Hurricane Information

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A tropical cyclone is a nonfrontal, low pressure weather system with organized convection characterized by inwardly spiraling winds. Tropical cyclones have central cores which are warmer than the surrounding atmosphere, with closed surface wind circulation about a well-defined center where winds are most intense.

Once formed, tropical cyclones are maintained by extracting heat energy from the ocean at high temperature and exporting this heat energy at low temperature into the upper troposphere. Tropical cyclones originate over tropical or subtropical waters and are sustained by warm and humid tropical conditions. Tropical cyclones spin counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Hurricane Katrina as a Category 5 hurricane in the Gulf of Mexico, one of the strongest hurricanes on record (Imagery: NASA Earth Observatory)

Tropical cyclones become hurricanes when their sustained winds reach 74 miles per hour (65 knots) or greater and they occur in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and eastern North Pacific Ocean. Tropical cyclones are called "typhoons" in the western Pacific and "cyclones" in the Indian Ocean.

Based on recent studies of historical hurricanes, many climate scientists from NOAA and elsewhere have determined that hurricane activity is likely cyclical. In the US, this means that due to numerous climatological and other factors, we can expect up to four decades of increased hurricane activity that began in 1995. While there is still debate on climate change and effects such as sea level rise and increased sea surface temperatures true impact on the frequency or intensity of hurricanes, other scientists have found evidence that gradually, more intense hurricanes may be headed our way.

International hurricane expert Dr. William Gray of Colorado State University discusses the cyclical nature of hurricanes at an annual hurricane conference (HPHC 2003). However, he does not seem to be convinced that there is much of a correlation between global warming and more frequent or intense hurricanes.

Color-enhanced Imagery demonstrating the fragile nature of southeast Louisiana's Mississippi Delta and wetlands (HPHC 2003, from the LAGIC Louisiana GIS CD)

Louisiana Hurricanes

Louisiana is one of the top states along with Texas, North Carolina and Florida to experience major hurricane strikes (Categories 3-5 on the Saffir-Simpson scale) in the US. For other state rankings through 1999, see also the Extreme Weather Sourcebook 2001.

Louisiana averages at least one major hurricane a decade - and we were hit with two in 2005. More about Louisiana's Hurricane History and A Brief Climatology of Tropical Cyclones in Louisiana can be found through the National Weather Service (Lake Charles, Louisiana).

For New Orleans, even prior to Hurricane Katrina in 2005, numerous historical hurricanes grazed the city via a nearby parish causing significant damage (1915: unnamed Category 4 and 1969: Camille, Category 5) or struck close enough to overtop the levees and flood the city (1947: unnamed Category 4 and 1965: Betsy, Category 3). Additional storms such as minor hurricanes, tropical storms and even major rainfall events have confronted the city as well, causing flooding and damage, such as the floods of 1995.

New Orleans flooding from
Tropical Storm Isidore (2002)

Did you know... hurricanes remember! Even though there is an associated storm surge for every category of hurricane (see the Saffir-Simpson Scale, below), due to forces of inertia, a once-Category 5 storm such as Hurricane Katrina that makes landfall as a Category 3 storm in terms of windspeed can put ashore storm surge at a much higher level.

Watch an animation demonstrating Saffir-Simpson categories (Associated Press)

Interstate 10 underpass, Isidore (2002)

Saffir-Simpson Scale
for Hurricanes

Category	Wind speeds (mph)	Approximate wind speeds in knots	Storm Surge	Damage level	Anticipated effects
1	74-95	64-82	4-5 ft	minimal	Damage to unanchored mobile homes, shrubbery, & trees; some coastal road flooding; minor pier damage
2	96-110	83-95	6-8 ft	moderate	Some roofing material, door, and window damage to buildings; considerable damage to vegetation, mobile homes, and piers; Coastal and low-lying escape routes flood 2-4 hours before arrival of center; Small craft in unprotected anchorages break moorings.
3	111-130	96-113	9-12 ft	extensive	Some structural damage to small residences and utility buildings; minor amount of curtainwall failures; Mobile homes destroyed; Coastal flooding destroys smaller structures; larger structures damaged by floating debris; Terrain <5 feet ASL may be flooded inland 8 miles or more.
4	131-155	114-135	13-18 ft	extreme	More extensive curtainwall failures with some complete roof structure failure on small residences; Major beach erosion; Major damage to lower floors of structures near the shore; Terrain <10 feet ASL may be flooded requiring massive evacuation of residential areas inland as far as 6 miles.
5	>155	>135	> 18 ft	catastrophic	Complete roof failure on many residences and industrial buildings; Some complete building failures with small utility buildings blown over or away; Major damage to lower floors of all structures located <15 feet ASL and within 500 yards of the shoreline; Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.

Additional references and information on hurricanes can be found at the following links:

NOAA AOML Hurricane Research Division --- Frequently Asked Hurricane Questions

NASA Hurricane Resource Page --- NASA Earth Observatory - Hurricanes: The Greatest Storms on Earth

NEW UPDATE! The Deadliest, Costliest, and Most Intense United States Tropical Cyclones From 1851 to 2006. Blake, Rappaport and Landsea, National Weather Service - National Hurricane Center, Miami, Florida, April 2007. See also the NHC/TPC Archive of Past Hurricane Seasons.

You can map historical hurricanes by conducting a query of Historical Hurricane Tracks using online database tools provided by the NOAA Coastal Services Center.

The Extreme Weather Sourcebook 2001 is Sponsored by the National Center for Atmospheric Research (NCAR), National Oceanic and Atmospheric Administration (NOAA), U.S. Weather Research Program (USWRP), National Science Foundation (NSF), and the American Meteorological Society (AMS)

LSU Hurricane Public Health Center ~ Suite 3221 Patrick F. Taylor Hall ~ Louisiana State University Baton Rouge, Louisiana 70803 ~ tel: (225) 578-0268 ~ fax: (225) 578-5263 ~ publichealth.hurricane.lsu.edu

webpage update: August 27, 2007