One winter I was vacationing in Beijing, Changchun, and Haerbin. Beijing was about -10C, cold I guess. Changchun was about -20C . I took a bus to Haerbin and saw the mercury fall to -40C (they have digital thermometers at the front on the long distance buses in NE China!). Anyways, there is a huge difference between -10C and -40C, when I got back to Beijing, it felt fairly balmy.
Yea I wouldn't recommend doing anything other than hopping between buildings or vehicles when it gets down to -40 (same in both C and F, which I think is neat)
-10C is 14F, which I agree is a pretty nice day if you're properly outfitted.
You can actually do stuff in -40C, just wear a lot of clothes and don't expect your digital camera to last very long when you are trying to take picture of carved ice sculptures and ice bars, etc...
Honestly, the coldest winters I spent were in Southern China, even though it was 5 to 10C out, no indoor heating really sucks and will beat you down quickly.
