Definition and key characteristics of tropical cyclones

Tropical cyclones are powerful, low-pressure weather systems characterised by heavy rainfall and strong winds that spiral around a central point. They are known by different names depending on the region, such as hurricanes or typhoons, but all refer to the same type of storm.

These systems are circular in shape, often spanning hundreds of kilometres in width, and typically persist for 7 to 14 days. Due to the Earth's rotation, they spin anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.

Conditions required for tropical cyclone development

Tropical cyclones form under specific environmental conditions, primarily in regions with warm temperatures and low pressure.

Essential conditions for formation

• Location and temperature - Most tropical cyclones develop between 5° and 30° north and south of the equator, where global atmospheric circulation creates a band of low pressure and warm sea temperatures of at least 27°C are present.
• Wind shear - A low difference in wind speed between the upper and lower atmosphere is necessary for storm development.
• Warm, moist air dynamics - Warm, moist air rises, leading to condensation that releases energy, fuelling the storm's power.
• Low pressure and surface winds - Rising air creates areas of low pressure near the surface, which increases wind speeds.
• Movement and spin - Easterly winds near the equator drive tropical cyclones westward, while the Earth's rotation deflects these winds, causing the characteristic spinning motion.

Strength, distribution, and seasonal patterns of tropical cyclones

Tropical cyclones vary in strength and occurrence across the globe, influenced by factors such as water temperature and seasonal changes.

Factors affecting strength and distribution

• Energy source - Cyclones gain strength from warm ocean water, leading to increased wind speeds; they weaken over land or cooler water as their energy supply diminishes.
• Geographical distribution - The majority of tropical cyclones occur in the northern hemisphere, particularly over the Pacific Ocean, with no significant change in their global distribution over time.
• Seasonal occurrence - Most storms form in late summer and autumn when sea temperatures peak.
• Frequency variations - The number of storms fluctuates yearly, with an observed increase in the Atlantic since 1984, though no consistent trend exists over the past 130 years.

Structural features of a tropical cyclone

The structure of a tropical cyclone is distinct, with specific zones that exhibit varying weather conditions.

Key components of a tropical cyclone

• The eye - At the centre, the eye is a calm area up to 50 km wide, formed by descending air. It features very low pressure, light winds, clear skies, no rain, and high temperatures.
• The eyewall - Surrounding the eye, the eyewall contains spiralling rising air, extremely strong winds (often around 160 km/h), dense storm clouds, torrential rain, and lower temperatures.
• Outer regions - Towards the edges, wind speeds decrease, clouds become smaller and more scattered, rainfall lessens in intensity, and temperatures rise compared to the eyewall.

Impacts of tropical cyclones as natural hazards

Tropical cyclones pose significant threats to life and property due to their intense weather conditions.

Major hazards associated with tropical cyclones

• High winds - Winds can reach speeds of up to 250 km/h.
• Intense rainfall - These storms can release trillions of litres of water daily, with rainfall becoming heavier closer to the eye.
• Storm surges - Low pressure and high winds cause a significant rise in sea level, which, if coinciding with high tide, can overwhelm flood defences.
• Coastal flooding - Strong winds drive large waves onto shore, combined with storm surges, resulting in flooding of coastal regions.
• Landslides - Heavy rainfall destabilises hillsides, increasing the risk of landslides.

Classification of tropical cyclones using the Saffir-Simpson Scale

Tropical cyclones are categorised based on their wind speeds using the Saffir-Simpson Scale.

Categories of the Saffir-Simpson Scale