nuclear winter(n = noun.phenomenon) - a long period of darkness and extreme cold that scientists predict would follow a full-scale nuclear war; a layer of dust and smoke in the atmosphere would cover the earth and block the rays of the sun; most living organisms would perish;
is a kind of cataclysm, catastrophe

   a hypothetical lowering of global temperature postulated as the aftermath of a full nuclear war. The mechanism of such an effect depends primarily on the injection into the atmosphere of small solid particles, as soot from burning forests, which reflect sunlight and cause a reduction in solar energy retained by the earth. [WordNet 1.5]

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