We rely on sunshine for almost all life on Earth. It grows crops we need to eat. It also allows plants and blue-green algae to make the oxygen we need to breathe. And it provides heat. If you reduce the amount of sunshine reaching the ground, the temperature drops a lot. One way of reducing sunshine is by having the sun lower in the sky, which is what happens in wintertime. Another way would be to block sunshine by having a lot of clouds or dust in the air, reflecting sunlight back into space before it could reach the ground. From the ground, it looks like the sky is dark. Nuclear winter is the term for having so much dust in the atmosphere that you block sunshine. Large nuclear bombs throw a lot of dirt up into the air--miles up--and this casts a long shadow. The dust is so fine that it floats for a long time, kind of like how dust in your house may keep floating for hours or days before it settles, especially if the air keeps getting stirred up. If there was ever a large nuclear war, it's predicted that nuclear winter would follow for several years.

"Nuclear winter" does not require nuclear bombs, though. A large asteroid hitting the Earth, or even a large volcano explosion, can cause enough dust to cause nuclear winter. For example, when the Tambora volcano erupted back in 1816, the dust cloud spread around the world, and stayed up for more than a year. The effect on the weather was so bad that they called it "the year without a summer."

(A large volcano releases more energy than all the world's nuclear weapons put together.)

A nuclear winter is an event that was proposed by Carl Sagan as a possible result of a nuclear war, which would be caused by large numbers of explosions kicking up enough dust into the atmosphere to at least partially blot out the sun and cool off the planet. He based this prediction off of another phenomenon, volcanic winter, which occurs when a volcano makes eruption so much ash into the atmosphere to achieve the same effect. Volcanic winters are known to happen; 1816 is known as "the year without a summer", and was caused by the eruption of Mt. Tambora in Indonesia. Asteroid impacts could also cause this sort of effects, and one major possible cause of the extinction of the non-bird dinosaurs would be the "meteoritic" winter caused by the Chixolub asteroid impact in Mexico 65 million years ago.

It is highly unlikely that a nuclear winter would be anywhere near as devastating as Carl Sagan suggested. Mankind simply does not have the firepower, not even with nuclear weapons. The Chixolub impact resulted in an explosion yielding approximately 10,000 times the combined nuclear firepower of the entire world combined. There have been many smaller, but still very large, impacts on the Earth since then, but they did not cause noticeable extinctions in the fossil record, despite being still hundreds or thousands of times the world's combined nuclear arsenal.