Methinks swimming in the outdoor pool is over for this summer. I emerged from the pool at lunchtime feeling that I’d just spent a day in Antarctica. I’m told that the peak daily temperature has now dropped to about 22 degrees.
Twenty-two? It doesn’t sound that bad. If the air temperature were 22 it would feel quite pleasant. So how come it feels so much nastier when you are in the water?
It’s simply that water does a far better job of removing heat from your body than air. How comfortable you feel depends on how quickly your body heat is being lost to the environment. Water is a thousand times denser than air – a litre of water weights a kilogram, but a litre of air weighs about a gram.
So there are many more water molecules than air molecules that can act in the process of taking heat away from your body. To maintain your body temperature at 37 degrees when you are surrounded by water at 22 degrees, your metabolism has to work overtime – whereas in air at 22 degrees its easy. ‘Windchill’ is a similar sort of effect; in a strong wind there are many more molecules that rush past you, and each can suck away a bit of heat from your skin. So you feel colder, even though the temperature of the air is the same.
These effects are used in your car engine’s cooling system. The cylinders generate heat because of the combustion of the fuel, and this heat is removed in the first instance by passing water around the engine. Then the heat is removed from the water through the radiator, where it gets moved to the air. The many blades on the radiator give a large surface area through which the heat can be transferred. And of course the faster the car is going, the better the cooling (the greater the windchill), since the more rapid the airflow over the radiator.
Unfortunately knowing this doesn’t help you when you are swimming. It’s cold – that’s all there is to it.