FORT LAUDERDALE, Fla. – They can bring torrential rains or clouds of dust. And sometimes they spin into monster hurricanes.

These are tropical waves that boil up over the Saharan region of Africa, drift west over the Atlantic Ocean and occasionally reach our shores.

Of the 80 to 90 waves produced during the course of the hurricane season, the ones that create the most concern are those that are moisture-laden and flare up into tropical storms.

A few of those roamed the Atlantic on Monday, but none appeared to be on the verge of strengthening, the National Hurricane Center said.

Many of the rest are dry waves that deliver Saharan dust as far as South Florida, as was the case over the weekend. The dust was expected to linger until Wednesday, when ocean breezes and afternoon rain should wash it away, said meteorologist Brad Diehl of the National Weather Service in Miami.

“We’re under a dome of high pressure, and that makes the atmosphere very stable,” he said. “With very stable conditions, it’s difficult for the dust to be removed.”

Today’s forecast calls for highs in the low 90s with a heat index, or “feels like” temperature as high as 102 degrees – and more dust.

Generally, Saharan dust outbreaks turn skies hazy, reduce visibility for pilots and produce spectacular sunrises and sunsets. They also can make breathing difficult for those with respiratory problems, although no problems were reported on Monday.

“If people want to be cautious, they should stay in an air-conditioned environment and limit their outdoor activities,” said Monica Pognon, a natural resource specialist with the Broward County Environmental Protection Department.

Because of the dust and heat, West Palm Beach officials on Monday waived admission fees at city pools and extended the hours at water playgrounds.

Whether a tropical wave – or an area of low pressure – carries rain or dust across the Atlantic largely depends on how it is formed while over northwest Africa, said Robert Rogers, of the National Oceanic and Atmospheric Administration’s Hurricane Research Division in Miami.

If the air over the Sahara is stable, the dust is allowed to build into a wave as it floats west toward the Atlantic. If the upper level wind flow over the Sahara is strong, the wave likely will pick up moisture as it interacts with the ocean, said Rogers, a research meteorologist.

“It’s a competition between the easterly wind flow and the Saharan air,” he said.

What creates that strong airflow isn’t quite understood, Rogers said. Some scientists believe it is the result of moist air moving up from the southern areas of Africa clashing with the dry air of the Sahara, creating a jet stream similar to that over the United States.

In any case, after a wet wave starts across the ocean, it will grow into a hurricane only if the water is at least 80 degrees and the atmosphere calm enough to allow thunderstorms to build, Rogers said.

“If you have enough thunderstorms associated with the wave, then a system can intensify and develop,” he said. “So it’s a lot of pieces that have to come together.”

Richard Pasch, a hurricane specialist with the National Hurricane Center, noted that even dusty waves can develop into monster storms. He cited Hurricane Elena, which hit Biloxi, Miss., as a Category 3 system in September 1985.

“That was a big swirl of dust when it came off the coast of Africa,” he said.

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(c) 2006 South Florida Sun-Sentinel.

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AP-NY-07-31-06 1834EDT