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• Many hurricanes in the Atlantic Ocean begin as a large complex of thunderstorms over tropical Africa called a mesoscale convective system (MCS).
• Once a storm moves westward into the Atlantic, it may begin to rotate and become more organized. At this stage, the storm is called a tropical depression. When the storm’s wind speed reaches 39 mph, it becomes a tropical storm, and is given a name (like “Katrina”). A hurricane has wind speeds greater than 74 mph. Hurricanes are classified into categories, from Category 1 (74-95 mph) to Category 5 (>155 mph).
• Hurricanes derive their “fuel” from the energy of warm ocean water. Hurricanes form and strengthen when surface ocean temperatures are warmer than 80 degrees Fahrenheit (°F).
• Ocean temperatures in the Gulf of Mexico just prior to Hurricane Katrina’s landfall were approximately 87°F, nearly 1° above average. Hurricane Katrina made landfall along the Louisiana/Mississippi border, to the east of New Orleans, as a Category 3 hurricane with wind speeds of 125 mph.

What are levees? Why are they necessary in New Orleans?

• A levee is an embankment built along a body of water, such as a river or lake, to prevent flooding.
• Levees were first constructed in New Orleans to confine the Mississippi River to its present channel. If the Mississippi River were not controlled by humans, it would shift its course 100 miles to the west.
• Levees were also built along Lake Pontchartrain to prevent flooding in New Orleans. These levees were designed to withstand a direct hit by a Category 3 hurricane.
• During Hurricane Katrina, multiple levees failed, flooding 80% of New Orleans.

Why is New Orleans below sea level?

• New Orleans is built on the Mississippi River delta. A delta forms at the mouth of a river where it flows into a larger body of water. Sediment carried by the river is deposited and builds upward toward the surface of the water, creating new land.
• Before humans began to control the Mississippi, the river would periodically shift its course. After a delta formed in one location, the river would seek a shorter, steeper path to the ocean, and the entire river system would shift to a new position.
• Today, sediment from the Mississippi is channeled directly into the Gulf of Mexico, bypassing New Orleans. Without any new sediment input, the existing sediments compress, leading to subsidence and causing New Orleans to sink.
• As New Orleans sinks, levees must be built higher and higher to maintain protection against flooding. At present, the river surface is 20-30 feet above the land surface in many locations.

How do wetlands help to reduce hurricane damage?

• A wetland is an area that is regularly flooded with water, containing plants adapted for life in a saturated environment. Wetlands trap sediment, recycle nutrients, purify water, buffer coastlines, and provide unique habitats to many birds, wildlife, amphibians, and fish.
• When a hurricane’s storm surge flows inland, wetlands slow the water and reduce the strength of the waves, and thus reduce hurricane impacts.
• Wetlands serve as a buffer for flood events. They soak up water like a sponge, and then release it gradually, absorbing the impact of the flood.
• Louisiana loses 40 square miles of wetlands every year through subsidence and drainage for agriculture. If this destruction continues, all of Louisiana’s wetlands will disappear within 200 years.
  

HURRICANE LINKS

Up-to-date Atlantic and Eastern Pacific hurricane information:

National Hurricane Center
Atlantic Tropical Weather Center

Educational pages:

Hurricane Research Division's Hurricane FAQ page
Hurricanes - Online Meteorology Guide (University of Illinois)
NASA's Hurricane Resource Page
Hurricane Hunters

Seasonal hurricane forecasts:

Colorado State University's Tropical Meteorology Project
NOAA's Climate Prediction Center
University College London

Hurricane data and information:

Global Hurricane Data, 1851-2005
Worldwide Tropical Cyclone Names

Hurricane Katrina:

Hurricane Katrina

Last updated:
A. Lachhab, Earth and Environmental Sciences
Susquehanna University, Selinsgrove, PA 17870
Telephone: 570-372-4215