5. Overview
The SN500SS is a four-component instrument with individual upward- and downward-looking
pyranometers and pyrgeometers. Each radiometer consists of a thermopile detector and filter
mounted in an anodized aluminum housing. Each radiometer is heated to minimize the effects
of dew, frost, snow, and ice on the filter and sensor head. Analog signals from each radiometer
are measured with an onboard voltmeter and converted to SDI-12 outputs, eliminating the need
for multiple analog data logger channels to make the four-component measurement of net radi-
ation. The SN500SS is small and lightweight to facilitate mounting.
Net radiation at Earth surface is the source of available energy that drives key processes, includ-
ing surface and atmospheric heating, evaporation, sublimation, and transpiration. Shortwave radi-
ation (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 incom-
ing (downwelling) and outgoing (upwelling) shortwave and longwave radiation. Net radiation at
Earth 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 con-
ditions. 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.
Net shortwave radiation is the difference between incoming shortwave (from sun, SW
i
) and out-
going shortwave (reflected by surface, SW
o
). Net longwave radiation is the difference between
incoming longwave (emitted by molecules in the atmosphere, LW
i
) and outgoing longwave (emit-
ted by elements at the surface, LW
o
). Net radiation is the sum of net shortwave and net longwave
radiation. Net radiation changes with solar zenith angle, atmospheric conditions (for example,
degree of cloudiness), and surface conditions (for example, bare soil, plant cover, snow).
Typical applications of net radiometers include measurement of net radiation on surface flux
towers and weather stations. Net radiation is a key variable in the surface energy balance and
influences turbulent fluxes, including evapotranspiration.
SN500SS Net Radiometer
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