Radiation instruments can accurately measure downwelling longwave radiation, also known as terrestrial or thermal radiation, which contributes to warming the Earth's surface. One common instrument used for this purpose is a pyrgeometer.
A pyrgeometer is specifically designed to measure the infrared radiation emitted by the Earth's surface and atmosphere. It is sensitive to the longwave radiation in the spectral range of approximately 4 to 100 micrometers, which corresponds to the thermal radiation emitted by objects at temperatures typically encountered on Earth.
Pyrgeometers work based on the principle of detecting the intensity of radiation using a thermopile detector. The thermopile generates an electric voltage proportional to the incoming infrared radiation, allowing the measurement of the downwelling longwave radiation.
These instruments are carefully calibrated to ensure accurate measurements. Calibration involves comparing the instrument's response to a known reference radiation source and adjusting the instrument's output accordingly. Calibration procedures and traceability to recognized standards are crucial to maintaining accuracy and reliability in radiation measurements.
It's worth noting that downwelling longwave radiation measurements are essential for studying and understanding the Earth's energy budget and climate system. They provide valuable data for assessing the radiative forcing and feedbacks associated with greenhouse gases and their impact on surface temperature and climate change.