Windswept_ The Story of Wind and Weather - Marq de Villiers [86]
The evolution from depression to full-blown hurricane usually takes a good four days. It took Ivan not quite three.
It is curious that with all this data available, and with so much attention being paid to hurricane tracks and intensities, that the actual birth of a hurricane still remains invisible. As I've said, we know where, we know when, and we know the necessary preconditions—but we still don't know why. What is the tipping point? What makes one system coalesce into a storm, another to dissipate? After all, a hundred Saharan thunderstorm systems drift into the Atlantic each year, but only a fraction even become tropical disturbances. Of those that do, only a fraction become storms, and not all of those become hurricanes. Globally, tropical cyclones are still uncommon. In any given year, the number will vary from thirty to one hundred, with somewhere around ten or twelve in the western Atlantic; of those, perhaps three or four will be defined as major. This may be changing: From 1951 to 2000, there was an average of ten named storms in the Atlantic each year; in the past few years the number has increased to about fifteen, and may still be going up—there are more tropical depressions to start with, and the ocean is two to three degrees warmer than in earlier decades. But the moment a storm becomes a hurricane is still hard to see.
No matter how closely scientists monitor the data, the exact instant eludes them. Only in what meteorologists call hindcasting, the after-the-fact scrutiny of the data, can they approximate it. The first hint is when they see wispy clouds lazily circling, drifting inward toward a point, an early sign of a system's struggle to overcome entropy, a sign of what they call, for obvious reasons, "organization"—a "well-organized storm" is a storm with serious potential. But even then, the why is mysterious. Storms are caused by dozens, perhaps hundreds, of forces that intersect and interact, sometimes directly, sometimes in ways so subtle they are hard to detect by even the most cunning of models. They have, in the scientific jargon, "a sensitive dependence on initial conditions." They are nonlinear, which mostly seems to mean they don't behave at all predictably.
In theory, it should be easy to track the beginning of a hurricane: Simply wind the film backward. We already know how its effects on the American coast (Step C) result from its westerly track across the Atlantic (Step B), which was caused by a tropical storm off the Sahara (Step A). Why not then follow it backward from C to B to A and then beyond to see how the whole thing began? With the vast array of data provided by the globe-encircling network of satellites, we seem to have plenty of information—those satellites can track phenomena to a resolution of a few yards. In practice, what you see when the film is unspooled is this: hurricane, smaller hurricane, tropical storm, tropical depression, thunderstorm, moist windy spot, then a set of weather conditions that in no way look any different from those that cause, well, nothing . . . What is it that energizes some of these warm moist spots into hurricanes? No one knows. All they can say for sure is that it must be very small, because it is presently beyond our ability to track.
Ernest Zebrowski Jr. in Perils of a Restless Planet explains how computer simulations of hurricanes, while crude, go some way to illustrating the phenomenon. A storm is created on screen using hypothetical initial data. Columns of numbers then yield wind speed, storm speed, barometric pressure, temperature, and other measurable variables. By itself, such a simulation yields no information of any value. But if you then conduct a second computer run, then a third and a fourth and a hundredth, giving each one