All objects with a temperature above absolute zero emit electromagnetic radiation. The wavelengths and intensity of radiation emitted are related to the temperature of the object. The atmosphere and terrestrial surfaces (e.g., soil, plant canopies, water, snow) emit radiation in the mid infrared portion of the electromagnetic spectrum (approximately 4-50 µm).
Pyrgeometers are sensors that measure the net longwave radiation difference between the detector surface and surface the detector is directed towards (typically, atmosphere or ground surface). Longwave radiation emitted by the surface of interest can be calculated with the additional measurement of detector temperature, typically accomplished via an internal thermistor or PRT.
Typical applications of pyrgeometers include:
- measurement of incoming longwave radiation from the sky and outgoing longwave radiation from terrestrial surfaces in atmospheric and energy balance studies
- incoming longwave radiation measurements from pyrgeometers for frost prediction models
- integration into net radiometers for measurement of net radiation at the land surface