It's a year before Katrina, and
Hurricane Pam is in full swing - striking New Orleans with 120 mph winds. Floodwaters are
surging into the city. But fortunately, this
hurricane is make-believe - merely the invention of scientist Ivor van Heerden of Louisiana State University in his supercomputer, using data from past storms.
Van Heerden's computer simulation is the focus of a disaster exercise held in Louisiana's capital, Baton Rouge. His simulation shows that New Orleans would be devastated: 61,000 people dead, over 175,000 injured and half a million homeless. But would the lessons from the Pam simulation be heeded when the real threat came along?
Tuesday 23 August 2005, 6 days before Katrina hits (T-6 days)
One year on and the hurricane season is in full swing. Thunderstorms are brewing off the west coast of Africa. Hurricanes are formed from centres of low atmospheric pressure and begin life as ' tropical depressions'. Above the warm waters of the eastern Atlantic, water vapour rises from the ocean, then cools - to form clouds and release energy. When this rush of air hits the stratosphere it flattens out, and, influenced by the Earth's rotation, the storm starts turning counterclockwise. As soon as the winds reach 39 mph, the depression is considered a tropical storm.
Six days before striking New Orleans - and still in the mid-Atlantic - Katrina is named. She is no more than a large tropical depression.
Wednesday 24 August, T-5 days
A day later, there's bad news: the Hurricane Center has upgraded the new depression to a 'tropical storm'. Approximately half of all tropical storms develop into hurricanes, so the team at the National Hurricane Center near Miami must predict quickly where Katrina will strike.
They turn first to data on atmospheric dynamics coming from remote-sensing satellites. But the team needs more data than even satellites can provide. So they send for an elite Air Force team whose mission is to fly right into the approaching storm. A new piece of technology has been added to the Hurricane Hunters' arsenal. A sonde is jettisoned out of the craft; then its GPS unit feeds back its position, relaying wind speeds at different points in the storm.
The data is fed to the Hurricane Center's supercomputers, which generate a prediction cone - a 5-day forecast of the hurricane's likely track. Initial predictions show that Katrina will cross into Florida and then head out again to the Gulf of Mexico. Katrina is still only a tropical storm. The key question is, will she become a hurricane?
At the bottom of the scale is a category 1 with wind speeds of up to 95 mph. A category 5 hurricane is the most feared with winds of more than 155mph: whole roofs are sheared off and houses destroyed. Hurricanes deliver a deadly double blow - not just high winds but a massive bulge of water called the storm surge. The high winds push down on the ocean's surface, causing the water to rise like an unnaturally high tide. This wall of water is so dangerous that 90% of deaths in hurricanes come from drowning.
Thursday 25 August, T-4 days
Today, Katrina finally grows into a category 1 hurricane: she forms an 'eye' - and she's heading straight for Southern Florida. Just three hours later, Katrina hits land. But with no warm water to fuel her fury, she dies down. There's more warm water ahead in the Gulf of Mexico, though, and beyond lies New Orleans…
The city was built between the Mississippi River to the south and Lake Pontchartrain, connected to the ocean, to the north. Over the aeons, the Mississippi's annual floods have deposited silt to create vast tracts of boggy marshlands. New Orleans itself was constructed on the only natural high ground, which would become the French Quarter.