Dramatic drops in temperature under the thick smoke produced by forest fires support the theory that the firestorm of a nuclear war could lead to an artificial winter of cold, crop failure and starvation, according to a study published Friday.

Alan Robock, a professor at the University of Maryland department of meteorology, said temperatures dropped as much as 36 degrees Fahrenheit, or 20 degrees Celsius, below normal in California mountain valleys that were shrouded for days last year by smoke from forest fires.The studies are based on temperature readings taken in the Klamath River Canyon of Northern California starting on Aug. 30, 1987, after lightning triggered forest fires that burned for more than a month.

Robock said smoke from the fires was trapped in the valleys of the area by a temperature inversion. The smoke blocked much of the sunlight from reaching the valley floors, and the temperatures there dropped dramatically.

"Virtually all the sunlight that is not reflected by the smoke is absorbed before it reaches the ground, thereby strongly cooling the ground while slightly heating the air," Robock wrote in the journal Science.

Temperatures, he said, dropped swiftly. By Sept. 7, a week after the fires started, the high temperature readings at Happy Camp, a point near the Oregon border, had cooled to 62.6 degrees, 36 degrees below the normal maximum for the season.

Happy Camp, in a valley covered with smoke, became much cooler than Slater, a town on a ridge to the north that was not covered.