Campbell Scientific CNR1 Net Radiometer - Manual (Page 29)

Appendix A. CNR1 Performance and
Measurements under Different
Conditions
Below, Table A-1, shows an indication of what one might typically expect to
measure under different meteorological conditions.
The first parameter is day and night. At night, the Solar radiation is zero. The
second column indicates if it is cloudy or clear. A cloud acts like a blanket,
absorbing part of the Solar radiation, and keeping Net Far Infrared radiation
close to zero. The third parameter is ambient temperature. This is included to
show that the "sky temperature" (column nine) tracks the ambient temperature.
Under cloudy conditions this is logical; cloud bases will be colder than the
ambient temperature at instrument level, the temperature difference depends
roughly on cloud altitude.
Under clear sky conditions it is less obvious that sky temperature "adjusts" to
the ambient temperature. This can roughly be attributed to the water vapor in
the air, which is a major contributor to the Far Infrared radiation.
It is assumed that when ambient temperature varies, the Net Far Infrared
radiation remains roughly the same, independent of ambient temperature. The
resulting measured values of the CG3's and CM3's are stated in columns 4 to 7.
These are indicative figures only, they depend strongly on other circumstances;
the CG3 results, of course, change with the sensor temperature. This is
indicated in column 8. During the day, the Pt-100 reading may rise due to
solar heating, up to 10 degrees above ambient temperature. During the night,
the sensor temperature may be lower than the ambient temperature due to Far
Infrared radiative cooling. The latter two effects do not influence the end result
of the calculations of Sky T and ground T. Therefore they are not taken into
account in the table. Actually in column 4 one might expect to see "0 to -50"
for all positions that are showing "0", in column 5 the "0" values may in reality
be "-20 to +20". The resulting sky temperature is indicated in column 9.
Under cloudy conditions this sky temperature is equal to ambient temperature.
Under clear conditions the sky temperature is lower than the ambient
temperature.
The ground temperature in column 10 is assumed to be equal to the ambient
temperature. In practice it may be higher during the day, due to solar heating.
Ground temperature may be lower than ambient during the night, due to Far
Infrared radiative cooling. The sky and the ground temperature can be
calculated from the measured values of the sensors using Equations 4.6 and
4.7.
A-1