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Katrina and Hurricanes

What exactly is a hurricane? How do they orginate? What havoc and destruction can they cause?

For the latest information about Katrina go to The National Hurricane Center (USA)

The following information is supplied by the superb Wikipedia (Open Source).

What is a hurricane?

In meteorology, a tropical cyclone (or tropical disturbance, tropical depression, tropical storm, typhoon, or hurricane, depending o­n strength and location) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.


The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h)


The formation of tropical cyclones is still the topic of extensive research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:

1. Sea surface temperatures above 26.5 degrees Celsius to at least a depth of 50 meters. Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.

2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.

3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through the world's tropical oceans.

4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan, the strongest storm to be so close to the equator, started its formation at 9.7 degrees north.)

5. Lack of vertical wind shear (change in wind velocity over height).

High levels of wind shear can break apart the vertical structure of a tropical cyclone.Tropical cyclones can occasionally form despite not meeting these conditions. A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to the creation of Typhoon Vamei at o­nly 1.5 degrees north of the equator in 2001. It is estimated that the factors leading to the formation of this typhoon occur o­nly o­nce every 400 years.

Structure of a Hurricane

A strong tropical cyclone consists of the following components.

* Surface low:

All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur o­n Earth's surface at sea level.

* Warm core:

Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.

* Central Dense Overcast (CDO):

The Central Dense Overcast is a dense shield of rain bands and thunderstorm activity surrounding the central low. Tropical cyclones with symmetrical CDO tend to be strong and well developed.

* Eye:

A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.

* Eyewall:

The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eye-wall replacement cycles, in which outer eye walls form to replace inner o­nes. The mechanisms that make this occur are still not fully understood.

* Outflow:

The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

Hurricane Katrina
(NEWS supplied o­n 28th August 2005)

Hurricane Katrina is a major hurricane expected to devastate southeastern Louisiana (especially the Greater New Orleans area), as well as southern Mississippi, southern Alabama and possibly well inland. At its second landfall, it is expected to be o­ne of the most powerful cyclones to hit the United States in recorded history. It is the eleventh named storm, fourth hurricane, and third major hurricane of the 2005 Atlantic hurricane season.Katrina formed o­n August 24, 2005 and made landfall near North Miami, Florida, United States, as a Category 1 hurricane o­n the Saffir-Simpson Hurricane Scale. An eye-restructuring event weakened it to a tropical storm; it then regained strength much faster than originally expected in the warm waters of the Gulf of Mexico, becoming a Category 5 hurricane

 The U.S. government's National Weather Service, as of 5:42 PM EDT, August 28, 2005, indicates that the system is rapidly expanding and will make landfall again in the area of New Orleans, Louisiana as a Category 5 hurricane at about 7 AM CDT (Louisiana local time) August 29, 2005.

The eye of the storm itself is 27 miles wide. As of 10 PM CDT, the storm is moving NNW at 10 mph and current models expect it to turn fully northward, eventually affecting not o­nly New Orleans but a large area of the Gulf Coast from the Florida panhandle to Louisiana. Hurricane force winds are predicted up to 150 nautical miles (278 km) inland, while the flooding rains will persist as far northward as the Great Lakes.A mandatory evacuation has been issued for the first time ever in New Orleans, as 70 percent of the city is below sea level and the storm surge is expected to be as high as 28 ft (8 m) above normal tide levels. An offshore instrument recorded waves of 36 ft (11 m) before becoming unresponsive. Disaster relief plans are in full operation in the affected communities.

Update : 1st September 2005

"Hurricane Katrina was a deadly and devastating tropical cyclone that was the cause of catastrophic damage in the southeastern United States and will likely become the worst natural disaster ever to hit the United States with major global economic consequences caused by the severe disruption of oil supplies, exports, and imports. Disaster relief plans are in operation in the affected communities. Some experts predict o­ne million people could become homeless as a result of the storm. Currently five million people are without power in the Gulf Coast, and it may be two months before all power is restored. The aftermath of the storm has compounded problems. In particular, the breeching of some the levees protecting New Orleans caused water to flow unabated into the City. There was also widespread looting, many people stranded due to flooding, and an ever present threat of contagious disease in affected areas."

Source :

Katrina may be the deadliest hurricane in the United States of America since the Galveston Hurricane of 1900, which killed around 8,000 (possibly up to 12,000) people. The death toll of Katrina so far is "in the thousands", stated by the New Orleans mayor Ray Nagin o­n August 31, but it will take weeks before accurate numbers are known. Katrina is also expected to be the costliest natural disaster in United States history.

[end of article]