1.3.2 Tropical Storms

• Global Distribution of Tropical Storms

  • Tropical storms are rotating, intense low-pressure systems (below 950 millibars)
  • 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.
  • They develop over the warm tropical oceans between 5° and 30° north and south of the equator.
• Tropical Storms & Atmospheric Circulation Relationship

  • 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 .
•  Intertropical Convergence Zone or ITCZ 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.
• Effect of the Hadley cell at the Equator.

  • Dry air descends creating high pressure at the surface.
  • High pressure zone creates a pressure gradients and air rushes to low - pressure zone generating the winds of the forming tropical storm.
  • Coriolis effect begins 5 degrees either side of the equator.
• Causes of Tropical Storms

  • Sea temperatures must be 27°C+ 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.
  • Rising air draws moist, warm air up from the ocean's surface, generating stronger winds
  • Upward spiraling air cools, condenses, and forms large cumulonimbus clouds, forming the storm's eye wall and causing heavy rainfall.
  • The sinking cold air at the center forms calm, dry eye of the storm.
  • Tropical storms dissipate without access to heat energy and moisture from the ocean.
• Characteristics of Tropical Storms

  • 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.
• -How tropical storms develop. (BBC)

  Tropical storms usually form between 5° and 30° latitude.

  • When the ocean surface waters reaches at least 27°C due to solar heating, the warm air above the water rises quickly, causing an area of very low pressure.
  • As the air rises quickly more warm moist air is drawn upwards from above the ocean creating strong winds.
  • The rising warm air spirals upward and cools. The water vapour it carries condenses and forms cumulonimbus clouds.
  • These cumulonimbus clouds form the eye wall of the storm
• Give one reason why the wind speed of a tropical storm may change as it reaches land. 

  Answer:

  • It loses its source of energy - either from the warm waters or loss of moisture over land [1]
  • As winds contact and begin passing over land/rough terrain, they become slower [1]
• Climate Change & Tropical Storms

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

  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]