In four different cases of extensive forest fire smoke the surface temperature effects were determined under the smoke cloud. In all cases, daytime cooling and no nighttime effects were found. The locations of smoke clouds from extensive forest fires in western Canada in 1981 and 1982, in northern China and Siberia in 1987, and in Yellowstone National Park in northwestern Wyoming in 1988 were determined from satellite imagery. As these smoke clouds passed over the midwestern Unived States for the Canadian and Yellowstone fires and over Alaska for the Chinese/Siberian fires, surface air temperature effects were determined by comparing actual surface air temperatures with those forecast by model output statistics (MOS) of the United States National Weather Serivce. MOS error fields corresponding to the smoke cloud location showed daytime cooling of 1.5¿ to 7¿C under the smoke but no nighttime effects. These results correspond to theoretical estimates of the effects of smoke, and they serve as observational confirmation of a portion of the nuclear winter theory. This also implies that smoke from biomass burning can have a daytime cooling effect of a few degrees over seasonal time scales. In order to properly simulate the present climate with a numerical climate model in regions of regular burning it may be necessary to include this smoke effect. ¿ American Geophysical Union 1991