I’ve often talked about vaso-constriction. As a reminder, upon immersion in cold water, in order to retain core warmth, blood flow to the extremities and skin slows. Upon exiting cold water a swimmer’s skin will feel really cold, even if the swimmer is experienced and may actually feel completely comfortable.
I watched the BBC’s Wild Swimming programme (synopsis, pretty poor). In talking about cold though they had a few useful images.
To set the scene, this was in an outdoor pool with temperature of 16 °C. (Really warm for many of us). But the presenter had no cold experience. At 16 °C she was gasping and going numb. After what seemed like two lengths, she had lost limb coordination and was having difficulty getting out of the pool. But that’s not why I bring it to your attention.
They had an infrared camera. Those images are useful for us, because regardless of your experience they apply at some stage. So I took some screen captures.
First was an image before immersion. You can see the higher heat radiative surfaces, particularly the arms and shoulders. Most of the body is at or well above 25 °C.
Next we have the image immediately after swimming. Look how cold the body surface is. From watching the program this seemed like it was only a few minutes. Most of the body temperature is below 20 °C, much of it at 16 °C to 18 °C. (The image was tilted that way in the original programme with half the temperature scale missing). Notice particularly the legs and torso, hands, nose, all around 16 °C. You can see the indicator on the upper torso at 17.2 °C.
You can see blood flow and heat returning quickly. There are hot spots in the forearm and biceps for example.
Just as a matter of interest, I used my infrared thermometer (€18 inc. shipping on eBay) on myself the other day. I’d swam for about 75 minutes in 10 °C water. I measured my lower torso at 18.1 °C.