Physical Hazards of Tropical Cyclones

High Winds (Primary Hazard)

• You can safely lean into a stiff breeze, but at $119\,\text{km/h}$, the wind becomes a destructive force capable of snapping trees and tearing off roofs.
• High winds are the defining hallmark of tropical cyclones, with Category 5 storms exceeding $252\,\text{km/h}$. The primary impact of these extreme winds is immense structural damage to buildings, power lines, and telecommunications infrastructure.
• Strong winds also turn loose objects into deadly flying debris and act as the main physical driver for pushing seawater towards the coast to create surges.

Storm Surges and Coastal Inundation

• Understanding atmospheric pressure explains why the sea level suddenly rises during a cyclone, often causing more deaths than the powerful winds.
• A storm surge is a temporary, rapid rise in sea level caused by low atmospheric pressure and high wind stress physically pushing water toward the coast. The process where sea levels rise as atmospheric pressure drops is called the inverse barometer effect.
• This massive wall of water causes severe coastal inundation, leading to overwhelming floods, coastal erosion, and the salinisation of freshwater sources.

Calculating the Inverse Barometer Effect

• For every $1\,\text{mb}$ drop in air pressure below the standard $1013\,\text{mb}$, the sea level rises by $1\,\text{cm}$.

• Step 1: Identify the central pressure of the storm (e.g., $913\,\text{mb}$).

• Step 2: Calculate the pressure drop from standard: $1013\,\text{mb} - 913\,\text{mb} = 100\,\text{mb}$.

• Step 3: Convert the drop into sea-level rise: A $100\,\text{mb}$ drop results in a $100\,\text{cm}$ ($1\,\text{m}$) rise in sea level due to pressure alone, before wind effects are even added.

Intense Rainfall and Landslides

• Imagine pouring a bucket of water onto a dry sponge; eventually, the sponge cannot absorb any more, and the water runs straight off the sides.
• Tropical cyclones produce torrential intense rainfall, often dropping $200\,\text{mm}$ to $400\,\text{mm}$ of water in just a few hours. When this precipitation exceeds the soil's infiltration rate, the ground becomes entirely saturated, leading to severe river and flash flooding.
• In mountainous or high-relief regions, this heavy saturation triggers a landslide, which is a highly destructive type of mass movement.
• Landslides are a dangerous secondary impact because they frequently block mountain roads, preventing rescue teams from delivering aid to isolated rural communities.

Assessing Hazard Significance

• Which hazard actually causes the most devastation during a tropical cyclone?
• The answer depends heavily on the location and development level of the affected area. Storm surges are widely considered the most significant hazard regarding loss of life, accounting for approximately 90% of all cyclone-related deaths due to drowning in coastal zones.
• For inland or mountainous areas where the ocean cannot reach, intense rainfall and mass movement become the deadliest and most significant threats.
• High winds generally cause the most widespread structural and economic damage, particularly in developed nations (HICs) with expensive infrastructure. In contrast, emerging and developing countries (LICs) suffer higher social impacts and fatalities due to less resilient housing and poorer drainage systems.