May 19th 2017, 2:57:03
So water is a special case because it has a very high specific heat capacity Cp[J/(Kelvin*kg)] and also quite high heat transfer coefficient h[W/(K*m^2]
When you talk about gases transporting heat less effectively that is very much dependent on pressure. At 200 atm, air will transfer heat probably 150 times better than at 1 atm.
When heat is transported in liquids and gases it is transported not only by conduction but also by convection and, depending on the temperature also by heat radiation. Heat radiation is usually not all that important at human body temperatures. It starts comming into play at around 330-500 Kelvin depending on the system. Ofc in space there is almost only radiative heat transfer.
In fact if a body is to be cooled down to hypothermia in air or water I would say it is 90% convection that matters.
Even convection can be split into two different kinds; Natural and forced convection. Natural convection around a body is the flow of fluid created by the temperature gradients that the body creates. Forced convection is stuff like wind, currents, waves and if the body is transported through the fluid. In theory you could have a system without forced convection, just as a closed unventilated room or a bathtub, but as soon as the body moves it will be forced convection to a degree.
So why does the density matter? Well because the heat transported by convection is a function of, among other things, the thermal inertia of the fluid that comes in contact with the body. Thermal inertia is density times heat capacity. While heat capacity is usually within an order of magnitude the density is about 3 orders higher in water than 1 atm air (1000 times).
Other things matter such as heat transfer coefficient and viscosity are also very different in gases versus liquids, but they also vary significantly between liquids.