Saudi Arabia’s Blistering Heat Will Soon Ease : Military tactics: With cooler weather, U.S. forces will enjoy a potentially critical advantage over Iraqi forces.

Until now, as the massive M-1 tanks trundled ashore and foot soldiers struggled to establish positions in the sandy wastes of Saudi Arabia, their most relentless foe has been the climate--heat so brutal that temperatures inside tanks soared to 140 degrees and even the toughest infantrymen could last only minutes in chemical warfare suits.

But all that is about to change. In a few short weeks, autumn will come to the Arabian desert--and with it a potentially critical advantage for U.S. forces if ordered to move into action against the vast Iraqi army.

The seasonal change, with sharp drops in daytime and nighttime temperatures, is changing the balance of the way time works for and against the two sides in the Persian Gulf crisis.

“Very shortly, it’s going to be quite cold out there,” Gen. H. Norman Schwarzkopf, commander of U.S. forces in Saudi Arabia, pointed out in a briefing for reporters Friday. With the hottest month now past, the general said with relief: “We’re on the down side.”

The abrupt end to the desert summer--sending daytime highs down about 13 degrees by Oct. 1--will come as U.S. troop strength hits its maximum in a coincidence some officials said could provide a prime opening for an offensive operation by the United States.

The waning of the searing afternoon heat will soften sun-baked sands, depriving Iraqi armored forces of traction they depend on. Heat-sapped American soldiers are expected to gain endurance. And with the cooler, drier days comes better visibility--a boon to U.S. pilots now hindered by the haze.

“A lot of the problems they’re facing are just going to disappear,” a desert warfare specialist involved in Pentagon planning said. “The climate will soon be on our side.”

The anticipated turnabout comes after weeks of anxiety in military circles as U.S. forces, already badly outnumbered, had to additionally contend with temperatures so high that many commanders ordered their troops to train only at night.

Now, with soldiers only slowly becoming acclimatized to a region in which conventional thermometers became useless by mid-morning, the break in the weather is expected effectively to speed up the clock in favor of the American troops.

“As the weather gets better,” said Army reserve Lt. Col. Pierf Wood, an analyst at the Center for Defense Information, “it becomes a clear plus on the side of favoring an offensive operation.”

The extraordinary change in the desert climate can come as a surprise to those who expect nothing but unending dunes of burning sands.

In his classic “The Seven Pillars of Wisdom,” T. E. Lawrence, the famed Lawrence of Arabia, wrote of treks in an Arabian desert in which “the air seemed cold enough to freeze anything, but did not: The wind, which had changed during the night, swept into us from the west in hindering blizzards.”

And in his history “The Kingdom: Arabia and The House of Saud,” author Robert Lacey writes of the Bedouin inhabitants: “The desert burns him by day and chills him by night.”

So quickly can weather change in the desert that Schwarzkopf, commander-in-chief of the U.S. Central Command, gently scolded reporters for their preoccupation with what has so far been virtually unbearable heat.

Indeed, the general announced: “One of the things we’re looking at right now is making sure that we have field jackets and sleeping bags because very shortly it’s going to be quite cold out there in the desert, and it’s not going to be hot.”

A well-known expert on desert warfare, David Syritt, a professor at Queens College in New York, said temperature variations can include “horrendous heat in the daytime, and then at night it can plummet down to severe cold.”

“Armies need to have virtually two sets of clothes,” Syritt said.

Experts on the desert climate at the Marine Corps Air Ground Combat Center in Twentynine Palms, Calif., said August is the hottest month on the Arabian Peninsula, with average maximum daytime temperatures of 108 degrees.

By October, the average high falls to a relatively balmy 95 degrees, with nighttime lows of 73. At the same time, winds virtually cease and humidity drops precipitously, leaving the air clear and dry. In December, temperatures swing between 73 and 57 degrees.

In military terms, the most significant consequence of the cooler weather is expected to be the softening of sands now baked to a hard crust. Iraqi tanks now capable of a quick attack across the terrain could find themselves bogged down.

“Assuming the United States stays on the defensive,” said Wood, a former infantry commander, “that’s to its benefit. It channels the attackers along predictable routes--killing fields.”

But for an American force less dependent on armor, the sloppy sands would not prove such a barrier, Wood and others pointed out.

In an American attack expected to revolve around air power, aircraft whose jet engines are made less efficient in the heat could fly combat forays with far less danger of malfunctions, said Barry Watts, a retired Air Force colonel.

“You get into a cooler time of year and you’re going to be able to turn airplanes a lot easier and quicker,” Watts said. “There’s a lot less stress on the airplane.”

At the same time, afternoon haze caused by heat rising from the desert--a summertime obstacle for fighter pilots--all but disappears in winter. Night-vision goggles and other thermal-imaging devices work more efficiently when a hot target stands in contrast to a cool background.

For ground troops, the experts said, even a five-degree drop in desert temperatures can mean as much as a 20% increase in endurance--a dramatic advantage in an offensive operation that would revolve around rapid maneuverability.

The difference, said Wood, the reserve lieutenant colonel who fought in Vietnam, “can be extraordinary. “When you’re humping stuff out there, you feel every damn degree.”

Times staff writers Maura Reynolds and Jenny Toth contributed to this article.