The latest understanding of nuclear winter is reviewed. Considerable progress has been made in quantifying the production and injection of soot by large-scale fires, the regional and global atmospheric dispersion of the soot, and the resulting physical, environmental, and climatic perturbations. New information has been obtained from laboratory studies, field experiments, and numerical modeling on a variety of scales (plume, mesoscale, and global). For the most likely soot injections from a full-scale nuclear exchange, three-dimensional climate simulations yield midsummer land temperature decreases that average 10° to 20°C in northern mid-latitudes, with local cooling as large as 35°C, and subfreezing summer temperatures in some regions. Anomalous atmospheric circulations caused by solar heating of soot is found to stabilize the upper atmosphere against overturning, thus increasing the soot lifetime, and to accelerate interhemispheric transport, leading to persistent effects in the Southern Hemisphere. Serious new environmental problems associated with soot injection have been identified, including disruption of monsoon precipitation and severe depletion of the stratospheric ozone layer in the Northern Hemisphere. The basic physics of nuclear winter has been reaffirmed through several authoritative international technical assessments and numerous individual scientific investigations. Remaining areas of uncertainty and research priorities are discussed in view of the latest findings.

The Ames airborne, autotracking sunphotometer has been operated aboard a Sandia Laboratories research aircraft to measure magnitudes, temporal/spatial variabilities, and wavelength dependence of optical depths in the near‐ultraviolet to near‐infrared spectrum of smoke from two forest fires and one jet fuel fire and of background air. The results were corrected for Rayleigh scattering and for estimated absorption by ozone and nitrogen dioxide. Characteristic differences in the aerosol optical depths of background atmospheres and of different types of smokes are the following: (1) the magnitude and wavelength dependence of “background” optical depths vary with the geographic location at which the measurements are performed; (2) the wavelength dependence of smoke optical depths depends on the fuels that feed the fires and on the residence time of the smoke cloud in the atmosphere. In general, the jet fuel smoke optical depths tended to be less wavelength dependent (near‐ultraviolet to near‐infrared) than background aerosol optical depths. Forest fire smoke optical depths showed a wide range of wavelength dependences, including incidents of wavelength‐independent extinction.

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[1] On the basis of a case study we outline a stochastic approach to investigate the internal structure of radiative properties of cirrus clouds and place it into the context of the state of the large-scale atmosphere. We analyze radar reflectivity h(t) measurements obtained with the ground-based millimeter-wavelength radar of the Atmospheric Radiation Measurements (ARM) Program of the Department of Energy at its Southern Great Plains facility. We demonstrate that the evolution of observed non-Gaussian, timedependent probability distribution functions of h(t) at each of various depths into cloud relative to cloud top is governed by the Fokker-Planck equation with linear drift D 1 (x) and stochastic multiplicative noise D 2 (x). The former is tentatively identified with largerscale forcing, and the latter is identified with in-cloud circulations and turbulence. Obtained quadratic dependence of D 2 (x) leads to a noise-induced drift that presents the influence of the small-scale noise on the slow, large-scale deterministic processes.Obtained larger values of the noise-induced drift for the middle 50% of cirrus versus both the upper and lower 25% is anticipated from an ice crystal growth and deposition region. We find that the probability distribution functions of the cirrus that developed above a low-pressure system exhibit behavior at larger scales, e.g., delay times of 2 h, that is consistent with the structure of cirrus based on aircraft in situ measurements and with results from ground-based Raman lidar studies of cirrus. In contrast, the tails of the probability distribution functions of the cirrus over high-pressure system do not show the structure of properties that is distinctive for the cirrus above low-pressure synoptics.

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