2
»min-1 = 0.069 W»cnr
2
1 J*cm
-2*
min-
1
= 0.238 cal»cm-
2
min-
1
= 0.0165 W * cm bb- 2
1 W •cm"
2
= 1 4 . 3 cal*cm
- 2
min-
1
= 6 0 . 5 J*cnT
2*
min-
1
- In order to ensure the reliability and comparability of
radiation measurements, the sensors have to be checked
frequently and calibrated at regular intervals against
reference instruments at national or regional centres as
recommended by WMO.
2 Instruments
2.1 Methods of measurement
211 To measure the duration of sunshine to the nearest tenth of
an hour, four physical principles are mainly used.
- radiation from the sun is focused to burn a trace in ab bchart
- radiation from the sun is made to record a trace on
photographic paper
- thermometric (bimetallic) switches controlling either
mechanical or electrical recording devices
- photoelectrical sunshine switches.
212 To measure solar and terrestrial radiation, two basic
physical principles are used
- the absorptive and emissive properties of black and white
surfaces resulting in temperature changes and differences
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