I was also under the impression that the whole flash-frozen thing was a bit of a myth. In any case, I came up with a completely non-professional
theory whilst reading this.
I watched a documentary on Disco channel the other day about toxic lakes, which have caused entire villages to be killed almost instantly. The way
these things work is that when a large body of water builds up in a volcanic crater, or other area with seismic activity, small vents at the bottom of
the lakes spray carbon dioxide into the water. However, with some lakes the water pressure at the bottom is so strong that the co2 cannot rise to the
surface as bubbles but dissolves into the water, specifically the bottom layers of the water. Eventually you have an incredible co2 rich layer of
water underneath a seemingly ordinary body of water, and in some instances these layers can be disturbed so that the co2 layer emerges, expelling huge
amounts of the gas into the air which drifts through settlements asphyxiating people.
A similar thing could happen with the atmosphere, whereby a fairly warm layer of air forms near the earth which puts pressure on the upper atmosphere,
which is forced to rise and cool more. As we know that different temperature air doesn't mix, this situation could remain static for some time before
an event such as a small asteroid collision causes an area through which the cold air could sink and the hot air rise with violent force. The cold air
might stream downwards freezing things fairly rapidly.
Not sure if this is very plausible...
