I've been in -40C before. It's colder than you can imagine.

A tip: If you're ever going anywhere that gets as low as -40 don't go to Mountain Equipment Co-op in the mall and buy one of those light shitty little nylon winter jackets that say "good to -40" on the tag. They're good to -40 only if you're wearing 6 of them or if you soak it with gasoline and light it on fire.