older iPhones useless below -30F (Score 5, Informative)

I used to ski with an iPhone 6s in my the breast pocket of a synthetic puffy jacket worn under a GoreTex shell and over at least two other layers. When temps dropped below -30F, I regularly experienced problems with the phone claiming to me overheating before shutting itself down when I pulled it out for a photo. No good idea of the temp inside that pocket, but it seems likely it was well above -30F. Expose to the ambient air was 1 minute before the "overheating" message appeared. Take home? Don't rely on an iPhone for back country navigation in the winter!