Extreme low pressure systems

Global distribution of cyclones and hurricanes

Cyclones, known as hurricanes in USA and typhoons in Asia, are violent tropical storms which occur during different seasons. They are examples of extreme low pressure systems. Hurricanes often occur at the start of the American summer from end of June until October.

A world map showing mean number of cyclones and their expected time (season) of occurrence.A map of the world showing the global distribution of cyclones

How do cyclones form?

For cyclones to occur and develop, the sea temperature must be 27°C for several weeks prior. Warm air then rapidly rises, creating low pressure. This leads to condensation and the development of large towering clouds and torrential rain. As warm moist air moves in to replace the rising air, it starts to spiral upwards towards the atmosphere. This spiral effect, from the rotation of the Earth, is known as the Coriolis effect.

Cyclone categories

Cyclones are categorised into five groups, from one, the lowest strength, to five, the strongest. In a category five cyclone, winds can exceed 300 kilometres per hour and the damage done is extreme. Property, transport structures, vegetation and infrastructure such as powerlines can be totally destroyed.

Cyclone Pam

In March 2015 Cyclone Pam devastated the island of Vanuatu in the Pacific Ocean. Pam was a category five cyclone where winds reached up to 320 kilometres per hour. Vanuatu is an extremely vulnerable remote island. National incomes are less than $3100 per year and most people are poor subsistence farmers who cannot afford to build well-built houses. The Government is also unable to afford hard or soft coastal defences. Due to climate change and sea-level rise the islands are extremely vulnerable.

Location and track of Cyclone Pam

A map showing the cyclone movement of Cyclone Pam to the east of Vanuatu.



  • 90 per cent of homes were destroyed leaving many homeless.
  • Water supplies were contaminated and wells destroyed.
  • 11 people were killed.
  • Schools were destroyed.
Two people looking through the wreckage of house.
A house destroyed in Vanuatu


  • Agricultural fields were badly damaged with up to 80 per cent of the island’s coffee growing crops totally ruined.
  • An estimated $2.5 billion of damage was done to farming businesses.


  • Heavy rain washed nutrients from soil.
  • Vegetation was flooded causing habitat loss.


Several nearby countries helped by sending emergency supplies, personnel and military support.

Aid items are held in large nets inside an aircraft. A man in the foreground is typing on a laptop computer.
Aid being transported to Vanuatu