Ok, so the simple real world answer is. Hold your hand or throw a digital thermometer on the output line or the head/cylinder of a larger air compressor, something you could run an impact wrench or air nailer off of. The metal line between the head and the tank is usually the hottest. You are compressing air from ambient to 110psi and that is roughly in the same range as the cylinder compression on a motorcycle engine. There is no fuel involved so you are only dealing with the heat caused by friction and the compression of air. BTW, they get friggin hot so be careful.