Two bodies at differing temperatures (and within sight of each other) will exchange heat energy via thermal radiation.
To calculate the radiation heat transfer between 2 bodies, enter the parameters below. The two bodies are assumed to be fully enclosed by a third nonconducting body, such that all radiation energy hitting this third surface will immediately re-radiate back into the cavity.
To simulate a single hot object fully surrounded by a large cavity (such as a hot brick sitting in a room), enter a View Factor of 1, and make sure that the second body has a much larger surface area than the first.
The default calculation is for a situation experienced by many living in cold winter climes. Ever wonder why a cloudless night sky feels so much colder than a cloudy night? The reason is that more body heat is lost to the cold clear sky; a cloud layer acts as a radiation barrier. Default values are for a hatless head at 95 deg F radiating to a cold clear sky at -150 deg F, with answers rounded to 3 significant figures.