Geoscience Reference
In-Depth Information
Important points in this chapter
Measuring solar radiation
: either estimated from cloud cover (reported by
an observer, or from a
Campbell-Stokes
recorder that focuses the Sun's rays
to burn a card when skies are clear); or measured using a
Thermoelectric
pyranometer
(from the heating it induces on a blackened surface), or a
Photoelectric pyranometer
(from the electrical output, from a silicone voltaic
detector, that it generates).
●
Measuring net radiation
: either measured as the difference in temperature
between blackened surfaces using a thermopile in a
net radiometer
, or by
measuring all four components of net radiation separately using two
pyranometers to measure (upward and downward) solar radiation and
two pyrgeometers to measure (upward and downward) longwave
radiation.
●
Measuring soil heat flux
: measured using the temperature difference across
a
soil heat flux plate
(a disk a few centimeters in diameter, a few millimeters
thick, with thermal conductivity similar to soil) inserted typically ~5 cm
below ground, with thermometers above to estimate flux loss in the soil
between the surface and plate.
●
Micrometeorological measurement of latent and sensible heat
: two
techniques remain in common use - both involve deploying sensors meters
or tens of meters above the ground.
—
Bowen ratio/energy budget method. H
and
l
E
are deduced by simul-
taneously measuring (a) all the other components of the surface energy
budget, to determine the
sum
of
H
and
l
E
; and (b) the gradients of
temperature (strictly virtual potential temperature) and humidity (often
measured as vapor pressure), to determine the
ratio
of
H
to
l
E
. Individual
values are calculated from the sum and ratio of the two. Sometimes, when
gradients are small, accuracy is improved by interchanging sensors
between levels or ducting air from different levels alternately to a common
sensor.
—
Eddy correlation method
.
H
and
l
E
are deduced by multiplying the
instantaneous fluctuation in vertical wind speed with the instantaneous
fluctuation in the volumetric heat content of the air to give the
instantaneous value of
H
, and with the instantaneous fluctuation in
the volumetric latent heat content of the air to give the instantaneous
value
l
E
. Time-average values are found by integrating these
instantaneous flux values. Rapid response sensors with stable
calibration are required, these typically being interrogated several
times per second.
●
Integrated water loss measurement of evaporation
: involves defining a sam-
ple of the evaporating surface for which all the water entering or leaving can
be measured over the sampling period. Common measurements include the
following:
●
