of Atlantic hurricanes originate from African easterly waves |
| Sufficient moisture in the mid-troposphere | Dry air in the mid-levels inhibits convection and prevents storm development |
graph TD
subgraph "Cross-Section of a Tropical Cyclone"
A["Eye<br/>10–60 km diameter<br/>Calm, clear, warm<br/>Descending air"] --> B["Eyewall<br/>Most intense winds<br/>Heaviest rainfall<br/>Strong updrafts"]
B --> C["Spiral Rainbands<br/>Bands of thunderstorms<br/>spiralling inward<br/>Decreasing intensity outward"]
C --> D["Outflow at tropopause<br/>Diverging upper-level winds<br/>carry air away from storm"]
end
| Feature | Description |
|---|---|
| Eye | A roughly circular area at the centre, 10–60 km in diameter. Air descends in the eye, creating calm conditions, clear skies and warm temperatures. The eye is surrounded by the eyewall |
| Eyewall | A ring of intense cumulonimbus clouds surrounding the eye. Contains the strongest winds (up to 300+ km/h) and heaviest rainfall. Updrafts in the eyewall can reach speeds of 10–20 m/s |
| Spiral rainbands | Curved bands of thunderstorms extending outward from the eyewall, sometimes hundreds of km from the centre. These produce heavy rain, gusty winds and occasional tornadoes |
| Upper-level outflow | Air that has risen in the eyewall and rainbands flows outward at the tropopause (~12–15 km altitude). This outflow is essential — it removes air from the storm, maintaining the low pressure at the surface |
| Warm core | Tropical storms are warm-core systems: the eye and eyewall are warmer than the surrounding atmosphere at all levels. This warmth is generated by the massive release of latent heat during condensation |
The Saffir-Simpson Hurricane Wind Scale classifies tropical cyclones by their sustained wind speed:
| Category | Wind Speed (km/h) | Damage | Storm Surge |
|---|---|---|---|
| 1 | 119–153 | Minimal: some roof and siding damage | 1.2–1.5 m |
| 2 | 154–177 | Moderate: major roof damage; shallow flooding | 1.8–2.4 m |
| 3 (major) | 178–208 | Extensive: structural damage; flooding inland | 2.7–3.7 m |
| 4 (major) | 209–251 | Extreme: severe structural damage; major flooding | 4.0–5.5 m |
| 5 (major) | > 252 | Catastrophic: complete destruction of residential buildings | > 5.5 m |
Key Point: The Saffir-Simpson scale only measures wind speed. Some of the deadliest tropical storms have been lower-category events that produced catastrophic rainfall or storm surges. Hurricane Harvey (2017) made landfall as a Category 4 but its primary impact was rainfall flooding (1,539 mm at one location in Texas over four days).
| Hazard | Description | Relative Danger |
|---|---|---|
| Storm surge | A dome of seawater pushed onshore by the storm's winds and low pressure. The most lethal hazard — responsible for approximately 90% of tropical cyclone deaths historically | Highest in low-lying coastal areas |
| Extreme winds | Sustained winds exceeding 119 km/h (Category 1+); gusts can exceed 350 km/h in extreme storms | Destroy buildings, uproot trees, create flying debris |
| Intense rainfall and flooding | Tropical storms can produce 250–500+ mm of rain in 24 hours; slow-moving storms produce even more | Causes river and flash flooding far inland |
| Tornadoes | Outer rainbands can spawn tornadoes; these are typically weaker than Great Plains tornadoes but still destructive | Most common in the right-front quadrant of the storm |
| Landslides | Intense rainfall saturates hillslopes, triggering mass movements | Particularly dangerous in mountainous tropical islands |
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