Celestron CG-4 User Manual download pdf (Page 35)

OPERATION

4.1.1 Example

During field measurements the pyrgeometer is exposed to varying

atmospheric conditions with typical radiating properties. Therefore we

define the two most common conditions known as, cloudy overcast

sky and clear sky.

4.1.2 Cloudy overcast sky

Typical for a cloudy overcast sky is that radiation emitted by the earth

is absorbed 100%. Therefore the overcast sky will re-emit the

radiation (L

) 100%.

In theory the net radiation (Uemf / S) will be zero, so the pyrgeometer

thermopile output voltage (Uemf) will be zero. Practically the

thermopile output shows a little negative voltage (a few Watts per

meter square), due to a small heat exchange between a relatively

warm pyrgeometer and a colder sky.

In this case the calculated atmospheric long wave radiation (L

) shows

a relatively large positive value. In the case of rain, the thermopile

output will read zero, because water deposited at the pyrgeometer

window is a perfect infrared absorber. A cloudy overcast sky condition

is illustrated in figure 4.1A

Figure 4.1A Cloudy overcast sky condition

Downwelling radiation

(Relative large value)

Atmosphere T

atm

Earth surface T

earth

> T

atm

Upward radiation

Net radiation (from 0 to -5 W/m

)

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Celestron CG-4 User Manual Page 35