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Unit 2 Section C - Causes and effects of tropical storms and responses to them

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Saved by K J Hutchinson
on April 17, 2010 at 9:58:04 pm
 

Lesson 1 - What is a tropical storm? Where do they form?

 

Learning objectives:

- to know what a tropical storm is

- to be able to explain the physical processes leading to the formation of a tropical storm

- to be able to describe the locations in which we find tropical storms

- to know how the Saffir-Simpson scale is used to classify tropical storms

 

Note: Depending on where you are in the world, you may call the tropical sotrm by a different name. In the USA, they are referred to as hurricanes; in Bangladesh, they are called tropical cyclones; in Japan, people speak of typhoons; in Australia, they have willw-willies. All of these terms refer to the same weather event. We will call them tropical storms unless we are referring to specific storms, in which case they will be given local names (eg. Hurricane Katrina; Cyclone Nargis).

 

Tropical storms are areas of intense low pressure. On a satellite image, a tropical storm will show as a huge, swirling mass of cloud. The eye (calm area) is at the centre. You can see this in the image below. It shows Tropical Storm Edzani over the Southern Indian Ocean and was taken on 7th January 2010.

 

 

There are several stages in the formation of a tropical storm. These storms occur over water in areas where the sea surface temperature is at least 27 degrees. Essentially, this means that they form between the Tropic of Cancer and the Tropic of Capricorn (hence the name 'tropical' storm). They rarely occur within 5 degrees either side of the equator because the Coriolis force is too weak here.

 

Tropical cyclones use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises rapidly upward from near the surface. This means that there is less air left at the surface (i.e. low pressure). Air from surrounding areas with higher air pressure pushes in to the low pressure area to try to equalise the pressure. Then that “new” air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds (because there is a huge amount of condensation). The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the surface.

 

Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. As the storm system spins faster and faster, an eye forms in the centre. It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.

 

The video clip below explains this process in images.

 

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When the winds in the rotating storm reach 39 mph, the storm is called a tropical storm. When the wind speeds reach 74 mph, the storm is officially a tropical cyclone, or hurricane. The strength of a storm is measured using the Saffir-Simpson scale, as shown in the table below.

 

 

Tropical cyclones usually weaken when they hit land, because they are no longer being fed by the energy from the warm ocean waters. In the northern hemisphere they track (i.e. move) westwards due to the Coriolis effect. The video clip below shows Hurricane Dean, from birth to death.

 

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Useful weblinks:

BBC animation - the formation of a hurricane

NASA animated guide to the development and structure of tropical storms

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