Kipp&Zonen CNR 4 Net Radiometers - Manual (Page 13)

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1.1.6.4 Calculation of the albedo for solar radiation
The albedo is the ratio of incoming and reflected Solar radiation. It is a figure somewhere between 0 and 1. Typical
values are 0.9 for snow, and 0.3 for grassland. To determine albedo, the measured values of the two pyranometers
can be used. The pyrgeometers are not involved, as they do not measure Solar radiation. Do not use measured
values when solar elevation is lower than 10 degrees above the horizon. Errors in measurement at these elevations
are likely and thus yielding unreliable results. This is due to deviations in the directional response of the
pyranometers.
Albedo = (E lower pyranometer) / (E upper pyranometer)
(1.3)
In the above formula, E is calculated according to formula 1.1.
Albedo will always be smaller than 1. Checking this can be used as a tool for quality assurance of your data. If you
know the approximate albedo at your site, the calculation of albedo can also serve as a tool for quality control of
your measured data at this specific site.
1.1.6.5 Calculation of the Net Solar radiation
Net Solar radiation is the incoming Solar Radiation minus the reflected solar radiation. It equals the solar radiation
that is absorbed by the earth's surface.
Net Solar radiation = (E upper pyranometer) - (E lower pyranometer)
(1.4)
In this formula E is calculated according to formula 1.1.
Net Solar radiation will always be positive. Checking this can be used as a tool for quality assurance of your
measured data.
Calculation of the Net Far Infrared radiation, soils temperature and sky temperature
Net Far Infrared radiation is, like Net Solar radiation, the part that contributes to heating or cooling of the earth's
surface. In practice most of the time, Net Far Infrared radiation will be negative.
Net Far Infrared radiation = (E upper pyrgeometer) - (E lower pyrgeometer)
(1.5)
In this formula E is calculated according to formula 1.2. From this equation the term with T cancels.
The E measured with the pyrgeometer, actually represents the irradiance of the sky (for the upward- facing
pyrgeometer) or the ground (for the downward-facing pyrgeometer). Assuming that these two, ground and sky,
behave like perfect blackbodies (actually this is only in theory), you can calculate an effective "Sky temperature" and
an effective "Ground temperature".
Sky temperature = ((E upper pyrgeometer)/ 5.67⋅10
-8 ) 1/4 (1.6)
Ground temperature = ((E lower pyrgeometer)/ 5.67⋅10
-8 ) 1/4 (1.7)
As a rule of thumb, for ambient temperatures of about 20 degrees Celsius, you can say that one degree of
temperature difference between two objects results in a 5 Watts per square metre exchange of radiative energy
(infinite objects):
1 degree of temperature difference = 5 Watts per square metre (rule of thumb)