Actinometric Measurements near Stockholm 1930-1936

Summary

After 1930 current actinometric measurements have been carried out under the supervision of statens Meteorologisk-Hydrografiska anstalt at experimentalfältet near Stockholm. The place of observation is situated about 3 km north of the city of Stockholm at comparatively plane ground. The nearest surroundings consist of parks and suburbs of villa residences. The air is here comparatively free from the products of combustion emanating from the industries and residences of the city.  Yet at low sun in winter the sun even in the middle of the day is not higher than about 10° above the horizon, and the radiation valucs will then probably to some extent be influenced by products of combustion adhering to the city.
The measurements were after May 6, 1931 taken with aid of the Michelson
Actinometer No. 535 delivercd early in 1931 by G. Schultze, Potsdam. Before
that time the Michelson 129 was generally used. These instruments have been compared at intervals with two Ångström compensation pyrheliometers, namely No. 152, kept as standard instrument, and No. 158. These latter instruments have both been standardized through comparison with tbe international normal instrument at Uppsala.
The constant of the Michelson instrument was at the delivery by thc Institute
in Potsdam referred to the Smithsonian Scale as standard; at the time of the delivery Marten makes the statement in a paper included in "Bericht uber die Tätigkeit des Preussischen meteorologischen instituts im Jahre 1930", that the difference between the Smithsonian and Ångström scales as represented by secondary standards kept at Potsdam is in 1930 found to be 3.85 per cent, while the difference in 1913 was determined to 3.4 per cent. The value 3.85 per cent may consequently be regarded as the diffcrence to be expected for the Michelson instrument at the delivery. On the 12th of April 1934 a careful determination of the difference between the Michelson instrument and the two Ångström pyrheliometers was carried out at Experimentalfältet. The Ångström instruments differed with about 0.3 per cent from one another, and their readings averaged 3.3 per cent below the Michelson No. 535.
On May 3rd 1937 a carefui comparison was made again. The difference between Michelson 535 and Ångström 158 was found to be 3. 75 per cent and for the difference between Michelson and Ångström No. 152 exactly the same value of 3. 75 was obtained.