Why do tropical cyclones form only under certain conditions, and why are their impacts so uneven between places?
Explain the conditions for tropical cyclone formation and structure, and assess why the scale of their impacts varies between locations
A focused answer to the H2 Geography outcome on tropical cyclones. The formation conditions, the structure of eye and eyewall, the hazards of wind, storm surge and rain, and why impacts differ with vulnerability and resilience.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SEAB wants you to explain the conditions a tropical cyclone needs to form, describe the structure of a mature storm, and assess why the impacts of similar storms differ so much between places. The central insight is that the storm is a heat engine driven by warm seas, but the human toll is set less by the storm's strength than by the vulnerability and resilience of the people it hits.
The answer
Conditions for formation
A tropical cyclone forms only where several conditions occur together:
- Warm ocean water, at least about 26.5 degrees Celsius to a depth of roughly 60 metres, to supply heat and moisture.
- Sufficient Coriolis effect, so a latitude more than about 5 degrees from the equator, to impart the rotation the storm needs.
- Low vertical wind shear, so the growing storm is not torn apart by winds changing with height.
- A trigger, some pre-existing low-level convergence such as an easterly (tropical) wave.
- An unstable, humid atmosphere through a deep layer, to allow sustained ascent.
The energy source: latent heat
The cyclone is a heat engine. Warm seas evaporate moisture; rising air condenses this moisture and releases latent heat, which warms the core, lowers surface pressure and draws in more air, which rises in turn. This positive feedback intensifies the storm while it remains over warm water. Crossing land or cool seas removes the moisture supply, so the storm weakens.
Structure of a mature cyclone
- The eye: a calm, clear, low-pressure centre where air gently descends.
- The eyewall: a ring of the tallest cumulonimbus and the most violent winds and heaviest rain, where the strongest ascent occurs.
- Rainbands: spiral arms of cloud and rain extending outward from the core.
The hazards
A cyclone delivers three main hazards: extreme winds that damage structures; a storm surge as low pressure and onshore winds pile water against the coast, the biggest killer on low coasts; and torrential rain causing river and flash flooding and landslides.
Why impacts vary
The same magnitude of storm produces very different impacts depending on the place:
- Physical factors: storm category, surge height (greater on shallow, funnel-shaped coasts), rainfall totals, and the storm's track and speed.
- Human factors: population density and exposure of low-lying settlement, building quality and code enforcement, early-warning systems and evacuation capacity, healthcare and emergency services, and overall wealth shaping preparation and recovery.
For the death toll, vulnerability often matters more than magnitude.
Examples in context
Example 1. Typhoon Haiyan, Philippines (2013). One of the strongest landfalling cyclones recorded, Haiyan drove a storm surge over five metres into Tacloban on Leyte, a low, funnel-shaped coast. More than 6,000 people died, with mortality concentrated in poorly built, densely settled coastal areas where warnings underplayed the surge. It is a clear case of physical magnitude meeting high vulnerability.
Example 2. Why Singapore largely escapes cyclones. Singapore sits within about 1.5 degrees of the equator, where the Coriolis effect is too weak to spin up a cyclone, so it is rarely struck directly. This makes it a useful counter-example: the formation conditions explain not only where cyclones occur but also where they do not, and why nearby places such as the Philippines are far more exposed.
Try this
Q1. State three conditions necessary for a tropical cyclone to form. [3 marks]
- Cue. Sea-surface temperature of at least about 26.5 degrees Celsius to depth; a latitude more than about 5 degrees from the equator for Coriolis effect; low vertical wind shear (and a humid, unstable atmosphere with a trigger).
Q2. Explain why a tropical cyclone weakens after making landfall. [2 marks]
- Cue. Over land it is cut off from the warm ocean that supplied moisture, so the latent-heat feedback driving it stalls; friction also disrupts the circulation, so it decays.
Q3. Explain why storm surge causes most deaths on low-lying tropical coasts. [3 marks]
- Cue. Low pressure and onshore winds pile seawater against the coast; on shallow, flat, funnel-shaped shores this surge rises high and floods far inland, drowning densely settled, poorly protected areas before people can escape.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original12 marksExplain the conditions necessary for tropical cyclone formation and the structure of a mature cyclone.Show worked answer →
Argument: tropical cyclones form only where several conditions coincide to allow a self-sustaining engine of rising moist air, and the mature storm has a distinctive structure that reflects this engine.
Conditions to explain: warm ocean water of at least about 26.5 degrees Celsius to a depth of around 60 metres, supplying heat and moisture; latitudes more than about 5 degrees from the equator so the Coriolis effect can impart rotation; low vertical wind shear so the developing storm is not torn apart; an existing trigger of low-level convergence such as an easterly wave; and an unstable, humid atmosphere through a deep layer.
The engine: warm seas evaporate moisture; as this air rises and condenses it releases latent heat, which warms the core and lowers surface pressure, drawing in more air, which rises in turn. This positive feedback intensifies the storm while it remains over warm water.
Structure: a mature cyclone has a calm, clear, low-pressure eye where air descends; a ring of the most violent winds and tallest cumulonimbus, the eyewall, where the strongest ascent and heaviest rain occur; and spiralling rainbands extending outward.
Markers reward the full set of formation conditions, the latent-heat feedback as the energy source, and an accurate description of eye, eyewall and rainbands.
Original12 marksA category 5 typhoon strikes a low-lying, densely populated coast in a lower-income country, while a similar storm crosses a wealthier, well-prepared coastline. Assess why the impacts of tropical cyclones vary so widely between places.Show worked answer →
Argument: physical magnitude sets the potential, but the realised impact depends heavily on the vulnerability and resilience of the place, so similar storms can produce very different outcomes.
Physical factors: storm category and wind speed, the height of the storm surge (greater on shallow, funnel-shaped coasts), rainfall totals and the resulting flooding, and the speed and track of the storm. Low, flat coasts amplify surge.
Human factors that explain the variation: population density and the proportion living in exposed low-lying areas; building quality and enforcement of codes; the existence and reach of early-warning systems and evacuation plans; healthcare capacity and emergency services; and overall wealth, which shapes preparation and recovery. A poorer, densely settled, low-lying coast with weak warning systems suffers far higher mortality and slower recovery than a wealthy, prepared one hit by a comparable storm.
Evaluation: a strong answer argues that vulnerability often matters more than magnitude for the death toll, using the contrast between, for example, Typhoon Haiyan in the Philippines and storms of similar strength making landfall on better-prepared coasts. Markers reward a structured split of physical and human factors and a clear judgement that resilience and vulnerability govern the human impact.
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