Campbell Scientific CNR4 Net Radiometer - Manual (Page 25)

CNR4 Net Radiometer
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 a specific site.
7.1.5 Calculation of Net Short-wave Radiation
The net short-wave solar radiation is equal to the incoming (downwelling)
short-wave radiation minus the reflected (upwelling) short-wave radiation.
Net Short-wave Radiation = (E upper Pyranometer)
– (E lower Pyranometer) (7-4)
In the equation above, E is calculated according to Equation 7-1.
Net short-wave solar radiation will always be positive. This can be used as a
tool for quality assurance of your measured data.
7.1.6 Calculation of Net Long-wave Radiation
The net long-wave far infrared radiation is the part that contributes to heating
or cooling of the earth’s surface. In practice, usually the net long-wave far
infrared radiation will be negative.
Net Long-wave Radiation = (E upper Pyrgeometer)
– (E lower Pyrgeometer) (7-5)
In the equation above, E is calculated according to Equation 7-2. According to
Equation 7-5 above, the terms that contain the sensor body temperature, T,
cancel each other. Therefore, if one is only interested in the net long-wave
radiation, instead of separate upper and lower components of the long-wave
radiation, the CNR4 temperature measurement is not required.
The E measured with the pyrgeometer actually represents the irradiance of the
sky (for upward-facing pyrgeometer) or the ground (for downward-facing
pyrgeometer). Assuming that these two, ground and sky, behave like perfect
blackbodies, theoretically, one can calculate an effective “sky temperature” and
an effective “ground temperature”.
4 / 1
8 10 67 . 5
r Pyrgeomete upper E
e Temperatur Sky
⎥
⎦
⎤
⎢
⎣
⎡
⋅
=
−
(7-6)
4 / 1
8 10 67 . 5
⎥⎦
⎤
⎢⎣
⎡
⋅
=
−
r Pyrgeomete lower E
e Temperatur Ground
(7-7)
As a rule of thumb, for ambient temperatures of about 20 degrees Celsius, one
can say that one degree of temperature difference between two objects results
in a 5 W/m
2 exchange of radiative energy (infinite objects):
1 degree of temperature difference = 5 W/m 2 (rule of thumb)
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