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?)