A Pyrgeometer is an instrument that can measure Downwelling Longwave Radiation. They are not part of basic meteorological observation stations, but may be included at first-order stations. 

Any object that has a temperature greater than absolute zero emits electromagnetic radiation. The wavelengths and intensity of the emitted radiation are proportional to the temperature of the item. The pyrgeometer is an instrument that measures the rate at which these emitting surfaces lose heat to space by measuring the downward longwave (infrared) radiation in the mid-infrared region of the electromagnetic spectrum (about 4-50 µm).

Pyrgeometers measure downward longwave radiation with a resistance-changing element. When the element is placed in the path of infrared radiation, its resistance/voltage changes proportionally to the amount of energy transferred. The thermopile sensor is incased in a black material to ensure the most infrared radiation possible is absorbed. When infrared radiation hits the thermopile sensor, two wires of different materials, typically nickel and copper, absorb energy and heat to different temperatures. This difference creates the voltage difference. Downward radiation is measured based on the voltage changes of a thermopile which changes in proportion with the amount of energy transferred. Measurements from pyrgeometers take into account other atmospheric variables such as water vapor and clouds that can absorb or reflect longwave radiation. Finally, the data is used to calculate the infrared radiation flux, which is a measure of radiation in $//<![CDATA[ \begin{array}{l}\frac{W}{m^2}\end{array} //]]>$.

Required Parameters

At least one radiation gage with observed data must be input to use this method. The user will assign a radiation gage to each subbasin within the Meteorologic Model editor.

Note

This is the recommended choice for use with the Priestley Taylor Evapotranspiration Method, where an effective radiation is used which includes both shortwave and longwave radiation.