Comparison of the standard pyrheliometer Å 158 with the SUB-standard instruments Å 70 and Å 171

Abstract

It is well known that the opening angle of a pyrheliometer has some
effect upon the measurement of the direct solar radiation. The reason
is that the instrument receivcs, in addition to the radiation from
the sun's disc itself, apart of the circumsolar sky radiation. This
part is the greater the larger the opening angle of the instrument.
The circumsolar sky radiation is greatest very close to the sun's
disc, it is increased by aerosols, i.c. by the turbidity of the air.
When Courvoisier in 1957 discussed the differcnce between the Ångström
Pyrheliometric Scale and the Smithsonian Scale, he pointed out
a difference of 1.77% between comparisons made close to the sea level
at Stockholm and at Davos, altitude 1590 m. This difference is
obviously due to the fact that the atmospheric turbidity in average
is greater at Stockholm than at Davos. Courvoisier expected that
there should be, in consequence, a correlation between the turbidity
variations on one hand and the variations of comparison readings at
Davos on the other hand. But he did not succeed in finding such a
relationship.

Nor at Stockholm, there is a striking effcct of the variations of
turbidity to be found in the series of pyrheliometer comparisons.
Nevertheless, such an influence might be expected since the aperture
of the reference instrument Å 158 is larger than that of modern
instruments. But, random errors of other kinds are generally greater
than the variation of the circumsolar sky radiation, this variation
being comparatively small since the comparisons are mostly made
under cloudless sky when the atmospheric conditions are rather
identical.

Lindholm (1963) pointed out the affect of the aperture in differences
between series of cornparisons madc at Davos, Stockholm and Visby
between the Swedish reference standard Å 158 and the Smithsonian
instrument S 14. He found that the Ångström reference instrument
gave 1.2% higher readings at Stockholm than at Davos because of its
larger opening angle.

The question of the affect of the opening angle came inte new
considerations after the international comparison meetings at Davos
in 1964. Firstly from August 24 to September 5, there was a comparison
of working standards from the whole world. This meeting was
followed by a regional one with participants from European countries.
During the first mentioned meeting, the standard pyrheliometer Å 158
was the instrument referred to. At the same time, the working standard
of the Davos observatory wes checked, and it served as reference
instrument during the European meeting. However, some deviations
difficult to explain happened during the latter meeting and the
discussions about them became rather animated. Ångström & Rodhe
presented 1966 a review on the affect of the circumsolar sky radiation
and tried to give a quantitative estimate on a theoretical as
well as empirical basis. The discussions about the comparisons at
Davos were closed at a symposium in Brussels in May 1966 and were
published in a Technical Note No 85 (WMO). In one of the lectures
at the symposium, Schöne (1967) presented data from measurements at
Potsdam indicating that the affect of the solar sky radiation might
have been overestimated by Ångström & Rodhe.

It was mentioned above that the affect of the circumsolar sky radiation
was difficult to prove in series of measurernents fixed to one
location, especially if the variations of atmospheric turbidity
there are small. Better results should therefore be achieved if the
investigations were performed at two sites under very different
atmospheric conditions. At the Swedish Astrophysical Observatory at
Capri it should be possible to make the comparisons under high
turbidity conditions. After some inquiries to the International
Foundation in Bern of the Jungfraujoch Scientific Station, the other
place where the turbidity is extremely low, was proposed: The Swiss
mountain hotel Gornergrat-Kulm at 3.130 metres above sea level. On
this top in the neighbourhood of Zermat and Matterhorn, the foundation
had recently equipped a branch station for astronomical investigations.