Asked by: Mark Myerson, Exmouth
Water has a very high volumetric heat capacity. Raising the temperature of a cubic centimetre of water by 1°C takes more than 3200 times as much energy than you’d need to heat the same volume of air by the same amount. This means that the layer of water surrounding your body heats up only very slowly, the temperature gradient between the water and you stays very steep, so the rate of heat loss remains high.
Worse, because water is very dense, even a slight current will penetrate through clothes and between the hairs on your skin. This displaces the water you have already warmed up. On the other hand, air is easier to trap in a stationary boundary layer that can be warmed up and this acts to reduce the steepness of the temperature gradient. Neoprene wetsuits keep you warmer in water because their pores are fine enough to hold the layer of water next to your skin fairly still.