An important factor in such estimates, all agree, is the quantity of combustible material that would contribute to the global pall of smoke. Based on estimates by various research groups, Dr. Turco assumes the total mass of material burned, including wood, plastics, petroleum and vegetation, would be 5,075 ''teragrams'' (trillions of grams), or about 6.8 billion tons.

But another leading investigator, Dr. Richard D. Small, a thermal science expert at Pacific-Sierra Research Corporation, a Los Angeles research organization, says he disagrees strongly with this estimate, which he believes is much too high.

Dr. Small estimates that a maximum of 1,475 teragrams of material would be burned in the United States, provided all the weapons in the Soviet arsenal were successfully launched and detonated, and that all combustible material was actually ignited. Comparable figures for burned material in Europe and the Soviet Union would be proportionately less, ''because those regions simply have less combustible material in homes, businesses and industries,'' he said.

''Our estimate is based on rigorous analysis of blueprints and other records of real homes and commercial and industrial structures,'' Dr. Small said. ''We add up every possible ingredient available for burning to estimate a weighted total.''

The greatest uncertainty in the article's assumptions, he said, is in the amount of smoke that would be injected into the atmosphere, remaining aloft long enough to reduce global temperatures.

Other uncertainties include the amount of smoke that would be removed from the atmosphere by rain, and the height to which smoke would be lifted by fires ignited in a nuclear exchange. These and other factors could radically change the atmospheric effects. Biggest Uncertainty ''Perhaps the largest uncertainty is timing,'' Dr. Schneider said. A nuclear exchange in late spring or summer might have a significant effect on temperature, while a war in late fall or winter would have no appreciable effect, because sunlight is already reduced and temperatures are already low, he said. The growing season, moreover, would be over, and therefore unaffected by cooling.

Dr. George Rathjens, a professor of political science at the Massachusetts Institute of Technology, charged in an interview that ''all the hype about a lot of freezing following a nuclear exchange is hyperbole.'' He said his own calculations, using the latest of Dr. Small's numerical smoke estimates in a standard mathematical model, result in a temperature drop of only about 9 degrees centigrade (16 degrees Fahrenheit) after a full-scale nuclear war in the Northern Hemisphere in midsummer.