Running

The temperature is low (5C / 41 F) and I'm starting to suffer from freezing hands (even with the gloves). 

When it's this cold, the veins in your wrist tend to tighten and the capillary profusion used by the sensor in the watch start to not be enough for reliable measurements. 

If you see strange reading of your bpm, there are 2 things you can do to mitigate the issue:
- try to keep the wrist warm (gloves and long sleeves)
- tighten the watch strap

hope this helps

Would this typically result in a higher heart rate reading?

Can’t answer on this specifically but at least the reading are consistent