Fig. 5.4 shows the structure of a tropical cyclone (hurricane). These storms bring strong winds, heavy rainfall and storm surges that can cause widespread d...

(a) The minimum sea surface temperature required for a tropical cyclone to form is 27 degrees C. [1]

This threshold provides enough thermal energy to heat the air above the ocean surface, driving the evaporation and convection that fuel the storm system.

(b) How a tropical cyclone forms and develops: [5]

1. Warm ocean water (above 27 degrees C) heats the air directly above it. This warm air absorbs large quantities of moisture through evaporation, becoming warm and humid. [1]
2. The warm, moist air rises rapidly by convection, creating an area of very low atmospheric pressure at the sea surface. Surrounding cooler air is drawn inward to replace the rising air. [1]
3. As the rising air ascends, it cools and the water vapour it carries condenses into clouds and rain. This condensation releases latent heat, which warms the surrounding air further and drives it upward even faster, creating a self-reinforcing cycle of energy transfer. [1]
4. The inflowing surface winds do not travel in a straight line towards the low-pressure centre. The Coriolis effect (caused by the Earth's rotation) deflects them, causing the winds to spiral. This gives the storm its characteristic rotating structure. [1]
5. At the very centre of the storm, air descends rather than rises, creating a calm region of clear skies and light winds called the eye. Surrounding the eye is the eye wall, where the strongest winds and most intense rainfall occur because this is where the upward convection is most vigorous. [1]

(c) Hazards associated with tropical cyclones at landfall: [5]

• Storm surge: The combination of extremely low atmospheric pressure and powerful onshore winds pushes a wall of seawater onto the coast. This surge can raise sea levels by several metres, inundating low-lying coastal areas and causing severe flooding. Storm surge is typically the deadliest hazard of a tropical cyclone. [1]
• Extreme winds: Wind speeds in a strong cyclone can exceed 250 km/h. These winds destroy buildings, uproot trees, bring down power lines and turn loose objects into dangerous projectiles. Infrastructure damage can leave communities without electricity, water and communications for weeks. [1]
• Intense rainfall and flooding: Tropical cyclones release enormous quantities of rain. In hilly terrain, this can trigger flash floods and landslides. Inland flooding may persist for days as rivers overflow, even after the wind has subsided. [1]
• Water contamination and disease: Floodwaters mix with sewage, agricultural chemicals and industrial waste, contaminating drinking water supplies. Standing water provides breeding grounds for mosquitoes, increasing the risk of water-borne diseases such as cholera and vector-borne diseases such as dengue fever. [1]
• Coastal erosion: The combined force of storm surge, powerful waves and high winds strips sediment from beaches and undermines coastal infrastructure such as sea walls, roads and buildings, causing permanent damage to the shoreline. [1]