Something is brewing up storm after storm in the Atlantic this autumn, and hurricane experts find themselves at a loss to explain what it is.
Like swallows that fly low before rain, high atmospheric winds sometimes behave in ways that signal stormy hurricane weather ahead.
And while hurricane forecasters use indicators "a little more sophisticated" than low-flying swallows, said Neil Frank, former head of Miami's National Hurricane Center, exact cause-and-effect relationships are notoriously hard to pin down.
Hurricane Roxanne, which sent tourists fleeing from southern Mexico last week, is the 17th named tropical storm this season--the most active year for Atlantic storms since 1933.
Colorado State University meteorologist William Gray predicted such a bonanza season last November. This August, he fine-tuned his estimate, warning that residents of the country's southern and eastern coasts would probably face nine hurricanes this year. Roxanne marks the 10th.
The flurry of storms this year has made this most recent one the first to get as far as the letter "R" in the official storm alphabet. Names are given to storms whenever their winds reach 39 m.p.h. It takes 75-m.p.h. winds to achieve hurricane status.
The hurricanes are back, said meteorologist Gray, not because something different happened this year, but largely because of what didn't happen.
That is, the extreme differences in wind speed, called wind sheer, that chop off the heads of potential hurricanes were noticeably absent over the Atlantic this year. So storms seeded in the rains of West Africa were carried unhindered over the ocean and allowed to grow to disastrous proportions.
A hurricane, said climate scientist Nick Graham of the Scripps Institute of Oceanography, is "just like a family. It needs an organized environment to live." The wind sheer slices the growing hurricane in half before it has the time to mature.
The phenomenon of "global warming" does not appear to be linked to the abundance of hurricanes.
But several other conditions are statistically correlated with hurricanes.
The absence of El Nino warming off the coast of Peru seems to be associated with the formation of hurricanes. So does wet weather over West Africa.
But all, Gray said, relate in some way to the winds. "That's the main ingredient."
Other meteorologists are not so sure there is such a cause-and-effect relationship.
"The atmosphere is not simple," said Frank, who is now a weatherman with a CBS affiliate in Houston. "The observations [of correlations] appear to be valid, but I'm not sure that anyone, including Bill Gray, has the physical explanation."
The fact is, stormy weather correlates with a lot of things--including the O.J. Simpson verdicts. "Not to pooh-pooh the science, but there are statistical correlations with any number of factors," said Frank Lepore, a spokesman at the Miami Center. "It's hind-casting more than anything else."
Lepore points out that there are 8 million square miles of the Atlantic Ocean, and air pressure readings are taken from only one spot, and only twice a day. "That leaves a large data gap."
Hurricanes obey the basic laws of physics, explained Frank, who compares a brewing hurricane to a pan of hot oatmeal on the stove. The sun is the burner and heats up the atmosphere above it. When you heat oatmeal, you get bubbles. In the atmosphere, Frank said, those bubbles are clouds.
"And you know what happens when you leave the oatmeal on the stove and forget about it: It boils over." Boiling over in the atmosphere produces a thunderstorm. And if the oatmeal boils over with a lid on it, it can explode. "That's a hurricane," Frank said.
Hurricanes get their swirling winds from the rising hot air from the Equator, which sinks as it moves toward the poles, and the rotation of the Earth.
Toward the middle, the winds spin faster for the same reason that water spins faster as it hits the center of the drain. "Hurricanes obey the conservation of momentum," Frank said. "If you pull your arms in, you spin faster."
But weather prediction is complicated by what scientists call its "non-linear" properties. In short, there is no simple relation between cause and effect.
"It's not like shooting a billiard ball and predicting where it's going to be a few seconds later," said UCLA meteorologist Warren Brier.
Gray began to wonder about the patterns that give rise to hurricanes in the early 1980s. "We'd always ask ourselves, 'Gee, is this going to be an active year or not?' And we never were able to tell."
The problem, he said, is that prognosticators were thinking locally, not globally.
Once they looked at global patterns, they found the connection between the absence of El Nino in the Pacific and hurricanes in the Atlantic. And that led them to suspect the winds.
"Some people study the physics, and then go look at the data," he said. "But sometimes you do a lot better if you first find the links, then go and find the physical explanation."
It helps, he said, "to know a little physics, so you know where to look."
As of late Wednesday, Roxanne had been demoted to a tropical storm. But the hurricane season isn't over, experts say, until it's over.
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As hurricane after hurricane blasted the Gulf Coast and vicinity this summer and fall, forecaster scrambled for explanations. Most blame an absence of wind sheer, in which layers of wind traveling at different speeds chop up the storms before hurricane can develop. Just why things were so quiet this year in the upper atmosphere, however, is still up in the air.
Doppler radar image of Hurricane Opal, above, was taken at 3:30 p.m. as the eye made landfall along the Florida Gulf Coast on Oct. 4. Below, the paths of storms recorded for the 1995 Atlantic hurricane season are available on the Internet at http://grads.iges.org/pix/alhurr.glf.
Sources: National Weather Service/Weather Data Inc. and institute of Global Environment and Society, Center for Ocean-Land-Atmosphere Studies