[LorenPechtel]

Nuclear winter requires two things.

1) Big bomb. The mushroom cloud must go up to above the maximum altitude for rain.

2) Soot. You need a city or the like underneath the bomb to produce a bunch of soot to get lifted into the sky.

Either one by itself (we've seen really big bombs, but they're not dropped on places that will make a bunch of soot, Saddam's oil fires created enough soot but nothing lifted it above the rain, the bombs on Japan weren't big enough to lift the crud above the rain) has little effect.