My first assumption was based on "evaporation causes cooling" but I realized that it is not the case as it is cooler even if I am submerged under it. Are all liquids generally cooler than air ? Just curious.
Firstly, to make a valid comparison between how water and air 'feels' on your skin, two conditions would need to be met:
- Both water and air would have to be at exactly the same temperature.
- That temperature would have to be lower than human body temperature (strictly speaking skin temperature).
If those conditions are met then water would certainly feel cooler than air. Several factors are responsible for this.
- Water has a much higher Specific Heat Capacity than air, making it a far better coolant than air.
- More intimate contact between water and skin, compared to air and skin, results in a higher Heat Transfer Coefficient which makes water again a better coolant.
- In the case of fairly thin layers of water evaporative cooling also takes place in the case of water on skin. As Latent Heat of Vaporisation is carried off the water will cool down and eventually skin will cool too due to heat transfer.
Two things to consider:
- You do not have temperature sensors sticking out into the environment. What you feel is not the temperature of the air or water; it is the temperature of the cells around the spot(s) where the temperature sensors are buried in the skin. That's why your face can "see" a hot stove element. The radiant energy from the hot metal reaches your skin and warms it slightly. As you turn your head, different skin cells get heated, allowing you to localize the hot source.
- There is a constant flow of heat energy from the body into the environment (as long as you're alive). A lot from the core, but also from each and every muscle in the arms and legs as well; the blood supply helps to move it around. Even when the outside temperature is well above body temperature, evaporative cooling can keep you at normal body temperature. A CSI at a murder at an oasis in the Sahara would need to measure how much the liver has warmed up to determine time-of-death.
So the question to answer is: how does the temperature and nature of the surrounding medium affect the temperature of the adjacent skin cells?
We're warm blooded so we will always be warmer than the surroundings. Water sucks heat out of your body faster than air of the same temperature. Water only conducts heat 25 times better than air. However, it can suck heat out of you much more than 25 times faster. Water even feels colder than glass of the same temperature even though it's actually only half as good a heat conducter. Heat conductivity is volumetric heat capacity times thermal diffusivity. For any gas, its thermal diffusitivity is approximately a constant times Kelvin temperature squared divided by pressure divided by molar mass and volumetric heat capacity is approximately another constant times pressure divided by Kelvin temperature divided by molar mass. That means heat conductivity for a given gas remains constant as pressure changes at constant temperature. Since water has a similar molar mass to that of air but conducts heat 25 times better, it can't be treated like a heavily compressed gas. For an infinitely heat conductive object suspended in a stationary fluid in the absense of gravity at a given amount higher temperature than the fluid, its rate of heat loss varies solely as the heat conductivity of the fluid. For any infinitely heat conductive object of a given amount higher temperature than the fluid its in, above a certain speed where heat diffuses a distance equal to its size in the time it takes to move its own length, the thickness of the fluid layer it's losing heat through varies as the inverse square root of its speed so its rate of heat loss varies as the square root of its speed. Also, given that's it's moving through a fluid then another fluid at the same speed which for both fluids is above the speed that that relationship holds. The thickness of the conducting layer varies as the square root of the thermal diffusivity and is independent of the volumetric heat capacity. That means the rate of heat loss is heat conductivity divided by square root of thermal diffusivity. Since water conducts heat 25 times better then air but also has a lower thermal diffusivity, it sucks heat more than 25 times faster. You don't conduct heat infinitely well but because of circulation, you're pretty close to doing so. That means the rate you lose heat will be the rate you would be predicted to lose heat up to a certain limit above which the rate of cooling of your surface becomes faster than that predicted if you conducted heat infinitely well. When you go into water, the water is moving around you and the rate of cooling predicted if you conducted heat infinitely well indeed is above that limit so your surface cools even faster until it reaches the water's temperature. That's why water sucks heat out of you so much faster than air.