3.4.1 Storm Hazards Flashcards
Tropical storms
Intense low pressure weather systems that develop in the tropics. They have strong winds and torrential rain.
They develop over warm water. As warm, moist air rises and condenses, it releases energy that increases wind speed.
What are the conditions for tropical storms to form?
- A disturbance near the sea-surface that triggers the storm (e.g. an area of low pressure).
- Sea water that’s warm (above 27°C to at least 50m below the surface), so lots of water will evaporate.
- Convergence of air in the lower atmosphere – either within the ITCZ or along the boundary between warm and cold air masses. This forces warm air to rise.
- A location at least 5 from the Equator. They don’t form 0-5° either side of the Equator because the Coriolis effect isn’t strong enough to make them spin.
Distribution of tropical storms
Tropical storms form in the tropics because the water there is warm enough.
They occur in the Caribbean Sea (where they’re called hurricanes) in the Bay of Bengal (where they’re called cyclones), in the China Sea and Pacific Ocean (where they’re called typhoons) and in Northern Australia.
How does the power of tropical storms change?
Tropical storms lose strength when they move over land because their supply of warm, moist air is cut off.
Direction of tropical storms
They initially move westwards due to the easterly winds in the tropics, e.g. trade winds move cyclones west across the Atlantic Ocean.
They move away from the Equator because of the Coriolis effect.
The shape of tropical storms
Tropical storms are circular in shape, hundreds of kilometres wide and usually last 7-14 days. They spin anticlockwise in the northern hemisphere, and clockwise in the southern hemisphere.
At the centre of the storm is an area of very low pressure called the eye.
Rising air spirals around the eye in the eyewall, causing strong winds.
Near the top of the storm, there is an outflow of moisture-laden air, so cloud cover extends for a long distance either side of the eye.
Storm Magnitude
Storms are classified using the Saffir-Simpson Scale, which is based on wind speed.
- Category 5 is the strongest (with winds over 250 km/h) and Category 1 is the weakest (with winds of 120-150 km/h).
The Saffir-Simpson Scale also estimates how much damage a storm of a given magnitude will do, from limited damage at Category 1 to catastrophic damage at Category 5.
Criticism of the Saffir-Simpson scale
It only predicts damage from wind. However, nearly 90% of hurricane related deaths in the USA were as a result of water, not wind. E.g. Storm surges, flooding, heavy rain.
Frequency of tropical storms
Tropical storms are quite frequent - around one hundred occur each year. Some of these never reach land, so they never develop into a major hazard. Storms are more frequent in the northern hemisphere between June and November, and in the southern hemisphere between November and April.
How can tropical storms be predicted?
Certain cloud formations in tropical areas can be identified from satellite imagery and used to tell when a tropical storm is forming. The storm can then be tracked using satellite imagery and models, helping scientists to work out when and where it is likely to hit land. The path of a tropical storm can therefore be predicted fairly accurately.
