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Le but de ce blog est d'eduquer et de discuter a propos des desastres naturels avec un focus sur l'activite et la vulnerabilite sismique, de reporter des informations generales relatives au tremblement de terre d'Haiti du 12 janvier 2010 et aux tremblements de terre du monde. Il met l'accent sur les efforts de reconstruction d'Haiti et la necessite d'utiliser des techniques de conception des structures de batiments et construction parasismique dans la construction des infrastructures physiques. / Les dossiers

Monday, September 5, 2011

Hurricane and its categories

Hurricane. Definition
A hurricane is a tropical cyclone occuring in the North Atlantic Ocean  or in the Northeast Pacific Ocean
What is a tropical cyclone?
A tropical cyclone is a storm system characterizes by a large low pressure center and numerous thunderstorms that produce high winds and heavy rains. Tropical storms strenghten when water vapor evaporated by the ocean is released as the saturated air, resulting in condensation of the condensation of the water vapor contained in the moist air.
What is a low pressure area?
A low pressure area is a region where the atmospheric pressure at the sea level is below that of the surrounding of the surrounding areas
What is atmospheric pressure?
Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth's atmosphere

Formation of Hurricanes

Graphic showing how movement of warm, moist air and cooler, dry air combines in a hurricane

Hurricanes start when strong clusters of thunderstorms drift over warm ocean waters.
In the Atlantic and eastern Pacific they are called hurricanes, but in the western Pacific they are called typhoons.
In the Bay of Bengal and Indian Ocean they are known as cyclones.
The very warm air from the storm combines with the moist ocean surface and begin rising. This creates low pressure at the surface.
As trade winds hit those within the storm, the whirling winds cause the storm to start spinning. Rising warm air leaves low pressure above the surface.
Air rises faster and faster to fill this low pressure, in turn drawing more warm air off the sea and sucking cooler, drier air downwards.
As the storm moves over the ocean it picks up more warm, moist air. Wind speeds start to increase as more air is sucked into the low-pressure centre.
It can take hours or several days for a depression to grow into a fully-formed hurricane.
Hurricanes are made up of an eye of calm winds and low pressure surrounded by a spinning vortex of high winds and heavy rainstorms.
When a hurricane hits land it often has devastating effects.
The Saffir-Simpson scale was devised to measure hurricanes around the Americas and is increasingly used to categorise typhoons and cyclones, too, although some regions still use different scales.
The effects:
Category 1:
  • Minor flooding
  • Little structural damage
  • Storm surge 1.2-1.5m above normal
Category 2: • Roofs damaged
• Some trees damaged
• Storm surge 1.8-2.4m above normal

Category 3:
• Houses damaged
• Severe flooding
• Storm surge 2.7-3.7m above normal
Category 4:
• Some roofs destroyed
• Major structural damage to houses
• Storm surge 4-5.5m above normal
Category 5:
• Serious damage to buildings
• Severe flooding further inland
• Storm surge more than 5.5m above normal

Hurricane formation Source: BBC News

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