Celestron CG-4 User Manual download pdf (Page 36)

OPERATION

4.1.3 Clear sky conditions

Clear sky conditions differ in the way that there is a relative large heat

loss caused by the atmospheric window. In this way the amount of re-

emitted radiation by a clear sky is smaller compared to the cloud

overcast sky condition. Because of the heat-lost in upward direction,

the sensors hot junctions will cool-down and show a relative large

negative net radiation value (from -90 to -130 W/m

). In this case the

calculated atmospheric long wave radiation (L

) shows a relative small

positive value. A clear sky condition is illustrated in figure 4.1B

Figure 4.1B Clear sky condition

4.1.4 Measurements during a sunny day

CG 4 allows accurate daytime measurements on sunny days without

the need for a moving shading device. Despite solar radiation up to

1000 W/m

the window heating will be less than 4 W/m

in the overall

calculated downward radiation.

Formula 1 can be applied without any problems with the following

exception: One must take note of the amount of Infrared radiation in

the solar spectrum. The amount of solar infrared radiation depends

Net radiation (from -90 to -130 W/m

)

Downwelling radiation

(Relative small value)

Atmosphere T

atm

Earth surface T

earth

>> T

atm

Upward radiation

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Celestron CG-4 User Manual Page 36