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

Tropical Storm

• 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, but winds are not constant across that, they vary with the strongest and most destructive winds being found within the eyewall 
• They develop in tropical regions between 5o and 20o north and south of the equator
• 

• 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

Formation of a tropical storm

• 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

Storm surge

• • 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