250
In Fig. 7 and Fig. 8 the values of global radiation Q measured using
different instruments are presented. To measure diffuse radiation a shading
ring was used but the corresponding correction is not taken into account,
which explains the disagreement of the results.
Chapter 25
4.
NET RADIOMETERS
Net radiation, as a difference between all incoming and outgoing
radiation fluxes, includes a wide spectral region containing both the solar
radiation and thermal radiation of the atmosphere and the ground surface.
Therefore, the correct measurement of net radiation requires instruments that
are non-selective in the spectral region
Moreover, it is quite
complicated to eliminate the influence of other parameters (wind speed,
temperature, etc.) other than radiation on the output signal. That is why net
radiation measurements are much more complicated in comparison with
short-wave (solar) radiation measurements, and why the comparison
between different instruments is unsatisfactory (Dehne et al., 1993, Halldin,
Lindroth, 1992).
An “ideal” net radiometer, as mentioned above, must be non-selective in
the wide spectral region between
Due to major technical
difficulties in the long-wave calibration, net radiometers are, as a rule,