When we are outdoors on a sunny day we feel the effects of heat on our skin even if the environment is at a low temperature. Likewise, when we are near the fire, we feel more heat on the surface of our body that is exposed to fire than on the opposite surface. These effects are caused by radiative heat transfer from the emitting source, the sun or fire, to your body. This radiative transfer is distinguished from other forms of heat transfer because it does not require a material medium to propagate. In material media heat transfer occurs by thermal conduction or convection. The thermal conduction is carried out in the solids by vibration of the crystal lattice and in electrical conductors also by free electrons. In liquids and gases it occurs mainly by the collision between molecules. The heat transfer by convection is that occurring by the macroscopic motion of matter.