Why so many hurricanes?

This summer's onslaught repeats a long-term cycle, bringing back warmer waters and more storms.

Forecasters sensed the 2004 hurricane season would be very active, but even storm veterans have been surprised by the past 30 days.

Last month marked the first time since the beginning of postwar hurricane reconnaissance flights that August generated three major hurricanes in the Atlantic. If the current forecast track for hurricane Ivan holds, it will be the third hurricane to strike Florida in a month.

Yet for all its fury, this season's burst of activity falls well within the bounds of past experience. What's surprising, say experts, is that the US and Florida haven't seen more major storms make landfall over the past few decades.

Despite the damage wrought by Charley and Frances, "we've been very fortunate," says William Gray, a tropical-meteorology specialist at Colorado State University who pioneered seasonal hurricane forecasting. He notes that since 1995, only 1 out of 7 major hurricanes spawned in the Atlantic have made landfall in the US, compared with the 100-year average of 1 in 3. The Florida peninsula alone saw 14 major hurricanes between 1926 and 1965. Since 1966, only three major storms have struck - Andrew, Charley, and Frances.

Now forecasters have their eyes on Ivan, which has devastated Grenada and Jamaica and at press time was bearing down on the Cayman Islands and Cuba with sustained winds near 155 miles an hour. Ivan has been blamed for 56 deaths in the Caribbean basin and, according to Red Cross estimates, 60,000 people on Grenada - two-thirds of the island's population - are homeless and 34 people have died. On Jamaica, where an estimated 500,000 people ignored warnings to evacuate, at least 11 were killed.

Several factors have converged to make this hurricane season one for the record books, researchers say.

For one thing, long-term cycles affecting the ocean and atmosphere are at play. Known as the Atlantic multidecadal signal, "these atmospheric conditions and warmer ocean temperatures can turn up for decades at a time," says Gerald Bell, a meteorologist with the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Md. Currently, long-term patterns favor hurricane seasons that yield more tropical storms and hurricanes than normal. Conditions are similar to those that held sway from the mid-1920s to the mid-1960s, another period of above-normal tropical cyclone activity.

Within those periods, he adds, storm activity season to season is affected by features such as El Niño episodes. Their long-range reach can generate wind patterns over the Atlantic that suppress the formation of hurricanes.

Forecasters see a weak El Niño beginning to build in the eastern tropical Pacific. But they add that it's unlikely to have much of an effect on this year's hurricane season. And if it remains weak, it could have little affect on next year's season.

In forecasting monthly activity for August, Dr. Gray says he and his colleagues missed unusually warm sea- surface temperatures in the eastern Atlantic, where hurricanes and tropical storms are born.

The team forecast above-average activity for the month, "but we could not have anticipated the unusually high amount of storm activity that occurred," he notes. August yielded eight named storms.

With the Atlantic basin in the midst of a long-term active phase for hurricanes, "undoubtedly [over] the next 20 years, we're likely to see much more damage than during the last 20 years," Gray says.

The reason: While hurricane activity is more or less readjusting to its long-term averages after a period of relative quiet, more people are placing themselves, their houses, yachts, and office high-rises in storm paths when they move to hurricane-prone states and their geologically fragile shorelines. In 1926, a hurricane struck Florida that - if it were to happen today - would cause $100 billion in damage, notes Roger Pielke Jr., with the Center for Science and Technology Policy Research at the University of Colorado at Boulder.

While no one advocates preventing people from moving to Florida or the Carolinas, the prospect of more growth in hurricane-prone areas puts the onus on residents to become familiar with preparing for hurricanes, on communities for enforcing building codes aimed at reducing damage, and on federal researchers to continue improvements in forecasting.

Yet even with improved forecasting tools, the public doesn't always put the information to best use, some researchers say. "We still can't explain why only some people leave and others don't," says Jay Baker, a geographer at Florida State University in Tallahassee who works on emergency-preparedness issues. Understanding that motivation may help officials devise education programs to encourage more people to heed calls for evacuation.

That response may have less to do with knowing the difference between a hurricane watch and a hurricane warning than it does with perceptions of individual risk, he says - a perception that can include misconceptions about how far a person needs to travel to avoid the storm. The solution can be as simple as more accurate hazard maps.

During hurricane Floyd in 1999, Dr. Baker says, more Floridians hit the highways - and clogged them - than was necessary. Since then, Florida counties such as Dade and Broward have drawn up precise topographical maps using airborne lasers to measure elevation changes as small as six inches.

These maps, Baker continues, help define evacuation areas when a hurricane and its attendant storm surge approach. In many cases, evacuations may not require more than a mile or two's travel to reach relative safety.

As Florida cleans up from Charley and Frances and eyes Ivan, "anyone has to be impressed at how well prepared all parties were" to handle the aftermath, says Dr. Pielke. Yet there are tremendous lessons to capture" about how to further reduce damage and loss of life. "We don't do fire drills with hurricanes, so we have to learn from experience."