Tropical Cyclone Hazards

Tropical cyclones are among the deadliest and most costly natural hazards on Earth. While the previous lesson explained how they form and their structure, this lesson focuses on the specific hazards they produce and the factors that determine how severe their impacts are. Understanding these hazards is essential for evaluating case studies and explaining why some tropical cyclones cause far more death and destruction than others.

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Primary Hazards

Primary hazards are the direct, immediate dangers caused by the tropical cyclone itself. They occur during the storm's passage.

High Winds

• Sustained wind speeds in a tropical cyclone can exceed 250 km/h (Category 5), with gusts potentially reaching over 300 km/h.
• High winds cause direct damage: tearing off roofs, collapsing buildings, snapping power lines, uprooting trees and turning loose objects into deadly projectiles (flying debris).
• The strongest winds are found in the eyewall, with wind speed decreasing outward through the spiral rainbands.
• Wind damage is proportional to the cube of the wind speed — this means a doubling of wind speed produces roughly eight times as much destructive force.

Storm Surge

Storm surge is often the deadliest hazard associated with tropical cyclones. It is a temporary rise in sea level caused by the combination of:

1. Low atmospheric pressure — the extremely low pressure at the centre of the storm causes the ocean surface to bulge upward (approximately 1 cm rise for every 1 millibar drop in pressure).
2. Onshore winds — the powerful winds push water towards the coast, piling it up against the shoreline.
3. Wave action — large waves driven by the storm are superimposed on top of the surge.

Factor Affecting Storm Surge Height	Explanation
Storm intensity	Stronger cyclones generate higher surges; Category 5 storms can produce surges exceeding 6 metres
Forward speed	Slower-moving storms can push water for longer, building higher surges
Coastal shape	Funnel-shaped bays and estuaries concentrate surge water, amplifying heights. The Bay of Bengal is particularly vulnerable
Coastal gradient	Shallow, gently sloping coastlines allow surge water to penetrate far inland; steep coastlines limit inland penetration
Timing with tides	If landfall coincides with high tide, the surge is added to the already elevated water level, producing devastating flooding

Exam Tip: Storm surge is the leading cause of death in tropical cyclones. The 1970 Bhola Cyclone in Bangladesh killed an estimated 300,000–500,000 people, primarily through storm surge flooding in the low-lying Ganges-Brahmaputra delta. Always mention storm surge when discussing cyclone deaths.

Intense Rainfall

• Tropical cyclones produce enormous volumes of rainfall. A single storm can drop 500–1,000 mm of rain in 24–48 hours.
• Hurricane Harvey (2017) dropped a record 1,539 mm (60.58 inches) of rain on parts of Texas over four days, making it the wettest tropical cyclone on record in the USA.
• This rainfall is produced by the rapid condensation of warm, moist air in the eyewall and spiral rainbands.
• Slow-moving storms are especially dangerous because they dump rain over the same area for an extended period.

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Secondary Hazards

Secondary hazards are consequences that follow from the primary hazards. They may occur during or after the storm.

Flooding

• The combination of storm surge and intense rainfall causes extensive flooding, both coastal and inland.
• Rivers and drainage systems become overwhelmed, leading to flash floods in urban areas and river flooding in low-lying areas.
• Flooding can persist for days or weeks after the storm has passed, particularly in flat, low-lying terrain with poor drainage.
• Floodwater contaminates freshwater supplies with seawater, sewage and chemical pollutants, creating public health crises.

Landslides and Mudslides

• Intense rainfall saturates hillslopes, reducing friction and triggering landslides and mudslides.
• These are especially dangerous in mountainous tropical regions with steep slopes and thin soils.
• Hurricane Mitch (1998) triggered mudslides in Honduras and Nicaragua that buried entire villages, killing over 11,000 people.
• Deforestation increases landslide risk because tree roots no longer bind the soil.

Disease Outbreaks

• Flooding creates ideal conditions for the spread of waterborne diseases such as cholera, typhoid and dysentery.
• Standing water provides breeding grounds for mosquitoes, increasing the risk of malaria and dengue fever.
• Damage to healthcare facilities, disruption of water supply and sanitation systems, and overcrowding in emergency shelters all contribute to disease outbreaks.
• In the aftermath of Typhoon Haiyan (2013), the Philippines reported outbreaks of gastroenteritis and leptospirosis among displaced populations.

Other Secondary Hazards