#3 CYCLONE FORMATION

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How do cyclones form?

The above figure shows how cyclones form. The green arrows show where warm air is rising. The red arrows indicate where cool air is sinking.

•           Tropical cyclones form only over warm ocean waters near the equator.

•           To form a cyclone, warm, moist air over the ocean rises upward from near the surface. As this air moves up and away from the ocean surface, it leaves is less air near the surface. So basically as the warm air rises, it causes an area of lower air pressure below.

•           Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then this new “cool” air becomes warm and moist and rises, too. And the cycle continues…

•           As the warmed, moist air rises and cools the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean’s heat and water evaporating from the ocean surface.

•           As the storm system rotates 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.

When the winds in the rotating storm reach 39 mph (63 kmph), the storm is called a “tropical storm”. And when the wind speeds reach 74 mph (119 kmph), the storm is officially a “tropical cyclone” or hurricane. Tropical cyclones usually weaken when they hit land, because they are no longer being “fed” by the energy from the warm ocean waters. However, they often move far inland, dumping many centimeters of rain and causing lots of wind damage before they die out completely.

Cyclone Categories

The Saffir-Simpson Hurricane Scale

•           Cyclones are divided into categories depending on the strength of the winds produced. There are many different classification scales but one you may be familiar with is the Saffir-Simpson Hurricane Scale. This scale is used to desribe storms in Hollywood movies, e.g. “Twister” and “The Day After Tomorrow”.

•           The classifications (1-5) are intended primarily for use in measuring the potential damage and flooding (storm surge) a cyclone will cause upon landfall.


Tropical Storms


Same Storm - Different Names…depending on where you are in the world tropical storms have different names

·         In the Atlantic and Pacific Oceans (eastern) they are called hurricanes.

•           average of 10 named storms per season

•           ~6 becoming hurricanes

·         Over the western Pacific Ocean they are called typhoons.

•           average of 16 named storms per season

•           ~9 becoming typhoons

•           The western Pacific ocean is a major source of tropical storms as the water is warmest and there are large distances between landmasses.

•           Over the Indian Ocean the storms are called cyclones. “Cyclone” is the best overall phrase to use for high intensity rotating storms.

•            Storms that form north of the equator spin counterclockwise. Storms south of the equator spin clockwise. This difference is because of Earth's rotation on its axis.

Tropical Cyclones


1.     Background:

–        Can be deadly!

•         For example, in 1991 a large cyclone in Bangladesh killed
>138,000 people in just two days!

2.     Tropical cyclones are a MAJOR natural hazard that can cause large numbers of fatalities and extensive damage.

3.     For example the 1991 Bangladesh cyclone, “Cyclone Gorky”, was among the deadliest tropical cyclones on record. On the night of the 29th of August the cyclone struck the southeastern coast of Bangladesh with winds of around 250 km/h (155 mph). The storm forced a 6 metre (20 foot) storm surge wave inland over a wide area, killing at least 138,000 people and leaving as many as 10 million homeless.

4.     Most widespread destructive weather hazard

–        For example: Hurricane Floyd (1999)

•           only a moderate level hurricane

•           caused US$5.6 billion in damage in the Bahamas and North Carolina (USA) and 57 fatalities

5.     Cyclones can devastate large areas, for example Hurricane Katrina (August 2005, USA) had gale force winds extending 120 miles (190 km) from the storm centre (“eye of the storm”) - remember cyclones are radial (circular) so that makes it approximately 240 miles (380 km) from one side to the other!

6.     In 1999 Hurricane Floyd caused 2.6-million coastal residents across the entire US state of Florida to evacuate their homes.

7.     The hurricane produced torrential rainfall that caused widespread flooding over a period of several weeks; nearly every river basin in the eastern part of the state exceeded 500-year flood levels.

8.     Hurricane Floyd was responsible for 57 fatalities and US$5.6 billion of damage.


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