W4400: Survival Guide – Hurricanes Prepared by Bali White and Emily Firth December 2006 I background information



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W4400: Survival Guide – Hurricanes

Prepared by Bali White and Emily Firth

December 2006
I Background information

Tropical cyclones, also called hurricanes and typhoons, are intense low pressure disturbances that form and migrate over the tropical ocean regions and are associated with intense winds and a very strong convective activity, which brings thunderstorms and large amounts of rainfall. They have the potential to cause major damage and loss of life when they make landfall.



These massive disturbances that grow in a time frame of a week or so, need specific and favorable conditions to occur, including high sea surface temperatures (at least 26°C) and weak vertical wind shears. Once they do, they spread over a radius of a few hundred kilometers.

Hurricanes are surrounded by rings of towering thunder clouds spiraling up to a small circle at the center of the storm, with a radius of 30-40 km. Here the winds can reach a speed of 100 km/hour and more and the most intense rainfall occurs. Inside this ring lies the eye of the storm, where the air is still and the convection is suppressed by slow downward motion (subsidence). As illustrated in the diagram the clouds form a banded structure and the eye at the center is approximately 30-40 km wide.



Recent hurricane events include:

  • 10 landfalling tropical cyclones in Japan in 2004

  • Cyclone Gafilo in Madagascar, 2004

  • Extremely active – record setting – 2004/2005 Atlantic tropical cyclone seasons including Hurricane Katrina, 2005

  • 5 tropical cyclones affecting the Cook Islands in a 5-week period in 2005

  • Cyclone Larry in Australia, 2006

  • Typhoon Saomai in China, 2006

  • Typhoon Durian in Philippines, 2006

  • Cyclone Catarina in Brazil, March 2004 – the first and only (to date) recorded Category 1 hurricane in the South Atlantic.

II Regional formation and migration of hurricanes

Hurricanes are active in the "trade wind" belts - the regions just north or south of the equator where the winds blow quite steadily from east to west (easterlies).

Here tropical disturbances generally form, initiated by weak pressure perturbations that exist all the time in the tropics. They move west with the trade winds in a steady, relatively slow motion (10-20 km/hour). During this phase they intensify mainly through the release of latent heat in the surrounding clouds and a small percentage reach full hurricane intensity. The hurricanes tracks curve poleward and they speed up north of ~30°N.



Hurricane development is predominantly in the following ocean basins:


Category

Wind speed

mph

(km/h)

Storm surge

ft

(m)

5

≥156
(≥250)

>18 (>5.5)

4

131–155
(210–249)

13–18
(4.0–5.5)

3

111–130
(178–209)

9–12
(2.7–3.7)

2

96–110
(154–177)

6–8
(1.8–2.4)

1

74–95
(119–153)

4–5
(1.2–1.5)

Tropical
storm


39–73
(63–117)

0–3
(0–0.9)

Tropical
depression


0–38
(0–62)

0
(0)
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