Tropical Cyclones

Distribution and conditions for formation

Tropical cyclones (called hurricanes in the Atlantic/NE Pacific, typhoons in the NW Pacific, cyclones in the Bay of Bengal/South Pacific) form between latitudes 5° and 20° N/S. They require:

1. Ocean surface temperature ≥ 26 °C to a depth of 50 m — provides the energy through latent heat.
2. Sufficient water vapour to fuel convection.
3. Low vertical wind shear — minimal change in wind speed/direction with altitude, so the developing vortex is not torn apart.
4. The Coriolis effect (at least 5° from the equator) to start the rotation — anticlockwise in the Northern Hemisphere, clockwise in the Southern.
5. A pre-existing weather disturbance (e.g. easterly wave) to provide initial uplift.

They do NOT form at the equator (no Coriolis) or in cold-water oceans.

Structure of a tropical cyclone

• Eye (20–60 km wide): calm, cloudless, descending air, very low pressure (~900 hPa). Deceptively clear.
• Eyewall: the most dangerous zone — towering cumulonimbus clouds, strongest winds (150–300 km/h), heaviest rainfall.
• Rain bands: spiralling bands of cloud and rain extending hundreds of km from the centre; destructive but less intense than the eyewall.
• Storm surge: dome of seawater (up to 9 m high) pushed ahead of the storm — the deadliest aspect of many cyclones.

As a cyclone moves over land or cooler water, it loses energy (cut off from warm ocean) and weakens rapidly.

Saffir–Simpson scale

Tropical cyclones in a warming world

• Warming ocean → more intense cyclones (higher categories) as more energy is available.
• Warmer atmosphere holds more moisture → heavier precipitation and flooding.
• Evidence: intensity of Category 4–5 storms has increased since the 1980s (Emanuel 2005, Knutson et al. 2020).
• Frequency: IPCC suggests total number of cyclones may remain similar or decrease, but the proportion reaching Cat 4–5 will increase.
• Range may expand poleward as 26 °C SST isotherm moves further from the equator.
• Sea level rise amplifies storm surge impact on low-lying coasts.

Case study A: Typhoon Haiyan, Philippines, November 2013 — LIDC/EDC

• Strongest landfalling tropical cyclone ever recorded: sustained winds ~315 km/h, gusts ~380 km/h.
• Storm surge: 7 m wall of water hit Tacloban City — the primary killer (6,300+ deaths).
• 14 million affected; 4 million displaced; 1.1 million homes destroyed.
• Response: Philippine government declared state of calamity; US military aid ("Operation Damayan") was critical; international NGOs (Red Cross, Oxfam) deployed. Slow to remote islands.
• Long-term: "Build Back Better" programme; PPP-funded resilient housing; improved early-warning systems.

Case study B: Hurricane Harvey, Texas, USA, August 2017 — HIC

• Category 4 at landfall; stalled over Texas for days, dropping 1.5 m of rain → catastrophic inland flooding (not storm surge).
• 68 deaths; $125 bn damage (costliest US hurricane at the time).
• Strong early warning, mass evacuation orders, FEMA rapid response. Deaths low relative to rainfall volume.
• Challenge: car-dependent Houston; 30,000 flood rescues by civilians (the "Cajun Navy").
• Demonstrates that even HICs suffer major economic loss — climate adaptation is needed at all development levels.

Edexcel B exam tip

The spec requires two contrasting case studies (HIC and LIDC/EDC). Examiners want: location, date, meteorological data, specific primary/secondary impacts, and explicit comparison of responses linked to development level.