In reply to simc:
Dealing with -20 is relatively child's play. No previous experience or theory really needed. Just an extension of -10 really.
Dealing with -30 is difficult... previous advice on layers, sweating and whatnot is all very valid. However, it's all very personal... everyone deals with it very differently. Experience pays dividends.
Dealing with -40 is just another planet. No book or other people's experience will ever prepare you for it. You just have to go out, deal with it and learn. Nothing works at this temperature.
And I'm talking about absolute temps... not including wind chill.