Tropical Cyclones

• Tropical cyclones, hurricanes, and typhoons are found in different parts of the world but they are all the same weather feature

  • cyclones are found in the Indian Ocean and Australia
  • hurricans are found in the Atlantic Ocean and eastern Pacific Ocean
  • typhoons are found in the Western Pacific Ocean
• Tropical cyclones are large areas of very low pressure with wind speeds of over 119 km h^-1
• Categorised using Saffir-Simpson hurricane wind scale

  • rates wind speed on a scale of 1 to 5
  • category 1 - when the wind speeds are betwen 119 and 153 km h^-1
  • category 5- when the speeds are over 252 km h^-1
• Super typhoon - term used in the western North Pacific

  • used for tropical cyclones with winds exceeding 241 km h^-1
• Names are rotated over 6 year period

  • if the storm is very destructive, the name is removed from the list and not usedd again
• Certain conditions are needed for tropical cyclones to form (1)

  • ocean surface temperature must be at least 27 C as warm water provides energy for increased evaporation, which rises, condenses and releases huge amounts of energy
  • warm ocean water must be at least 60m deep
• Certain conditions are needed for tropical cyclones to form (2)

  • conditions must occur between latitudes 5 degress and 20 degress north and south, as any closer to the equator would mean an insufficient coriolis force (rotation of the earth) to make the air spin, and at higher latitudes the oceans are too cold
  • should be very little wind shear (change in wind speed or direction with height in the atmosphere) as strong wind shear can stop the vertical development of a storm
• Conditions exist between May and November in the Northern Hemisphere and between November and May in the Southern Hemisphere
• Tropical cyclones start as clusters of thunderstorms that grow in size and start to spin

  • air rises as it is heated from below
  • it cools, condenses and releases latent heat - causes air to rise evenfaster, intensifying the low pressure area and more air is sucked in owards the centre of the storm
  • cumulonimbus clouds are formed
  • air cools at high altitudes and sinks forming the eye of the storm
• A tropical cyclone derives its energy by evaporating moisture from a warm ocean when it passes over land or a cold ocean current it loses the source of energy and the speed decreases
• A tropical cyclone can be up to 800km in diameter and up to 20km in height

  • usually last for a week
  • rotate in anti-clockwise direction around the eye in the Northern Hemisphere and a clockwise direction in the Southern Hemisphere
  • the eye is the calmest part of the storm
• Features of a tropical cyclone

  A) eye

  B) centre

  C) eyewall

  D) edge

  E) spiral rainbands

  F) rotating anti-clockwise

  G) eyewall

  H) eye

  
• Typical weather expected as a tropical cyclone passes over:

  • sky becomes cloudy, wind speed increases, rain with sunny intervals
  • air pressure falls, wind speed continues to increase to over 119 km h^-1 and large cumulonimbus clouds form and very heavy rain falls - this is the eyewall or vortex
  • in the eye of the storm, the sky is clear, winds are light and there is little rain, temperatures are warm
  • after the eye has passes cumulonimbus clouds form again, with the return of heavy rain and strong winds
  • wind speed and rainfall decreases, sunny intervals
• Tropical cyclones cause the following hazards:

  • strong winds → structural damage to buildings
  • heavy rainfall → river floodding and landslides
  • storm surges → flooding in low lying coastal areas, intense low pressure can raise sea levels and strong winds can push waves up to 5m high inland
• How deep should the ocean be for tropical cyclones to form?

  60 metres
• What are the angles required for a tropical cyclone to form?

  5 and 20 degrees north or south of the equator
• What is the surface temperature for a tropical cyclone to form?

  27 or 28