5 expected measurement results – Campbell Scientific NR01 Net Radiometer User Manual
Page 12
NR01 Four-Component Net Radiation Sensor
coefficient of 1). For example, equation 2.4-2 calculates, in Kelvin, the sky
temperature:
T
sky
= (LW
in
/5.67.10
-8
)
1/4
2.4-2
The NR01’s pyrgeometers are type IR01. Pyrgeometers are not classified by
the ISO or WMO.
The atmospheric LW
in
radiation essentially consists of two components:
1 Low temperature radiation from the universe, filtered by the atmosphere.
The atmosphere is transparent for this radiation in the so-called
atmospheric window (around 10 to 15 micrometer wavelength).
2 Higher temperature radiation emitted by atmospheric gasses.
Down facing instruments are presumably looking directly at the surface, which
behaves like a normal blackbody.
As a first approximation, the sky can, be seen as a cold temperature source
with its lowest temperatures at zenith and getting warmer at the horizon. The
uniformity of this LW source is much better than that in the solar (SW) range,
where the sun is a dominant and non-uniform contributor. This explains why a
pyrgeometer with 150 degrees field of view can perform a good measurement.
Table 2.4-2 summarizes the main measurement errors for the IR01. The error
in the directional response is caused by non-perfect field of view. The window-
heating offset occurs when solar radiation heats up the instrument window,
producing a positive sensor offset.
TABLE 2.4-2. Main Measurement Errors in the LW Signal
Source
Maximum Error
Directional response
8 W/m
2
on LW
in
at -100 W/m
2
LW
net
Window heating offset
+15 W/m
2
on LW
in
at 1000 W/m
2
SW
in
Temperature
dependence
+/- 5 % for the entire range
2.5 Expected Measurement Results
The average energy balance at the earth surface strongly depends on the:
• Latitude (mostly for SW)
• Local surface properties (SW and LW)
• Local surface temperature (LW)
Table 2.5-1 summarizes the average global values. The average net radiation at
the earth surface is positive, and the remaining energy is used for convective
heat transport and evaporation.
8