Be advised that the thesis of a "nuclear winter" has some inherent contradictions. Only bombs above a certain yield can produce mushroom clouds that carry soot particles (from burned forests and cities) to altitudes where they will not be washed out by precipitation, and can therefore linger indefinitely. But, above this yield, the mushroom cloud rises so fast, it does not gather soot from burning forests, etc. So, where a nuclear winter cloud would be possible, it could happen only for bombs that have low enough yield that the soot would be washed out by the first rain.