FEATURES,  View from the Water

10 degrees is not twice as warm as 5 degrees, but it’s useful to assume that is

Regular readers will have noticed that we’re sometimes a little fussy at H2Open about our choice of words. We don’t like to describe wetsuited swimmers leaping into 20 degree water as ‘brave’ or ‘hardy’, nor do we like to use the word ‘freezing’ for water that is far removed from zero degrees Celsius.
Another thing that annoys us is to see 10 degrees Celsius described as twice as warm as 5 degrees, or 30 as twice as warm as 15. It’s not. It’s just a result of the scale we use. We can see this is nonsensical if we switch to Fahrenheit.

  • 5 degrees Celsius = 41 degrees Fahrenheit
  • 10 degrees Celsius = 50 degrees Fahrenheit

And 50 definitely isn’t twice 41, even if it feels like it as you approach your fiftieth birthday, but that’s a different topic.
The reason is that zero degrees Celsius is not the bottom of the scale. The coldest possible temperature, or absolute zero, is -273.15 degrees Celsius, or 0 degrees Kelvin.
So, 10 degrees Celsius (and this shouldn’t be taken too literally) is in fact only about 1.8 per cent warmer than 5 degrees, and 30 degrees is only about 5.2 per cent warmer than 15 degrees. Half of 10 degrees would be a very chilly -131.5 degrees Celsius.
However, despite this small difference in absolute terms, for swimmers the difference is hugely significant. In practical terms, we could consider 10 degrees Celsius to be twice 5 degrees as can be seen from the expected survival times in cold water without protective clothing. At 5 degrees, expected survival time is around 30 to 90 minutes (this is for someone whose is kept afloat with their airway clear – loss of dexterity can take less than three minutes and loss of consciousness can occur within 15 minutes). At 10 degrees, expected survival time is one to six hours (with loss of dexterity in 10 to 15 minutes and loss of consciousness in one to two hours), while at 20 degrees it’s three hours to indefinite.
Now, while we know there is a lot of individual variability and that trained, experienced and closely supervised swimmers can greatly exceed these figures, they do highlight the dangers of cold water.
So, while 10 degrees is not twice as warm as 5 degrees, when planning your swimming a reasonable rule of thumb might be to assume that it is. As we saw last week with Davina McCall, cold water needs to be treated with respect.
Picture: Colin Hill prepares for a cold water swim

Stay up to date with The Dip, our free weekly outdoor swimming newsletter.

I created Outdoor Swimmer in 2011 (initially as H2Open Magazine) as an outlet for my passion for swimming outdoors. I've been a swimmer and outdoor swimmer for as long as I remember. Swimming has made a huge difference to my life and I want to share its joys and benefits with as many people as possible. I am also the author of Swim Wild & Free: A Practical Guide to Swimming Outdoors 365 a Year and I provide one-to-one support to swimmers through Swim Mentoring.