Focusing on Gilbert's extra eye.

Focusing on Gilbert's extra eye

As Caribbean and Mexican residents sort through the wreckage of their encounters with hurricane Gilbert, meteorologists sift through piles of data from satellites and from airplanes that flew through the storm. Aside from its notoriety as one of the stronger hurricanes on the books, Gilbert interests scientists because a phenomenon appeared within it that may offer a forecasting tool. The storm developed an outer band of clouds surrounding the central eye-wall -- forming an unusual feature that on satellite images resembled a bull's-eye pattern.

This circular outer wall of clouds formed at about a 70-mile radius from the storm center on Tuesday, Sept. 13, says Hugh Willoughby of the Hurricane Research Division of the National Oceanic and Atmospheric Administration in Miami. The outer band was concentric around the normal eye-wall, an inner ring of clouds that measured less than 15 miles in diameter on that day. The eye of a hurricane is a calm, cloud-free area encircled by a wall of clouds and the storm's fiercest winds.

Outside the eye-wall, winds usually grow weaker with distance from the center and clouds are organized into a spiral that rotates like a pinwheel. But 12 hours before the storm passed over Mexico's Yucatan Peninsula, the clouds of Gilbert's innermost spiral pulled together, shaping themselves into a circular band. The wind speed in this outer ring rose to about half that of the true eye-wall, so that planes flying into the storm had to pass through two walls to get to the eye, Willoughby says.

Double eye-walls have developed in many very intense hurricanes, including Allen in 1980, but scientists remain uncertain why or how commonly the pattern forms. In Allen, the outside ring of clouds migrated inward and replaced the decaying inner eye-wall. The storm weakened during this day-long replacement period, then reintensified. Willoughby and his colleagues have proposed that the migration process occurs in many strong hurricanes, suggesting forecasters could use this pattern to help judge whether a storm will weaken or strengthen.

According to Willoughby, Gilbert's outer wall also began to migrate inward, but the process was interrupted slightly by the storm's pass over the Yucatan. Although Gilbert did not definitively confirm the migrating eye-wall theory, Willoughby says, "I think we made some converts last week."

Robert Sheets, director of the National Hurricane Center in Coral Gables, Fla., isn't one. "I still haven't seen it documented whether or not there is a transition from an outer eye that becomes the inner eye," he says.

Meteorologists were surprised not just by Gilbert's second eye-wall but also by the narrowness of the eye itself. Hurricane eyes usually measure about 25 miles in diameter, but at times Gilbert's spanned little more than 9 miles. Its central pressure of 885 millibars (26.13 inches of mercury) was the lowest on record for the Western Hemisphere.