Operating Temperature
The official temperature range for Pebble smartwatches is -10 and 50 degrees C.
However, that would be the temperature of the watch, not necessarily the temperature of the weather you're wearing it in.
While it is on your wrist, your watch is unlikely to reach below -10 degrees C since it has your body heat and presumably a sleeve or glove to protect it. So, you'll have to be careful not to leave your watch by itself in those temperatures - i.e. don't leave it in your car overnight.