Thanks for the link and the explanation. I'm not sure if I'm going to get my head around dry adiabatic lapse rates and such without actually working through examples, but it's fun, and a start, to learn how to read the temperature - and apply it. Sort of, bearing in mind these are forecasts and not highly accurate.
Monday 400-500mb - short / dissipating contrails
Temp: -25°C
Dew Point: -41°C
http://i42.tinypic.com/2hs94p3.png
Tuesday 250mb - long / dissipating contrails
Temp: -58°C
Dew Point: -65°C
http://i42.tinypic.com/4gp4wk.png
Don't know what to make of that yet... need some more observations. Eventually the errors may even out and a pattern may emerge.