Kipp&Zonen CGR 4 Pyrgeometers - Manual (Page 12)

2.4.1. Overcast sky
Typical for a cloudy overcast sky is that radiation emitted by the earth is absorbed 100% by the clouds.
Therefore, the overcast sky will re-emit the radiation (L
d
) 100%.
In theory, the net radiation (U
emf
/ S) will be zero, so the pyrgeometer detector output voltage (U
emf
)
will be zero. In practice, the detector output shows a small negative voltage (a few W/m²), due to a
small heat exchange between the relatively warm pyrgeometer and the colder sky.
In this case the calculated atmospheric long-wave radiation (L
d
) shows a relatively large positive value.
In the case of rain, the detector output will read zero, because water deposited on the pyrgeometer
window is a perfect infrared absorber. A cloudy overcast sky condition is illustrated in Figure 5.
Figure 5: Cloudy overcast sky condition
2.4.2. Clear sky
Clear sky conditions differ in that there is a relatively large heat loss caused by the atmospheric window.
The amount of re-emitted radiation by a clear sky is smaller compared to the overcast sky condition.
Because of the heat loss in the upward direction, the thermopile hot junctions will cool down and show a
relatively large negative net radiation value (from -90 to -130 W/m²). Therefore, the calculated atmospheric
long-wave radiation (L
d
) shows a relative small positive value. A clear sky condition is illustrated in Figure 6.
Figure 6: Clear sky condition
Page 12 CGR 4 Manual