On the 23rd of August '05 the National Hurricane Center identified a tropical depression that had emerged as a result of two others combining (21MB AVI video, NOAA site). Two days later, TD12 (as the depression was identified) was upgraded to a Tropical Storm, then (on the 25th), to a hurricane. That evening, at 6:30 PM, it made landfall in SW Florida with speeds of 161 Km/h (100 mph). At this time, it was still a relatively moderate storm with a hope of degenerating before striking the Gulf Coast seaboards of Alabama, Mississippi, and Louisiana.

Hurricanes are a rather peculiar byproduct of three features of the planet earth: the rotation about its axis, shared by all planets in the solar system; the thick layer of gas on top of a large, solid core (which the earth shares with Venus and Saturn's moon, Titan); and the presence of both flat oceans and high mountain ranges. When a zone of low pressure develops in the tropical region, air that would otherwise fill it is deflected to the left, since lower latitudes are moving faster than higher latitudes. This is known as the Coriolis Effect, and it has opposite effects depending on the hemisphere. The air cannot reach the core, or eye, of the storm however fast it flows. This clockwise motion of the upper atmosphere is characteristic of normal weather patterns over huge, continent-sized regions, but inside a tropical system like Katrina, the extreme pressure differential drives winds of merciless fury.

At the same time that air forms clockwise streams over the major land and water masses, air near the equator tends to pushed upward. As it reaches the upper troposphere, it migrates north; at the same time, air over the poles tends to sink to the surface. Naturally, this conveyor belt of air tends to flow in a clockwise pattern (N. Hemisphere), and forms a bulge at about the 45-60th parallels. The jetstream flows through this bulge and plays a major role in the shape and propagation of weather systems. The extreme dynamism of the Gulf Stream (itself the product of several compounded circulatory systems) contributes to the propagation of tropical storms and hurricanes in the Gulf/Caribbean region.

Like most cyclones that strike this region, Katrina was initially a bubble of chronically low pressure created in the blazing sands of the Sahara desert. There, the surface temperature of the sand causes air at low elevations to expand and rise. This one, initially identified as TD10, seemed to diffuse at first into a complex of air pockets, but instead consolidated rapidly. Even after the merger that turned TD10 into TD12, it had to grow significantly before turning into a tropical storm. At this time, an upper-level trough running approximately between the Caribbean and the Gulf seems to have tended to push storms northward rather than into North America. This trough, by late July, had dissolved and been replaced by an upper-level ridge, which contributed three influences on weather systems:

1. Reduced shear and light winds in the upper-atmosphere, allowing for convection to hold together
2. Light but steady steering currents.
3. Upper-level high pressure to support outflow

The intensity of a tropical cyclone is determined by the pressure; Katrina's pressure at landfall was 918 mBars, making it the third lowest pressure of any cyclone to strike the USA (not counting past events when the measuring equipment failed to survive the hurricane). Windspeeds exceeded 280 Km/h at sea piled up a storm surge 9 meters high, the highest in US record. This storm surge, in turn, is generally believed to have ensured that the levees enclosing both the Mississippi River and Lake Pontchartrain (actually, an inlet of the Gulf of Mexico) would fail.

As you can see from the figure to the left, much of New Orleans is actually below both sea level and the (much higher) Mississippi River. The city is largely an outgrowth of the old French Quarter on the banks of the river. Then, between the River and the "Lake," there is a canal that runs the 6 Km between the two, and it was here that the surge broke through. Unfortunately for the Big Easy, this area is essentially a gigantic basin that was entirely inundated with contaminated water. Once the water flowed in, there was no chance of flowing out.

According the American Red Cross, a category 5 storm striking the area with a 20-ft (6 meter) surge was expected to cause between 25,000 and 100,000 deaths. At the time the article was written, however, the likelihood of such a direct hit on the city was regarded as much too small. This may have reflected a crucial error of reasoning, that a storm surge would not survive a dissolving storm, or that surges would be too narrow to score a direct hit.

(Part 2)

SOURCES: Wikipedia entry for Katrina; see also the entry for Tropical Cyclone; CaribWX offers an outstanding page on weather conditions, trends, in the Caribbean. See also this chart in the New Orleans Times-Picayune illustrating hurricane formation.