Tropical Cyclones (Edexcel GCSE Geography: B (1GB0))

• Tropical cyclones are rotating, intense low-pressure systems (below 950mb)
• Characteristics include:
  • Lasting 7-14 days
  • Heavy rainfall
  • High wind speeds (over 119 kmh)
  • High waves and storm surges
• Tropical cyclones 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 cyclones are rated on the five-point Saffir-Simpson scale based on wind speeds
• Tropical cyclones 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

Anatomy of a tropical cyclone

Stages of tropical cyclone formation

• In the right conditions, a tropical cyclone can form rapidly and follow a number of stages:
  • Warm, moist air rapidly rises, forming an area of low pressure
  • Air from high-pressure areas rushes in to take the place of the rising air
  • This air then rises, forming a continuous flow of rising air
  • As the air rises, it cools and condenses. This releases heat energy, which helps power the tropical cyclone
  • Air at the top of the storm goes outwards away from the centre of the storm
  • The Coriolis force causes the rising air to spiral around the centre.
  • Some of the air sinks in the middle of the storm, forming a cloudless, calm eye
  • The tropical cyclone moves westward from its source 
  • If a tropical cyclone makes landfall or moves over an area of cold water, it no longer has a supply of warm, moist air and loses speed and temperature; therefore, rainfall and winds decrease

• Tropical cyclones are known as:
  • Typhoons in the South China Sea and west Pacific Ocean
  • Hurricanes in the Gulf of Mexico, Caribbean Sea and the west coast of Mexico
  • Cyclones in the Bay of Bengal, Indian Ocean 
• They develop over the warm tropical oceans between 5° and 30° north and south of the equator

Distribution of tropical cyclones

• Tropical cyclones track west (owing to easterly winds) and slightly towards the poles
• Many tropical cyclones eventually move into areas dominated by westerly winds (found in the mid latitudes)
• These winds will reverse the direction of the tropical cyclone to an eastward path
• As the tropical cyclone continues to move poleward, it picks up speed and may reach 30 mph or more
• Tropical cyclones can travel about 300 to 400 miles a day, or approximately 3,000 miles before dying out
• Tropical cyclones mostly occur in the late summer to autumn when sea temperatures are at their highest
  • In the northern hemisphere, late summer to autumn is June – November
  • In the southern hemisphere, this is between November to April 
• The warm tropical ocean waters produce around 80 storms per year
• The highest number of storms are in the Pacific Ocean, followed by the Indian Ocean, then the Atlantic Ocean
• The most powerful storms occur in the Western Pacific
• The frequency of storms in the Atlantic has increased since 1995 
• There is evidence to suggest that tropical storms are becoming more intense due to global warming
• It has been calculated that the energy released by the average storm has increased by 70% in the past 30 years

• 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 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

How the Hadley Cell affects tropical cyclone formation

• 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

Dissipation of tropical cyclones

• A tropical cyclone will dissipate if it loses its source of energy - either from the warm waters or loss of moisture over land 
• When a tropical cyclone makes land, the winds become slower as it passes over rough terrain and over built up areas
• If a tropical cyclone stirs up deep, cold ocean waters, then it will lose energy and dissipate