Nuclear explosions trigger a horrendous chain reaction. The instantaneous outcome is the thermal and blast effect annihilating everything in and around the area. The mushrooming cloud due to the explosion rises high into the stratosphere, spreading out vast quantities of soot and radioactive debris. Some of the debris fall back to the ground as rain out. The soot and other particles suspended in the atmosphere would block sunlight and lower the global temperatures steeply to subzero levels, ushering in wintery conditions over the planet. This sequence of events has been termed as 'Nuclear Winter'

is a hypothetical climatic effect, most often considered a potential threat following a countervalue, or city-targeted, nuclear war.

Nuclear winter, the environmental devastation that certain scientists contend would probably result from the hundreds of nuclear explosions in a nuclear war. The damaging effects of the light, heat, blast, and radiation caused by nuclear explosions had long been known to scientists, but such explosions’ indirect effects on the environment remained largely ignored for decades. In the 1970s, however, several studies posited that the layer of ozone in the stratosphere that shields living things from much of the Sun’s harmful ultraviolet radiation might be depleted by the large amounts of nitrogen oxides produced by nuclear explosions. Further studies speculated that large amounts of dust kicked up into the atmosphere by nuclear explosions might block sunlight from reaching the Earth’s surface, leading to a temporary cooling of the air. (Smoke from such materials absorbs sunlight much more effectively than smoke from burning wood.) The TTAPS study coined the term “nuclear winter,” and its ominous hypotheses about the environmental effects of a nuclear war came under intensive study by both the American and Soviet scientific communities.

Nuclear winter (also known as atomic winter) is a hypothetical climatic effect, most often considered a potential threat following a countervalue, or city-targeted, nuclear war. Climate models suggest that the ignition of a hundred or more firestorms that are comparable in intensity to that observed in Hiroshima in 1945 would produce a small nuclear winter.[1] The burning of these firestorms would result in the injection of soot into the Earth's stratosphere, producing an anti-greenhouse effect. The models conclude that the magnitude of this effect from the cumulative products of 100 firestorms would reach sufficient extent to unmistakably alter the global climate, resulting in agricultural losses from the colder weather, and lasting for a period of years, whereas an all-out US-Russia war would cause catastrophic summer cooling by about 20 �C in core agricultural regions of the US, Europe and China, and by as much as 35 �C in Russia.[2]

On the fundamental level, it is known that firestorms can inject sooty smoke into the stratosphere, as each natural occurrence of a wildfire firestorm has been found to 'surprisingly frequently' generate minor 'nuclear winter' effects.[3][4][5][6] This is somewhat analogous to the frequent volcanic eruptions that inject sulfates into the stratosphere and thereby produce minor volcanic winter effects.

Nuclear winter is a predicted climatic effect of nuclear war. It has been theorized that severely cold weather and reduced sunlight for a period of months or years could be caused by detonating large numbers of nuclear weapons, especially over flammable targets such as cities, where large amounts of smoke and soot would be ejected into the Earth's stratosphere.