Net radiation at Earth’s surface is the source of available energy that drives key processes, including surface and atmospheric heating, evaporation, sublimation, and transpiration. Shortwave radiation (approximately 280 to 4000 nm) is emitted by the sun, and a fraction incident at Earth’s surface is reflected. Longwave radiation (approximately 4000 to 100,000 nm) is emitted by molecules in the atmosphere and land surfaces. Net radiation is the difference between incoming (downwelling) and outgoing (upwelling) shortwave and longwave radiation. Net radiation at Earth’s surface is spatially and temporally variable due to changes in position of the sun with respect to Earth’s surface, changes in atmospheric conditions, and differences in land surface conditions. Shortwave radiation accounts for a larger proportion of net radiation during the day when the sun is shining. Longwave radiation contributes to net radiation during the day and at night.
Typical values of the four components of net radiation (Rnet) for a clear summer day near solar noon over vegetation and a clear winter day near solar noon over snow are shown in the figure below (all units are W m-2 ). Net shortwave radiation is the difference between incoming shortwave (from sun, SWi) and outgoing shortwave (reflected by surface, SWo). Net longwave radiation is the difference between incoming longwave (emitted by molecules in the atmosphere, LWi) and outgoing longwave (emitted by elements at the surface, LWo). Net radiation is the sum of net shortwave and net longwave radiation. Net radiation changes with solar zenith angle, atmospheric conditions (e.g., degree of cloudiness), and surface conditions (e.g., bare soil, plant cover, snow).
Net radiometers are instruments designed to measure net radiation. Apogee Instruments SN-500 Net Radiometer is a four-component instrument, with individual upward- and downward-looking pyranometers and pyrgeometers to measure the individual components of net radiation.
Net radiation is a key variable in the surface energy balance and influences turbulent fluxes, including evapotranspiration. Applications include measurements on flux towers and weather stations.
Apogee Instruments' net radiometers are used by Alaska Electric Light & Power for avalanche forecasting. A live camera feed of the sensor mounted at Mt. Roberts shows the net radiometers ability to clear snow off the sensors through the low power heaters, the camera can be viewed here. Photo credit: Alaska Electric Light & Power