Tropical Storms (AQA GCSE Geography)

• Tropical storms are rotating, 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
• They develop over the warm tropical oceans between 5° and 30° north and south of the equator

Distribution of tropical storms

• The relationship between tropical storms and global circulation is mostly related to the Hadley cell, the Coriolis effect and equatorial trade winds:
  • The equatorial regions receive intense solar heating raising ocean temperatures 
  • This warm, moist air rises, leading to an intense low-pressure zone between the two Hadley cells - also known as the Intertropical Convergence Zone or ITCZ 
  • This generates thunderstorms, strong winds and intense rainfall at the surface
    • These are typical weather conditions at the rising arm of the Hadley cell
    • Dry air descends creating a high-pressure zone at the surface

    • This generates a pressure gradients and air rushes to the low-pressure zone generating the winds of the forming tropical storm

  • The (trade) winds move in a westerly direction from the equator 
  • Where the Coriolis effect starts the air spinning from 5° north and south of the equator
    • The effect is too weak at the equator to move the air
    • The spin is anti-clockwise (anticyclone) in the northern hemisphere but clockwise (cyclone) in the southern hemisphere as they are low-pressure systems
  • The greater the low-pressure, the greater the winds, the greater the spin and the larger the tropical storm becomes

Effect of the Hadley cell at the Equator

• Sea temperatures must be 27°C and above to allow warm air to rise quickly, causing an area of intense low pressure
• They only form between 5° and 30° north and south of the equator (warmest waters are found here)
• The rising air draws further moist, warm air up from the ocean's surface, generating stronger winds
• The air spirals upwards, cools, condenses and forms large cumulonimbus clouds 
• These clouds form the eye wall of the storm and produce heavy rainfall
• 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

• Characteristics include:
  • Lasting 7-14 days
  • Heavy rainfall
  • High wind speeds (over 119 kmph)
  • High waves and storm surges
• Tropical storms can vary in diameter (100-1000km) 
• Winds spiral rapidly around a calm central area known as the eye, with descending cold air, low pressure, light winds, no clouds or rain
• The winds of the storm are not constant across its diameter
  • The outer edges of the storm have lighter wind speeds, smaller and more scattered clouds, rain is less intense, and the temperatures begin to increase
  • The strongest and most destructive winds are found within the eyewall, with spiralling storm clouds, torrential rainfall and low temperatures 
• Tropical storms are rated on the five-point Saffir-Simpson scale based on wind speeds
• Tropical storms are considered major when they reach category 3 and have wind speeds between 111-129 miles (178-208 kilometres) per hour
• A category 5 storm can deliver wind speeds of more than 157 miles (252km) an hour

Formation of a tropical storm

• Global temperatures are set to rise as a result of global warming
• More of the world's oceans will be above 27° C, therefore, more places across the world will experience tropical storms
• Oceans will stay at 27°C or higher for longer during the year, which will increase the annual number of tropical storms 
• Higher temperatures will mean storms will be stronger, more frequent and cause more damage