Nature and Causes of Tropical Storms (AQA A Level Geography)

Distribution of Tropical Storms

• Tropical storms are intense low-pressure systems (below 950mb)

• They are known as:

  • Typhoons in the South China Sea and west Pacific Ocean

  • Hurricanes in the Gulf of Mexico, Caribbean Sea and west coast of Mexico

  • Cyclones in the Bay of Bengal, Indian Ocean and northern Australia

• Characteristics include:

  • Heavy rainfall

  • High wind speeds (over 119 kmph)

  • High waves and storm surges

• Winds spiral rapidly around a calm central area known as the eye

• Tropical storms can be as much as 2000 km in diameter

• Winds are not constant across the diameter, they vary with the strongest and most destructive winds being found within the eyewall 

• They develop in tropical regions between 5° and 20° north and south of the equator

Distribution of tropical storms

Formation of a Tropical Storm

• Several factors affect the formation of tropical storms:

  • Oceans – tropical storms gain large amounts of moisture from oceans for the formation of clouds and precipitation

  • High temperatures – sea-surface temperatures over 27°C are needed to provide energy to power the storm through high rates of evaporation and condensation

  • Low wind shear – if the wind shear is too high, it can disrupt the organisation of the storm and prevent it from strengthening

  • Atmospheric instability – in tropical regions, warm, moist air near the surface rises creating instability and leading to the formation of clouds and precipitation

  • Coriolis effect – the rotation of the Earth causes the air to move around the centre of the eye in a circular or rotating motion

• These factors also influence the distribution of tropical storms:

  • They form over oceans between 5° and 20° north and south of the equator where ocean waters are warmest and the Coriolis effect is strongest

  • At the equator, although it is warm enough, the Coriolis effect is too weak

  • Tropical storms lose energy when they reach land or areas of cooler water

Stages of Tropical Cyclone Formation

• There are several stages in the formation of a tropical storm:

  • Warm, moist air rises quickly forming an area of intense low pressure

  • Air from high pressure areas rushes in to take the place of the rising air, generating strong winds

  • This air then rises forming a continuous flow of rising air

  • As the air rises it cools and condenses forming large cumulonimbus clouds 

  • These clouds form the eye wall of the storm and produce heavy rainfall

  • Condensation releases energy in the form of latent heat which helps to power the storm

  • Air at the top of the storm goes outwards away from the centre of the storm

  • The Coriolis effect causes the rising air to spiral around the centre

  • Cold air sinks at the centre creating a calm, dry area known as the eye of the storm

  • Tropical storms will die out if the heat energy and moisture from the ocean are no longer available to drive the storm

Forms of Storm Hazard

• Tropical cause a range of different hazards that can threaten human life and property:

  • High winds over 119 km/h uprooting trees, damaging infrastructure, buildings, causing injury and loss of life

    • e.g. Typhoon Haiyan which struck the Philippines in 2013 had sustained wind speeds of 315 km/h

  • Intense rainfall leading to flash flooding, damaging property and injuring people from fast-flowing water

    • e.g. in 2017, Hurricane Harvey caused over 150cm of rain in Texas, USA

  • Storm surges occur when large volumes of water are forced inland by the low pressure (which causes the sea surface to rise) and strong winds associated with tropical storms

    • They can reach several metres high and cause extensive flooding, erode beaches, damage sea defences and contaminate farmland and freshwater

    • e.g. Hurricane Katrina which hit the USA in 2005 caused a storm surge of over 8m in some areas

• Coastal and river flooding caused by intense rainfall and storm surges can affect large areas of low-lying land including farmland and the tourist industry

  • e.g. Typhoon Haiyan which struck the Philippines in 2013 caused widespread flooding due to heavy rainfall and storm surges

• Landslides are triggered because soil becomes saturated due to intense rainfall, increasing the weight of the soil and making it unstable

  • In areas with steep slopes where the soil can no longer hold its position, a landslide is triggered

  • e.g. Hurricane Maria which hit Puerto Rico in 2017 caused extensive landslides and resulting in deaths and damage to infrastructure