By Richard Pyle DHAHRAN, Saudi Arabia (AP) - As the change of season cools the sun-scorched Saudi desert and fills the sky with clouds, U.S. military planners have a new weather worry - sandstorms.

A "sarrayat," Arabic for `the night comer', could occur at critical moments for ground forces, whose tactics are heavily geared toward fighting under cover of darkness.The night sandstorms are brief but violent and are sometimes accompanied by lightning and thunder.

Even in daylight, strategists must reckon with seasonal storms called "shamal," the Arabic word for north, which pick up sand and turn the sky yellow for hours or an entire day.

A U.S. Navy captain said recently that his ship had turned from the official gray to a subtle shade of brown after a few weeks in the Persian Gulf.

American troops have learned to protect their vehicles and aircraft from the incessant dust and sand - using such modifications as pantyhose for filters. But they have not yet experienced the fury of the sustained desert winds that sometimes reduce visibility to near zero and cause even the hardiest Bedouins and their camels to hunker down.

"We're not overly concerned, just aware that there's a possibility of dust storms," said 1st Lt. Todd Fasking, an Air Force meteorologist. "The percent of time that they occur is really very small, but if a storm comes at absolutely the worst time, the percentage doesn't matter."

Saudi experts say the desert storms this time of year are less severe than those of the "Khamseen," or 50 days, from late May through early July, when the Indian Ocean monsoons generate a low-pressure system over the desert.

American strategists have not forgotten the 1980 Iranian hostage rescue attempt that ended in disaster when the helicopters suddenly became caught in a sandstorm that one pilot later described as "like flying in a bowl of milk."

And history suggests that the northern Saudi region, where more than 150,000 U.S. troops are deployed, may be overdue for bad weather.

Jaber Jum Ah, a consultant on desert weather for Aramco, the Saudi-owned oil company, said severe storm seasons come every six or seven years. The last one was in 1983, he said. They usually occur between Oct. 10 and Nov. 10, he said.

The worst season in modern times was 1925, the year of the "sinkings" - so-called because hundreds of small craft were swamped and lost in a gulf storm that lasted about half an hour.

"The gulf is not so deep, and the seas can run as much as 15 feet - not serious for big ships but very dangerous for small ones," Jum Ah said.

While the storms may arise suddenly, meteorologists say threatening weather patterns can be predicted and monitored.

Fasking said winter frontal systems develop in Saudi Arabia much as they do in United States, generally moving from west to east, and can be observed by satellite.

"We can see a front coming and track it," Fasking said. "With the satellite we can see suspended sand as it kicks up out of northern Saudi Arabia into central Saudi."