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

Tropical storms develop:

• Over warm tropical oceans

• Between 5° and 30° north and south of the equator

• When winds converge (meet) near the ocean surface

• There is low vertical wind shear

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

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

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