I did my first cold-ish run yesterday in 42 degrees. It was the first time that my AW heart rate sensor was not totally awesome. I had a long sleeve shirt on but no gloves. Almost every time I glanced at HR during the first 1.5 to 2 miles, the watch either had no reading (grayed out) or it was 30 bpm to 40 bpm too high. After the first couple miles, it was fine the rest of the way. I was wearing another optical HR sensor at the same time that was working fine over the entire run. I remember reading a couple other posts from people having issues with HR in the cold. This is only my first, but I wonder if it will be a common thread as we enter winter. I would be curious to graph the HR from the AW against my Garmin, but there is no practical way to easily extract and graph AW HR data using Apple's app stack.