What conditions should an asteroid or comet impact need to meet in order to start an impact winter?

Is there a way to calculate if an impact from a given asteroid (say 5 miles in diameter, solid rock to which we know the mass) would start an impact winter? If yes, I would like to see a real example of an existing asteroid, if possible some calculations, and the conclusion that an impact winter would be imminent.

I'm not sure if I'm not forcing the boundaries of available knowledge, but I would like to be able to understand and deduce if a given asteroid impact would start an impact winter. I know what that is, and I've read the Wikipedia article Impact winter. I'm interested in a specific example, if possible.

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To what extent should the climate be affected to satisfy your requirements? – Grant Thomas Aug 27 '11 at 0:19
I would immagine there is a treshold from which an impact winter is imminent. My question is intended to deducing that treshold – Ioan Paul Pirau Aug 27 '11 at 8:33
I don't think you can narrow it down t just a few parameter. Only because it would all depend on the type of celestial body (conglomerate, solid, comet, etc), where it lands, what angle it hits at, etc. etc. etc. Too many variables to give a good answer. – Larian LeQuella Aug 28 '11 at 17:41
@Larian I did mention that I might be forcing the boundaries of available knowledge but as there was possible to make this impact simulator I think it might be possible to calculate the probability of an impact winter as well. – Ioan Paul Pirau Aug 29 '11 at 12:34
@All: If you think it is indeed not possible to calculate or the question is not good you should vote to close the question as not constructive and also provide a comment rather than just giving it a -1. Thank you. – Ioan Paul Pirau Aug 29 '11 at 12:35

I'd heard about a really detailed simulation of an India-Pakistan nuclear war, and found an oblique reference to it on Wikipedia. The money quote is "Five million tons of soot would be released, which would produce a cooling of several degrees over large areas of North America and Eurasia, including most of the grain-growing regions. The cooling would last for years, and according to the research could be 'catastrophic.'"

Hence, it sounds like five million tons is a maximum limit to the minimum size of an impacting body to create a global catastrophe. In fact, that is probably really optimistic, because a high speed astronomical impact could probably kick up much more soot than its own mass.

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