The Magic Number

Cold water swimmers have a finely tuned thermo-receptive sense, an ability to judge water temperature reasonably accurately, just from how it feels on our skin. It’s something that develops with time, experience and swimming.

As an experienced pool manager my friend Clare told me that regular bathers will notice a temperature difference in warm water of half a degree. So you can imagine that cold water swimmers will probably have a similar ability.

I’ve written and often referenced my Precise Open Water Temperature Scale, one of this site’s most popular articles. It was a humourous attempt to convey some of that precision and personal experience. And the search term that still brings people in for this subject and is another of the site’s other most popular articles “what temperature of water is too cold to swim in“.

We refine this innate temperature-sensing ability as a result of regular immersion and the training of various aspects of the physiological response; habituation lowering our pre-immersion heart rate and eventually lowering stress hormones, and acclimatisation improving our cold exposure times, and we also get better at evaluating our thermo-receptive reaction. Of course you can only say that water feels like twelve degrees, if you have an actual thermometer so that you can do the comparison.

In those previous posts I’ve deliberately not put a precise figure on the question of what is too cold, because there are too many variables: the person experience, height and weight, the ambient air temperature and the local climate, the wind, the person’s drive to swim and more. I once read a Philippines swimmer say that water temperature of 21 degrees Celsius was too cold to swim. Of course the climate and ambient there is not just warm, but hot. Relatively cold water occurs at a higher temperature the higher the air temperature. The burden of constant cold for open water swimmers in Ireland, the UK and some other locations such as the Atlantic coast of South Africa, having no choice but to swim in cold water, is also an advantage, as we must acclimatise.

Nonetheless it is a fact that here in Ireland and in the UK and many other places such as the swimmers of San Francisco or Chicago, there is a Magic Number. That number is ten degrees Celsius, which (helpfully) is 50 degrees Fahrenheit. 10°/50° we could call it.

The Magic Number comes and goes. It goes when we get it in November or early December when the temperature is dropping and there’s rarely any temporary recovery. Once the water drops to 10°C, winter is here and there is little impetus to try to extend swims.

On the other hand we wait anxiously for the Magic Number to arrive, for the water to reach that level after the six long months of cold. The cold isn’t gone but at ten degrees hour-long swims can occur, often much more. Ten degrees, fifty degrees tease us more. The Magic Number may slide up for a weekend or a week only for the water to dip back to nine degrees again. It will probably, in a normal year, make its first appearance in April and dip again in early May.

Locations that were out of reach over the winter are once more attainable. Tramore pier and back or Doneraile Head laps can resume. I can start swimming to the Metalman or around Brown’s Island again, each year like rediscovering those locations anew. The Sandycove swimmers will be double-lapping the island. Our swimming range extends and the world grows large again.