The Characteristics of Tropical Cyclones

• 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

Exam Tip

Make sure you know how storms develop, along with a few of the main characteristics and how climate change may affect them. 

Being able to draw and annotate the formation of a tropical storm will gain you credit, or you may be asked to complete a diagram in the exam. 

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

Exam Tip

Remember, conditions such as warm oceans and the Coriolis Force exist at all times, but tropical cyclones do not form all the time.

It is the combination of all the right conditions coming together that leads to tropical cyclone formation.

Distribution of Tropical Cyclones

• 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

Atmospheric Circulation & Tropical Cyclones

• 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

Worked example

Suggest one way the distribution of tropical storms could change if global ocean temperatures continue to rise.  

(1 mark)

Answer:

• They may affect areas further from the Equator. (1)

• They could affect parts of the sub tropics/the South Atlantic/NE USA. (1)

• They could have a broader distribution/effect on larger parts of the world. (1)