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Nuclear winter refers to a long-term cooling of the Earth's climate following a massive nuclear war. Nuclear weapons, as we all know, pack a lot of power into a small device. If a large number were detonated over short time period it would result in a lot of debris in the atmosphere. This debris would be of a type that would likely stay suspended in the atmosphere at high altitude. It is believed that this high-altitude debris would reflect a significant portion of the sun's rays, not allowing them to reach into the lower atmosphere or to Earth. As a result, the climate would cool significantly, resulting in a long, cold winter (depending on the amount of debris, one long cold winter season to several years or decades of very cold weather).
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Similar effects have been seen from large volcanic eruptions which through large amounts of fine ash into the air. Krakatoa, Pinatubo, and Mt. Saint Helens all resulted in raising the Earth's effective albedo (the amount of light reflected back to space) and produced a measurably cooler climate over large portions of the planet.
Due to some air pollutants released from chimneys of factories,the air will be filled with them.They assume the shape of a mushroom cloud and prevent sun's rays from reaching the earth.So it becomes cold. 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". As a consequence, photosynthesis would stop leading to the destruction of all green plants. Subsequently oxygen regeneration would cease and carbon dioxide would accumulate. Earth's radiation balance and heat budget would get altered leading to drastic changes in the global circulation pattern. the seasonal monsoons and tropiacl rains may disappear. the duration and extent of the nuclear winter scenario would depend on the location, season and intensity of the explosion.