It’s spring. We’re in the sea more or want to be, stretching out swimming times or wanting to. It’s probably more appropriate to talk about cold now than during the winter, because we are coming into the most difficult time of year for distance swimmers.
In the winter many stop sea swimming or switch to wetsuits. Those of us who keep swimming skin through the winter do it for the challenge, and only think about relatively short exposure times.
This is following a discussion on USMS time back. American swimmer Mike N_ (Hi Mike!), who engaged in a fantastic Open Water swimming holiday around Britain and Ireland…in January (crazy man)… directed me to a great paper on extreme cold adaptation in humans.
At various monthly meetings with Eilís for both of my Channel visits, and much to the annoyance of others swimmers wondering when I’d ever shut up (remember the recent introversion points), and going back further to my early days open water swimming, I kept coming up against one question no-one could really answer for me.
All things being equal, especially body size and insulation, why can one person stay longer in cold water than another?
Everyone prevaricates and says “experience”. I say why? They say “mentality”. Why? “Personal strategy”. Explains nothing really.
(Personal strategy is what I could call something like “I tell myself it’s warm“).
These were insufficient answers for me. Yep, geek. I wanted to know. Had the answer been something reasonable I would have forgotten about it. And remember, long swims in cold water, with that personality I mentioned the other day? Well, it’s hard to think about girls in bikinis on your tenth cold lap of Sandycove.
Here was my thinking: Think of a human as a “blackbody”. A blackbody is a physical method of simplifying calculation problems. Instead of dealing with a complex shape like the human body, think of it as a sphere. (It’s where physicist jokes about spheroid chickens in a vacuum* come from. Those are funny jokes by the way, just in case you are wondering).
So treat two people who are roughly the same size and weight as a sphere. Their surface area is the same so surface area to volume ratio is the same.
Let’s say they have around the same fat percentage. And that the water temperature is the same.
So we have similar figures which should all give a measurement of heat loss: The amount of heat loss of a sphere is directly proportional to surface to volume ratio (large bodies lose heat more slowly than small bodies. It’s why polar bears and some sea swimmers are big).
The rate will also be directly proportional to the temperature difference between the medium and the body. Heat loss will be mediated by insulation properties (fat).
Now setting aside the fact that there is little research that I could find in this area (but not none) predicting heat loss in humans (I even logged a question into the Wolfram Alpha forums), we have a hypothesis: heat loss in water is directly related to a short list of physical factors.
Here’s another way of looking at the same problem with which you’ll be more familiar. New or For Sale houses now in Ireland require a new Energy Rating. That figure gives how well the house performs from a heat loss point of view. If you combine this figure with the number of Degree Days (number of days in Ireland which the temperature is below 15.5C when you want the internal heat to be 18C) you can come up with an equivalent estimate for heating costs over a year, regardless of house shape.
But we can see that the issue of duration in cold water is not answered by treating people as a black-body radiation problem.
(If it was possible, I probably would have an answer to my Wolfram Alpha question, and we would have simple swimmer charts which say if you weigh W, have X body fat percentage, then your exposure duration in X degrees of water would be Y time.)
Something therefore must have been missing from my visualisation. Computer or calculation models are derived by coming up a certain set of basic principles, inputting some real world measurements, and seeing if the output mirrors the real world.
What did I not consider?
An answer is in the next post on this subject.
*A farmer noticed that his chickens were sick, and called in a biologist, a chemist, and a physicist to help diagnose the problem. The biologist observed the chickens, concluding, “I can tell you there’s something wrong with your chickens, but I don’t know what’s causing it.” The chemist took fluid samples from the chickens back to his lab, and returned saying, “I can tell you what’s infecting your chickens, but I don’t know how they got it.” Meanwhile, the physicist had been sitting on the floor, scribbling madly on several notebooks worth of paper. Suddenly, he jumped up, exclaiming, “I have the answer, but it only works for spherical chickens in a vacuum.”