WITH U.S. FORCES, SAUDI ARABIA, DEC. 29 -- Except for the wind, it's splendid weather for combat, U.S. military commanders say.

The staggering heat that greeted U.S. soldiers when Operation Desert Shield began in August has disappeared, and temperatures have steadied in the high sixties during the day and in the low forties at night. Military planners consider these the best days to make war.

"It's perfect," said Lt. Gen. Walter J. Boomer, commander of U.S. Marines in Saudi Arabia. "Some people have described it as Southern California weather. It's very similar."

But since mid-December, wind has replaced heat as the fundamental weather concern for the 300,000 U.S. combat troops confronting Iraqi soldiers in the Arabian desert. Sudden dust storms spring up almost without warning, sometimes grounding aircraft and turning a bright sunny afternoon into creamy gray twilight.

If the United States and its allies go to war against Iraq this winter, wind could affect almost every aspect of the fighting. Infantry will have difficulty seeing the enemy; artillery spotters will have problems finding targets; missile settings will have to be recalculated; aircraft will have trouble flying. In August the Iraqis worried that the heat might destroy their chemical warfare agents; in January they will worry that the wind might disperse them.

The weather, Boomer said, "will be the same for both sides." In August "the heat affected us both equally, and it's no different now. No one has an advantage."

Desert Shield got a taste of what the wind could do during the pre-Christmas visit of Defense Secretary Richard B. Cheney and Army Gen. Colin Powell, chairman of the Joint Chiefs of Staff. An ambitious itinerary of visits to various military installations around northern Saudi Arabia had to be sharply curtailed because sandstorms grounded the VIP helicopters. Dust over one air base forced the visitors' airplane -- the military version of a Boeing 707 -- to abort one landing attempt before touching down on the second try.

On Christmas Day and again on Friday, visibility at the same air base shrank to one mile, closing it to some types of aircraft for nearly two hours. Other military airports have reported visibility of as little as a half-mile on some occasions.

"To the pilots it might as well be fog," said Air Force Capt. Judy Dickey, meteorologist for the 1st Tactical Fighter Wing. "It's the visibility, rather than the wind, that they worry about."

Dickey, 25, of Anaheim, Calif., sits in a tiny airport office filled with machines that spew rivers of weather reports, satellite photographs and other meteorological information. Her office briefs every pilot who takes off and most of those who come in.

Every hour the Patriot ground-to-air missile battery guarding the airbase calls for wind data to feed into the computer that controls the missiles. Every six hours the Air Force's chemical warfare defense installation calls for a "downwind message": wind direction, strength and whether the weather system is stable or unstable.

On most days, forecasting the weather is simple. "It's cold, windy, dry and sunny with high cirrus clouds," Dickey said. "The temperature is not expected to get above 70 until February."

The military appears to have solved its earlier wind-driven worry: the ubiquitous, destructive desert sand. The basic difficulty for aircraft and most other vehicles and equipment was that Arabian sand is, in fact, dust, almost as fine as talcum powder and nothing like sand found in the United States.

Desert Shield vehicles needed finer-mesh filters and got them. Pantyhose worked in the interim. Helicopter rotors blew the dust into choking clouds that scarred the leading edges of the blades. Now the blade edges have been covered with erosion-resistant epoxy tape.

For tank drivers, infantrymen and other ground forces, acclimating ultimately has simply meant "pulling a little more maintenance on our vehicles," said Marine Sgt. Maj. Earnest W. Jones.

The number of dust storms is likely to increase with the onset of winter, Dickey warned. In general, she said, they will be short-lived and will be confined to specific areas. Dust may darken one major airport in eastern Saudi Arabia for hours but leave another airport 20 miles away bathed in sunshine.

The dust storms begin as cold fronts over the Mediterranean near Lebanon. They move eastward into Iraq, drawing energy from the sun-baked plains until cool sea breezes become whistling desert winds.

Dickey said the wind has to reach 25 mph before it starts to suck up dust and fling it heavenward, sometimes casting a haze for several thousand feet. Pilots can often easily see the ground when they fly through the dust. Fly above it, however, and the dust looks like a billowing, opaque cloud of fog.

The strongest winds come in the afternoon, when the contrast between cold air and sun-warmed sand is greatest. And once the sun sets, desert temperatures plunge dramatically and the wind calms, the soldiers' worries are far from over. "The worst period comes at this moment," Dickey said, "because the dust starts to settle and becomes denser."

What Dickey and other meteorologists watch most closely are pronounced Mediterranean cold fronts that may indicate the onset of shamals, full-fledged desert windstorms that can last up to five days. A full-strength shamal travels at 45 mph, carrying with it a stinging dust cloud up to 100 miles wide, 60 miles long and more than 3 1/2 miles high. Visibility shrinks to little more than 100 yards.

Windstorms blow most strongly across the Arabian desert's northeastern quadrant, Dickey said, an area that includes the southern half of Iraq, all of Kuwait and northeastern Saudi Arabia.

Nearly a million armed men and women are stationed in the quadrant, and everybody involved in the desert confrontation will have to cope with the wind -- in different ways.

The wind moves in a southeasterly direction, picking up speed -- and dust -- as it travels. The Iraqis, Dickey said, "won't get it as bad because they don't have the momentum. But they get it first."