**Ask a monkey a physics question thread**

[wufwugy]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

[Savy]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

It's a hypothetical thing that probably doesn't ever happen.

[MadMojoMonkey]

It is my understanding that the threat of nuclear winter comes from burning of material after detonation such that a funnel is created such that smoke and debris reaches the stratosphere, where it would disperse and linger for a long time. Why didn't the Hiroshima or Nagasaki bombs cause this? Were the cities not sufficiently dense to have enough burning material? Were the bombs too small to create sufficiently enough burning? What is the lowest level and placement of nuclear bomb that could create nuclear winter? (Ex: could a well-placed suitcase nuke in NYC be enough or would it take some serious monster bombs igniting half the city?)

We (humans) have not yet made a bomb that big. Even the Tsar Bomba isn't big enough for that.

We're talking energy release on the order of a 15 km diameter asteroid impact, such as is likely to have caused (or at least heavily contributed to) the Cretaceous Paleogene extinction event which killed off all the non-avian dinosaurs.

It would take a full scale international nuclear war to release that kind of energy, and I'm not sure it would be a long enough winter to compare with the one mentioned above.

Hiroshima and Nagisaki were altogether tiny explosions on the scale of nuclear weapons. We had much better technology in the following decades (see Tsar Bomba), and are many more decades past that, now.

IDK about the density of the cities as pertains to this discussion. The bombs themselves didn't create much burning. The widespread destruction of buildings and power lines is what created the ensuing firestorm. The Japanese didn't even believe that the bombs were a problem. They were convinced that we sent in massive waves of fire bombers behind the nukes, which of course we did not have the logistical capacity to have done.

I don't think any single nuclear explosion can trigger nuclear winter.

I don't think you can fit an actual nuclear weapon in a suitcase, either. You can fit a dirty bomb in a suitcase, but initiating a multi-stage nuclear event is not child's play.