I wouldn't rely on the artificial simulations of freezing conditions -- I'd bet you they use pure water. And clouds aren't pure water!
You can have supercooled water on surfaces -- liquid well below freezing -- that will, when it lets go, become ice immediately.
http://www.agu.org/cgi-bin/SFgate/SF...00&=%22A13B%22
Until recently nobody looked for anything smaller than dust particles as condensation nuclei. Now we know that far smaller bacteria and even viruses are present above ocean water
http://www.sciencedirect.com/science...14c63f9909a268 and can get swept up into clouds.
We've known what happens sailing ships through water that's full of living things. I wonder if anyone's looked inside engines for the kind of very thin, but very hardy, films of bacteria that are such problems in hospitals and food processing plants. Among other things they can alter the freezing point of water.
It's a very odd thought. But if you ever turned on a distilled water faucet in a lab, medical or dental office and saw white guck come out -- those are bacteria that make quite a good living off the trace minerals left in very pure distilled water. Clouds have a lot more in them than pure distilled water. Besides natural biological material they've got big loads of sulfates and other industrial pollution, downwind from many areas. It'd be interesting to find out where the clouds associated with engine shutdowns originated and what was in them besides water.
http://www.sciencedirect.com/science...1eff16041d047c
http://www.agu.org/cgi-bin/SFgate/SF...22A13C-0929%22
I just had to wonder if a microbiologist has ever gone over the inside of a jet engine as carefully as they would the inside of a food processing plant or a hospital operating room, to see what lives in those extreme conditions and what it might do when it grew a layer thick enough to start flaking off.