I suspect it's more a case of when you get cold blood is naturally drained away from your skin surface, it's part of the body's way of trying to keep your core temperature correct, it's why people tend to look pale when they are cold. This makes the heart rate difficult to detect accurately.
My new FR35 is reporting a HR about 30 bpm higher than it really is. This is all the time. This is NOT a function of not wearing it properly, skin conditions, temperature, etc. IT IS DEFECTIVE! I am returning it and Amazon is sending me another one under warranty. Hope it works properly!
My Garmin FR35 has also displayed high heart rate intermittently for more than a year. In my case my resting heart rate is 60 and my exercise target heart rate is between 100 and 110. Today my FR35 displayed 140 after I returned from a brisk walk. At the same time I also measured my HR on an AliveCore KardioMobile 6L EKG device, which showed my HR to be 74. This problem and difficulty syncing data between my FR35 and my Samsung Galaxy Note 5 cell phone are why I will replace my FR35 with a more accurate device if I can identify one. I have been planning on replacing my Note 5 with a Note 20 and my FR35 with the new Galaxy Watch 3, but only after Samsung gets the HR/BP/EKG apps working and only if they turn out to be more accurate than my FR35.
Buy a Chest Strap for your FR. The Wrist Based HR Machines can pick up the "Cadence" of the Workout and report it as HR. A Chest Strap based Watch will give more accurate readings except that the strap can get over soaked with sweat and also fail to give accurate readings but is easily sorted out by wiping it down.
Wearing the Watch Strap a little tighter and further up the wrist will help as well.
When I do Weight Training I wear mine on the Ankle and it works 100% then.
I still want to try on the ankle with a long sock over it for running.
My Forerunner 35 has been reporting super high heart rates when I'm doing easy runs. I've had it and worn it every day for close to two months now. I was worried because it was recording near max heart rate even when I wasn't pushing my pace, and I was getting concerned that I was way out of shape - it was telling me my VO2 max is in the bottom 20 per cent of my age and gender and I couldn't figure out what was going on, because I certainly wasn't feeling like I was going to pass out or like I was exerting that much effort. This is happening both in warmer and colder temperatures outside over the course of two months, so I don't think it's connected to temperature. I am a pretty active woman, and train 5 or 6 days a week.
Today I participated in a virtual "dance party" for fun and wore my watch. I noticed that even though I was exerting myself quite a bit, my heart rate never climbed higher than 30 below the highest rate I've tracked while running, even when I was feeling quite out of breath.
So, I'm wondering whether my arm movement or cadence while running is affecting my heart rate. Is that possible? And if so, is there a way to fix this?
TIA
Make sure the watch is worn properly, tight but not too tight. If you move the watch your skin should move in the same direction. https://www8.garmin.com/manuals/webhelp/forerunner35/EN-US/GUID-F2E7E0A9-FB44-4297-BF4D-D0C31C400C45.html
Arm movement should be no problem as long as the watch doesn't slide on your skin, hope my explanation is clear, I'm Dutch, not a native English speaker...
With my FR35 some sharp HR spikes occurred while walking, never happened while cycling, also not while riding on cobblestones with my road bike.