The air heats up, becomes less dense and starts to rise. This movement of matter carries your body heat away with it, and the rising air is replaced by new cool air for your body to heat up again. This is natural convection - when the wind starts to blow, whipping the air layer around your body away, this becomes forced convection. Either way, if you are inadequately prepared you will eventually suffer hypothermia. 3rd Edition 1990.
Thermal conductivity is defined by the rate at which heat flows through a certain area of a body.
Heat is normally conducted by atoms literally bumping into each other. In metals, electrons also add to the flow which is why metals tend to have high thermal conductivities. Thermal radiation is the transfer of heat energy by electromagnetic radiation - typically infrared, but visible or even shorter wavelengths for very hot bodies. The rate of radiative heat transfer varies as the difference of the fourth powers of the temperatures of two objects, and on the amount of surface area exposed, and on the emissivity of the surfaces. Emissivity, e , is a number between 0 and 1 which indicates how good a material is at emitting thermal radiation.