Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 9)

For measuring atmospheric radiation with some pyrgeometers,
such as the CGR 3, it is desirable to shield the instrument from
the direct short-wave solar radiation which may heat up the
pyrgeometer dome or window and cause significant thermal
offsets. The direct solar radiation is intercepted by a small disk or
sphere. The shadow of the disk must cover the pyrgeometer dome
completely. However, to follow the sun's apparent motion, a
power-driven tracking device is necessary.
This can be done using a Kipp & Zonen sun tracker, such as the
model 2AP, designed to track the sun accurately under all
weather conditions. More information about the combination of
pyrgeometer and tracker is given in the sun tracker manual.
Alternatively, a static shadow ring can be used to intercept the direct solar radiation; but it is less
accurate and may require periodic manual adjustment. At times the shadow ring also intercepts a
proportion of the diffuse sky radiation. Therefore, corrections for this to the recorded data are necessary.
Kipp & Zonen produces a universal shadow ring, model CM 121, which is suitable for use at all latitudes.
In the CM 121 manual, installation instructions and correction factors are given.
In practice the CGR 4 does not require shading from direct short-wave solar radiation because the
dome-heating effect, when suitably ventilated, is negligible due to the unique construction of the
pyrgeometer.
2.2.5. Installation for measurement of net long-wave radiation
A net pyrgeometer measures both the downward atmospheric long-wave radiation and the upward
long-wave radiation from the surface below. It can be configured from two CGR series pyrgeometers and
a suitable mounting plate. In the case of the CGR 3, two instruments can be simply mounted back-to-back
and an optional mounting rod fitted.
The requirements for installation of the upper and lower
pyrgeometers are the same as in paragraphs 2.2.1 and 2.2.3 for
downward and upward long-wave radiation.
A typical arrangement is shown in Figure 4. According to the
WMO the height should be 1 m to 2 m above a uniform surface
covered by short grass.
The mast shown intercepts a fraction D/2πS of the radiation that
is coming from the ground. In the most unfavourable situation
(sun at zenith) the pyrgeometer shadow decreases the signal by a
factor R²/H².
When determining the net long-wave radiation, it is not strictly
necessary to record the pyrgeometer housing temperatures.
Assuming that the temperatures of the upper and lower housings
are equal, it can be cancelled from the equation for net-radiation.
Figure 4: Net radiation configuration
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2.2.4. Installation for shaded measurement of downward long-wave radiation
Figure 3: 2AP Sun Tracker with
shaded pyrgeometer
CGR 4 Manual