Looking for changes in the internal eyewall of a hurricane may facilitate forecasters defeat one of their most complicating challenges: predicting gradual strengthening or weakening. The ability to forecast what path a hurricane will follow has ameliorated dramatically in recent years, but anticipating sudden changes in intensity has remained a problem.

And knowing a hurricane's strength is critical to making decisions about evacuating areas when a storm comes near. Now, a research team led by Robert A. Houze Jr., a University of Washington professor of atmospheric science, is announcing evidence that clouds around the eyewall of a storm can cause sudden modification in intensity. The findings, in Friday's issue of the journal Science, are based on studies of data accumulated in 2005 in storms Katrina and Rita that devastated New Orleans and portions of Mississippi, Alabama, Louisiana and Texas.

The most power full winds in a hurricane circulate in the cloud wall that surrounds the relatively tranquil and clear eye of the storm. Assuming measurements from aircraft flying into these storms, the researchers directed by Houze found that occasionally a moat of clear air will take shape outside the eyewall. Winds moving toward the center of the storm will then become a new eyewall outside the original one, passing thru the storm center from the incoming flow of energy and destroy the old eyewall. Because the new eyewall is bigger than the old one, its winds circulate more decelerated, thus reducing the intensity of the storm. But the new eyewall can then begin to engage, spinning faster and faster and changing the storm's intensity.

Using aircraft to study these changes could help meliorate forecasts of storm intensity, Houze said in a telephone interview.

He noted that while Hurricanes Rita and Katrina moved along similar paths, Rita experienced eyewall replacement and Katrina did not. Rita weakened from a category 5 storm to a 3 or 4 after the eyewall was modified. Katrina was a 5 just offshore but was somewhat less powerful when it came ashore. Now the researchers are cutting into the detailed information to see if they can find out what caused the eyewall replacement in one storm and not the other. Right now, they're not sure what triggers eyewall replacement.

They are looking at a couple of ideas, he said, "but nothing definitive can answer this yet." One answer focuses on the rain bands that travel away from the center of the storm, he said. There appears to be a dangerous distance from the center of a hurricane that they cannot go beyond, and some weather experts think an accumulation of rain bands can form the secondary eyewall.

Another possibility, Houze said, associates to the pattern of humidity. If the storm is very moist in its interior but has lower humidity away from the center, the rain bands may be decrease within the humid zone, he said.

Hugh E. Willoughby of Florida International University, who was not part of Houze's team, declared in a commentary on the report, "Earth's atmosphere is still fiendishly capricious." Developing a way to foretell eyewall replacement is crucial to forecasting, he said.

In a separate report, meantime, other researchers noted further evidence that global warming has contributed to stronger hurricanes in the Atlantic Ocean.

The report in Geophysical Research Letters by scientists at the University of Wisconsin-Madison and the government's National Climatic Data Center in Asheville, N.C. added more details to studies last year that established connections between warming and increasingly intense hurricanes. "The data say that the Atlantic has been trending upward in hurricane intensity quite a bit," said leading expert on the team, researcher James Kossin of Wisconsin. That was not true for other oceans, however.

Sea-surface temperatures may be one motiv the Atlantic Ocean is like no other, Kossin said. "The average conditions in the Atlantic at any given time are just on the edge of what it takes for a hurricane to form. So it might be that only a small change in conditions makes a much better chance of having a hurricane." Both research efforts were funded by the National Science Foundation.