Hurricane Katrina and other hurricanes occur when air above the oceans is heated by the water and sun (usually at tropical latitudes near the equator). The hot air rises due to convection, and this causes atmospheric low pressure known as a tropical depression [1]. When the hot, moist air rises, some of the water vapor condenses to liquid water (rain), and this phase change releases more heat. This heat causes air to rise even faster and form lower pressures in a feedback loop, making the hurricane stronger and stronger. Heavy rains come from the condensation of water in warm, moist air. Strong winds result form warm air rushing to the low-pressure eye of the hurricane. [1]

Katrina was a category 5 hurricane (winds up to 175 miles per hour) as it moved across the Gulf of Mexico [2]. When it hit land in Louisiana, it has dropped to a category 3 with winds up to 125 mph. It was not the strongest hurricane to ever make landfall, but it was extremely destructive. It was so destructive primarily because levees around New Orleans, Louisiana failed. Levees are water barriers built to prevent flooding (parts of New Orleans have an elevation that is lower than sea level). When the levees failed, huge areas of the cities flooded. Very heavy winds also contributed to the damage, but flooding was the most destructive aspect of the hurricane.

Katrina was the most destructive hurricane in the U.S. in terms of monetary loss, with an estimated $60 million in damage [2], but it was not the largest in the world. The Bhola tropical cyclone of 1970 is the most destructive hurricane ever. This hurricane struck the Bay of Bengal and caused approximately 500,000 deaths in Bengladesh and India [3].

[1] Pipkin, B., Trent, D.D., Hazlett, R., Bierman, P. (2008). Geology and the environment. Belmont: Thomson Brooks/Cole.

[2] katrina[3]

katrina on Wikipedia

Katrina might be the most expensive hurricane, based on the cost of the stuff that was destroyed.I heard a talk about Katrina by a famous scientist named Roger Pielke. He says the damage from Katrina was so expensive, because so many expensive buildings were built in New Orleans, where the Mississippi River flows into the ocean. Some people think it is stupid for us to build so many expensive buildings by rivers and oceans, where floods happen, and stuff gets destroyed.

In terms of lives lost, the 1959 Mexican hurricane was maybe the worst. 1500 or more people died.

An even more deadly storm was the 1970 Bhola Cyclone. About 400,000people died in that one. Here's the website:

I like the info about on that website about their #5, Hurricane Iniki:"Not surprisingly, the Hawaiians handled the effects of Hurricane Iniki amazingly well. Communities held parties to cook all the perishable food, since the power was knocked out. Grocery stores offered free food to anyone who needed it, while most insisted on paying anyways."

Or, in terms of how expensive the stuff would be today, the Great Miami Hurricane of 1926 was even more expensive than Katrina,according to Wikipedia.com.

The strongest hurricane was maybe the Great Labor Day Storm of 1935.It was a Category 5 hurricane.

Different websites have different lists of the biggest storms, so not everyone agrees.

I don't know about the weather conditions that make big hurricanes.Roger Pielke says that the damage from big hurricanes is because we keep building more expensive stuff along sea costs, not because of Global Climate Change. But we don't know for sure whether Global Climate Change caused Katrina.

Keep asking questions!
Best wishes,

Hurricane Katrina was devastating because it hit New Orleans. That was a disaster waiting to happen, ever since the French governor who built New Orleans over two hundred years ago ignored the advice of his own engineer not to build the city where he did. Hurricane Katrina itself was a big hurricane, but definitely not the biggest - in the Atlantic, I believe that still remains the title of Hurricane Gilbert, although Typhoon Tip in the Pacific was substantially more powerful yet, and is the most powerful hurricane-like storm on record. Ironically, Tip did fairly little damage, because it spent most of its lifetime over empty ocean. Gilbert, by contrast, did a lot of damage, although not nearly as much as Katrina, because Gilbert did not hit New Orleans.