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Appendix A. CNR4 Performance and

Measurements under Different

Conditions

Below, Table A-1 shows what one might typically expect to measure under

different meteorological conditions.

The first parameter is day and night. At night, the solar radiation is zero. The

second column shows if it is cloudy or clear. A cloud acts like a blanket,

absorbing part of the solar radiation, and keeping net far infrared radiation

close to zero. The third parameter is ambient temperature. This is included to

show that the "sky temperature" (column nine) tracks the ambient temperature.

Under cloudy conditions this is logical; cloud bases will be colder than the

ambient temperature at instrument level, the temperature difference depends

roughly on cloud altitude.

Under clear sky conditions it is less obvious that sky temperature "adjusts" to

the ambient temperature. This can roughly be attributed to the water vapor in

the air, which is a major contributor to the far infrared radiation.

TABLE A-1. Typical output signals of CNR4 under different meteorological conditions.

Explanation can be found in the text.

1 2 3 4 5 6 7 8 9 10

Day

night

Cloudy

clear

+20ºC

-20ºC

Pyrgeo-

meter

Pyrgeo-

meter

low

Pyrano-

meter

Pyrano-

meter

low

100

sky T

ground

d cloud +20 0 0 0-500 0-150 20 20 20

d cloud -20 0 0 0-500 0-150 -20 -20 -20

d clear +20 -100* 0 0-1300 0-400 20 1* 20

d clear -20 -100* 0 0-1300 0-400 -20 -53* -20

n cloud +20 0 0 0 0 20 20 20

n cloud -20 0 0 0 0 -20 -20 -20

n clear +20 -100*** 0 0** 0 20 1*** 20

n clear -20 -100*** 0 0** 0 -20 -53*** -20

* Values may suffer from the so-called window heating offset; the sun heats the pyrgeometer window causing a

measurement error of + 10 Watts per square meter (maximum).

** Values may suffer from negative Infrared offsets, caused by cooling off of the pyranometer dome by far

infrared radiation. The maximum expected offset value is 15 Watts per square meter.

*** Values may suffer from dew deposition. This causes the pyrgeometer-up values to rise from -100 to 0 Watts

per square meter.

A-1

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 59 60

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