Using the Apple Watch Activity application and the heart rate monitor was working fine when running outdoor until mid november. Then the temperatures got lower around 7°C (45 °F) and the heart rate is giving "stupid" values with a 90 bpm when it should be around 150 bpm. I wear the watch the same way whatever the external temperature is. For historical reasons I also run with a MIO which gives proper results.
Apple Watch, iOS 9.1
I've been wearing my Watch while cross country skiing in temps as low as -10 F and have experienced no problems with the heart rate monitor. I do wear a shirt and jacket when skiing in those low temps. If you are not wearing a coat that covers the Watch I suggest you try that. If you don't want to wearva coat perhaps just a wristband would suffice. If the Watch still does not provide reliable heart rates try resetting the Watch. Press and hold both the Side button and the Digital Crown continuously until the Apple logo appears. Then release the buttons and let the Watch restart.
If none of these suggestions helps make an appointment at an Apple Store to have a technician examine your device. Or contact Apple Support.
Hi
For the best results from the heart rate sensor, Apple Watch should be worn snugly against the top of your wrist. It may help to tighten your watch band for the duration of each running workout, loosening it again afterwards.
Also, when you are exercising in the cold, skin perfusion in your wrist may become too low for the heart rate sensor to get an accurate reading.
More information:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Heart rate not working in low temperature
