I’ve been using the Apple Watch series 5 now for a few months and heart accuracy has definitely dipped during colder runs. Sadly I no longer trust it during the winter.
The picture below shows the first 7km of a run with the Apple Watch recoding HR (as my wife’s device grabbed my external Polar OH1) and then a huge dip as we disconnected her watch and connected my watch to the Polar OH1.
![](https://i0.wp.com/greghilton.co.uk/blog/wp-content/uploads/2020/01/img_1398.png?resize=640%2C1385&ssl=1)