You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Spec mapping (AQA 7037): Paper 1, §3.1.5 Hazards — "the nature of tropical storms and their underlying causes; their distribution, frequency and magnitude (Saffir–Simpson scale); the sequence of formation and development; the structure and features of a tropical storm; primary and secondary hazards (high winds, storm surges, coastal flooding, river flooding and landslides); impacts (social, economic, environmental, political); short- and long-term responses; the use of the Park model and the Hazard Management Cycle; the relationship of magnitude and frequency to development and governance." It applies the atmospheric-systems lesson (SSTs, Coriolis, latent heat) to a specific named hazard and requires a detailed case study of a recent tropical storm in a developed and developing-world context to illustrate the relationship between physical factors, vulnerability and resilience. It links synoptically to §3.1.1 (latent-heat transfer; coastal/water systems), §3.2.1 (globalised impacts) and §3.2.x (coastal population density and informal settlement as vulnerability). Assessment objectives: AO1 (formation, structure, hazards), AO2 (applying these to explain contrasting impacts and reaching judgement), AO3 (Saffir–Simpson data, surge/wind figures, deaths-vs-development comparison).
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. The unifying physics is that a tropical cyclone is a heat engine: it converts the thermal energy of a warm ocean into the kinetic energy of wind via the release of latent heat, which is why every formation condition below traces back to warm water and the ability of moist air to rise and rotate.
Tropical storms have a distinctive global distribution that follows directly from their formation requirements. They occur in seven main "basins" — the North Atlantic, eastern, central and western North Pacific, the North Indian Ocean, the South Indian Ocean and the South Pacific — all between roughly 5° and 20–25° latitude over oceans warmer than 26.5 °C. They are absent from the South Atlantic and southeastern Pacific (where SSTs are too low and wind shear too high), and they cannot form on the equator (zero Coriolis). The western North Pacific is the most active basin, which is why the Philippines and Japan are so frequently struck (the Philippines averages ~20 cyclones a year). Each hemisphere has a distinct season — the late summer/autumn of its respective hemisphere, when SSTs peak (June–November in the North Atlantic; November–April in the South Pacific) — because the ocean needs the summer to warm sufficiently. This seasonality and basin geography is itself examinable as a distribution pattern explained by the underlying physical conditions.
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 |
The six conditions are not a checklist of unrelated facts — they describe the assembly of a self-sustaining heat engine. (1) Warm water (> 26.5 °C) drives intense evaporation, loading the lower atmosphere with moisture and latent heat. (2) Around a pre-existing disturbance, this warm, moist air rises and condenses, releasing latent heat that warms the air column, lowers surface pressure and draws in yet more moist air — a positive feedback that intensifies the system. (3) The Coriolis effect (f=2Ωsinϕ, hence the > 5° latitude rule) deflects the inflowing air into a rotating spiral. (4) Low wind shear allows the vertical "chimney" of the storm to remain stacked and organised rather than being torn apart. The storm matures into the familiar eye/eyewall structure and is sustained as long as it sits over warm water; it weakens rapidly on making landfall (fuel cut off, friction increased) or over cool water. This sequence is the AO1 spine of any tropical-storm essay.
Worked AO3 skills exemplar — interpreting a Saffir–Simpson / damage relationship. Given the Saffir–Simpson table below: (i) Describe: damage potential rises non-linearly with category — each step up the scale represents a far greater than proportional increase in destructive power. (ii) Manipulate: because the kinetic energy of wind scales with the square of wind speed, a Category 5 (~252 km/h) carries roughly (252/119)2≈4.5 times the wind energy of a Category 1 (~119 km/h), and damage (which rises even faster, ~with the cube or more of wind speed) escalates dramatically — a one-category jump can multiply losses several-fold. (iii) Explain: this is why rapid intensification near landfall is so dangerous, and why the surge column also grows with category. (iv) Evaluate: stress the scale's key limitation — it measures wind only, so it systematically understates the threat from low-category but slow-moving, rain-heavy or surge-heavy storms (Harvey 2017's flooding; Bhola 1970's surge). The wind category is a poor proxy for total impact.
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 structure matters operationally because the worst conditions are concentrated in the eyewall, and the passage of the eye produces a deceptive lull: as the calm, clear eye passes overhead, people may believe the storm has ended and venture outside — only to be caught by the returning eyewall winds from the opposite direction. The right-front quadrant (in the Northern Hemisphere) is typically the most dangerous, because there the storm's forward motion adds to its rotational winds and it is where surge and embedded tornadoes are most likely. A further structural concept is the eyewall replacement cycle, in which a mature storm grows an outer eyewall that chokes off and replaces the inner one, temporarily weakening the winds but enlarging the storm's wind field and surge potential. These structural details turn a generic description into precise, examinable AO1.
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 |
A storm surge is produced by two combined effects: the "inverse barometer" effect (the storm's intensely low central pressure literally allows the sea surface to bulge upward — roughly 1 cm of sea-level rise per 1 hPa pressure drop) and, dominantly, the wind set-up (the storm's powerful onshore winds physically pile water against the coast). The surge is amplified by a shallow, gently sloping continental shelf (which lets the water "stack up"), by a funnel-shaped coastline or bay (concentrating the water), and by the surge coinciding with high tide. This is why the deadliest surges occur over wide, shallow deltas — the Bay of Bengal (Bhola 1970, ~300,000–500,000 deaths) and Tacloban's funnel-shaped bay (Haiyan 2013, ~7.5 m surge). The surge, not the wind, is historically the great killer, which is the single most important corrective to the "wind = danger" assumption built into the Saffir–Simpson scale. | 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 |
It is essential to classify these correctly as primary versus secondary hazards — a common exam discriminator. The primary hazards come directly from the storm's physical structure (extreme winds, the storm surge driven by wind and pressure, and the torrential rainfall of the eyewall and rainbands). The secondary hazards are triggered consequences: river and flash flooding as the rainfall runs off, landslides as saturated slopes fail (devastating in mountainous Central America and the Philippines), waterborne disease (cholera, typhoid, dengue) from contaminated supplies, and the economic and food-security crises that follow the destruction of crops and livelihoods. Slow-moving storms are disproportionately dangerous because they prolong every rainfall-driven secondary hazard — the reason a "weak" Category 1 that stalls can kill more people than a fast-moving Category 5.
| Category | Details |
|---|---|
| Deaths | 6,300 confirmed; over 1,000 still missing |
| Injuries | 28,689 |
| Displaced | 4.1 million people |
| Buildings | 1.1 million houses destroyed; 600,000 damaged |
| Storm surge | Up to 7.5 m in Tacloban City — effectively a tsunami-like wall of water that destroyed everything in its path; the surge arrived with almost no warning |
| Infrastructure | Tacloban airport destroyed; roads impassable; power grid destroyed across affected islands; water supply contaminated |
| Agriculture | 600,000 hectares of farmland damaged; 33 million coconut trees destroyed (coconuts are a primary livelihood) |
| Economic cost | $12.9 billion (approx. 5% of Philippines GDP) |
| Health | Risk of cholera, typhoid and dengue from contaminated water; overwhelmed hospitals |
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.