[thedoc (OP)]

Stefan Jonsson asked the Naked Scientists:
   Hi!
I listened to your show when you tried the freezing shower to see if a person could get better at coping with cold.
I'm a person who handles cold better than most of my friends. I can be good in a T-shirt (even outside for a short while in winter) when my friend wants an extra sweater indoors. But she will happily go swimming when I feel it's much to cold to jump into the water. Why can that be? Or am I just a wimp when it comes to taking a swim outside? ;-)
Thanks for the show!!
What do you think?

[evan_au]

Less body fat?
- In air, your skin temperature is maintained by your core temperature
- In water, you lose too much heat through your skin, and your core temperature is affected sooner? (a drop in core temperature is bad news!)

On another tack: How is your skin temperature in cold air (as measured by another person)?
- If your peripheral blood flow reduces by itself to maintain core body temperature, your fingers will feel abnormally cold.
- But your skin could feel severely cold in water.

[puppypower]

When you jump into cold water there is a sudden cold rush due to rapid heat transfer. With cold air, the heat transfer is much less, allowing your body to maintain temperature, so the same level of sensory rush does not occur.

A loose analogy is fire walking. The trick is to build a layer of sweat on the bottom of your feet to buffer the heat transfer. Without the sweat as a buffer the heat transfer is higher and can burn the skin.

Your skin can buffer cold air easier, than cold water, due to lower heat transfer coefficient. If we add wind to the cold air, to get convection, that can increase the heat transfer, so you get colder faster. But liquid water is has even better heat transfer especially since the quick plunge adds convection.

The Polar Bear Club will jump in the ocean during the winter. This is assisted by the cold air causing the skin temperature to fall before you do the plunger thereby lowering the thermal gradient at the skin.

[Colin2B]

......But she will happily go swimming when I feel it's much to cold to jump into the water.

The clue is in the 'she'. Women have more subcutaneous fat than men - average fat for a woman is 20-25%, man 15% - this not only acts as an insulating layer keeping heat in, but stops heat warming the skin and as Evan points out the temperature sensors in the skin tell us whether we are cold. So a woman's hands and feet are colder than a man's when the air temperature is cold. Research published in The Lancet medical journal in 1998 reported that women's hand temperatures were, on average, 2.8 degrees lower than men's.  "The blood flow to the hands and feet changes very rapidly so that we can lose or conserve heat. The blood vessels to the hands and feet are the first to constrict when we feel cold. So a woman will constrict the blood flow to the feet and hands before a man will, and this is why she feels colder." Despite this, The Lancet study also showed that women's core body temperatures are, on average, 0.4 degrees higher than men's – meaning they literally have cold hands, but a warm heart.
This also means that women have a more even response to cold eg when going into water, and can survive longer than men in cold water due to the insulating layer.

[Paul25]

It intrudes on the space of warmed air surrounding your body