Geoscience Reference
In-Depth Information
(b)
(a)
Downward radiation
(all wavelengths)
Dry Nitrogen
to inflate
domes
Polythene domes
(c)
+
−
Treated surfaces
absorbs radiant
energy at all
wavelengths
“Thermopile” measures
temperature difference
between surfaces
Upward radiation
(all wavelengths)
Figure 7.4
(a) Schematic diagram of a thermoelectric net radiometer which measures the difference in radiant energy at all
wavelengths arriving from above and below; (b) A simple thermoelectric net radiometer; (c) Kipp and Zonen CNR 4
thermoelectric net radiometer which measures all four components of net radiation using four separate sensors. (From Kipp
and Zonen, 2010.)
the two active surfaces, see Fig. 7.4b. Sometimes these domes are inflated using dry
nitrogen at slightly greater than atmospheric pressure.
Net radiation can also be obtained by measuring all four components of net
radiation (i.e., upward and downward shortwave radiation, and upward and
downward longwave radiation) separately. Instrument packages can be obtained
comprising four separate radiometers, two pyranometers to measure the solar
fluxes and two pyrgeometers to measure the longwave fluxes, see Fig. 7.4c. These
radiometers are thermoelectric sensors operating similarly to thermoelectric
pyranometers but with appropriate wavelength filtering to select the required
wavebands.
Measuring soil heat flux
Soil heat flux is measured using soil heat flux plates. These are circular disks a few
centimeters in diameter and a few millimeters thick made of material with a
thermal conductivity that is broadly similar to that of soil. The assumption is that
because the thermal conductivity is similar, when the disk of material is inserted
horizontally into the soil the flow pattern of heat in the soil is not greatly disturbed.
There are shortcomings in this assumption because the thermal conductivity of
soil changes substantially with soil moisture content. A thermopile with the bimetal
