-40 C is the temperature at which mercury freezes, making most thermometers useless. When I was a kid in Moscow, Russia, -35 C meant that we didn't have to go to school. -40 C was even more fun - it felt like an entire winter break.
At temperatures like these I remember being clothed in 2 pairs of pants, 3 sweaters, 2 hats, and an incredibely heavy fur coat. Movement was a problem. It also looked very funny: imagine stuffed animals trying to jump across mounds of snow in the dead of winter.