Geography 1.2.1 Tropical Cyclones

What are some characteristics of tropical cyclones?

What are some 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 

Anatomy of a tropical cyclone

Anatomy of a tropical cyclone

Stages of tropical cyclone formation

• 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

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

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 and 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

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