Lets put it in nerd terms, shall we? Water has a high specific heat, meaning it takes a lot more energy to raise the temp of water than of air. Part of this is due to density. A cubic foot of water weighs in at a nice 62.425 pounds. A cubic foot of air only weighs a fraction. According to this source:
FINAL ANSWER: 1 cubic foot of air at standard temperature and pressure assuming average composition weighs approximately 0.0807 lbs.
Density isn't the only factor though. A cubic foot of iron weighs about 450 pounds depending on composition.
However iron only has a specific heat of 0.108 (kcal/kgoC)
Water has 4.184J/g oC
That is joules per gram so we multiply by 1000 for a kilogram to get 4184 and convert joules to calories to get 0.999976 (kcal/kgoC)
1 joule = 0.000239005736 kilocaloriesFor simplicity's sake we'll say Iron has a specific heat about 1/9th that of water even though its far more dense.
The point of all this jiggery-pokery with the calculator is that water dissipates heat much more quickly than air at a given temperature. When you exert yourself your body generates heat by friction and the actions it uses to break down ATP and fat and all that good stuff. Any conversion of energy, in the case of exercise its chemical to mechanical, there is waste heat produced.
On a bike or a treadmill or anything based in air your body temperature will rise, you'll sweat and the sweat will by evaporation mitigate the buildup of heat to some extent. When surrounded by water you still generate the same heat, more if you're burning more calories in a given time span, but that heat is constantly fed into the surrounding water.
Anyone who's ever popped the cap on a hot radiator knows that water can absorb a load of heat. Cars are water cooled, boats are ironically air cooled. In PC terms the specific heat of a liquid that is mostly or all water will be able to move more heat away from a hot processor or gpu than air alone.
Without that rise in body temperature, the body is actually cooled faster than it can produce heat to warm itself. If you spend long enough in an 80 degree (Farenheit) pool then you can actually get hypothermia. It will take hours but the same effect will not happen in air unless you're starving or their is a strong wind blowing (more on convection in just a bit)
Actually lets talk about convection right now. When a fan is 'cooling' its really just moving air around. Blowing air over a neutral object that isn't producing any heat will not cool it. If you spritz water on that same object the evaporation will cool it. If the object is an animal or other heat producing object then the generated heat will be lessened due to convection.
While running the flow of air that you feel is partially due to your pushing through it. If the air is at a constant temperature then it will absorb some heat and then quickly be replaced by air at the default temperature. The same is true of water as you push through a pool. If you were in a confined room with a fan blowing then started doing jumping jacks you would feel a rise in the ambient temperature in just a few minutes.
In a pool this would take MUCH much longer because though the energy you're exerting is the same the medium that is absorbing it takes much more energy to raise or lower its temperature. Remember a kilocalorie, or food calorie, is the amount of energy it takes to raise one kilogram of water by one degree (at zero degrees Celsius) a 25x25x1.5 yard pool contains about 25312.5 cubic feet of water. That amount of water has a mass of about 716748.75 kilograms and so you'd have to burn over 700,000 calories to produce A SINGLE DEGREE temperature rise.
That's impressive, since it would take nearly a full gigacalorie to produce even such a modest goal. So, since we are constantly losing heat to the water and its constantly cooling us we experience no body temperature rise and therefore no secondary loss of appetite associated with running or other dry land activities.
I can attest to this today as all I have had to eat so far has been a diet sundrop and a protein shake. 200 calories and I have been up for almost 4 hours. If I had been swimming today I would be ravenous and probably would have needed a snack or a bigger breakfast than just some whey protein in milk.
It will be interesting to monitor this phenomenon over the coming weeks. Its also at least partially indicitive of why swimming burns so many calories compared to other stuff. We have to constantly maintain our body temperature or crap starts shutting down and the body is probably burning extra fuel just to keep you from freezing in the pool.
06/17/2008, 249 lbs, 13.8 MC