Geoscience Reference
In-Depth Information
(a)
(b)
Figure 7.1
(a) Campbell-
Stokes sunshine hour
recorder; (b) a card after it has
been removed from the
recorder showing burns made
on the card by the focused
solar beam when there was
bright sunshine during the
day. The length of the burns
are measured to determine
the number of bright sunshine
hours. (From Fairmount
Weather Systems, 2010.)
Length of burns
are measured
Photograph of a burnt card
sky while 8 oktas, or 10 on the decimal scale, indicates overcast sky. Estimates
made in this way are, of course, only representative of conditions within the range
of visibility of the observer. Problems associated with this method include inability
to make observations when the visibility is very low, when it is foggy for example,
and difficulty in estimating the correct fractional cover for clouds that are near the
visual horizon. Quite often just one visual estimate of cloud cover is made each
day, often at the same time as other daily measurements (such as rainfall), although
several observations through the day are required to give an estimate of the daily
average because there is often a marked diurnal variation in cloud cover.
John Francis Campbell (1822-1885) invented the first simple instrument to
estimate daily average cloud cover indirectly in the form of the number of hours of
the bright sunshine expressed as a fraction of the maximum number of hours for
which bright sunshine is feasible on the day observations are made. The device he
invented, which is often called the
Campbell-Stokes
recorder, comprises a sphere of
glass that serves to focus the Sun's rays onto a card, see Fig. 7.1a. When the Sun is
exposed it has sufficient energy to burn the card and as it moves in the sky, the
length of the burnt trace on the card can be later measured and interpreted in
terms of the time without cloud during the day, see Fig. 7.1b. At the time of writing,
this is by far the most common radiation instrument used at agro-climate stations
worldwide.
Thermoelectric pyranometers
Figure 7.2 shows a thermoelectric sensor of solar radiation often called a
Kipp
pyranometer. When solar radiation is incident on a surface which has properties
close to those of a blackbody, the surface temperature rises. The warmed surface
loses energy to its surroundings and an equilibrium is established between the
incoming solar energy and this outgoing heat loss. The equilibrium temperature of
the absorbing surface is related to the strength of the solar beam. In the Kipp
