Pyrgeometer

A pyrgeometer is a device that measures the atmospheric infra-red radiation spectrum that extends approximately from 4.5 µm to 100 µm.

It measures the resistance/voltage changes in a material that is sensitive to the net energy transfer by radiation that occurs between itself and its surroundings (which can be either in or out). By also measuring its own temperature and making some assumptions about the nature of its surroundings it can infer a temperature of the local atmosphere with which it is exchanging radiation.