Hypothermia starts at relatively high temperature (naked, below 25°c if I remember well)
There are records of people dying after hours of being exposed to 14°c without proper clothing (actually in the Philippine, if memory serves me well)
Our body (without technology like clothing or homes) are really just adapted to live around the equator at constant 27°c :-)
The Ph. are just off the equator. I’ve been in the water in northern Australia and the water was 30c and on the verge of being too warm. Definitely not refreshing, felt like a salty bath tub.
Due to water being a way better conductor of heat than air you lose heat up to 75% faster than in air. Even 30°C warm water can therefore quickly lead to hypothermia if unprotected. Ever seen scuba instructors in warm countries? While their customers would often submerge in nothing but a swimsuit the instructors would often wear 6mm full body wetsuits due to longer exposure. And yet a lot of them are shivering in the evenings.
I've been shivering in the water at 27 degrees Celcius in Australia (diving, no wetsuit, sitting on the bottom). Admittedly I have a very slender build, but clearly 27 degrees is below my equilibrium temperature when not in activity.
There are a few adaptions to colder conditions like Brown Fat and shivering which directly converts calories into heat and work fairly well.
Body Size and insulating fat at can also make a huge difference. People in warmer climates are simply less adapted to cold conditions and therefore at greater risk for a given temperature.
PS: Some people can actually walk around without clothing or becoming cold in 40f weather and be perfectly comfortable. It's sleeping that's the greatest risk as you reduce metabolic activity while having a large surface area in contact with the ground.
There is wide variety in human resistance to cold, including cold water. It isn't surprising that someone who had lived in Russia could spend a great deal of time in the tropical ocean. Some people who live in the tropics can do that!
Incidentally, this variation seems not to be genetic, or maybe it is epigenetic. When visiting India during the "winter" with a friend who was born there but had lived in USA for decades, his family was shocked at how both of us were comfortable in short sleeves while they shivered in heavy coats.
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