On Pyrheliometric Measurements

Abstract

Pyrheliometric measurements are now introduced in the meteorological practice to an extent which makes a condensed survey over the present status desirable.

In the following I have combined the main content of the survey on the pyrheliometric scales, given by the International Radiation Commissions, with a description of the present modernized type of the Ångström Pyrheliometer. I have added a critical examination of its function and the sources of error which ought to be avoided at its operation. A detailed survey of its construction seems justified by the fact that it is the only standard instrument at present available in the market, where institutes can easily determine its physical constants, thus controlling the basis for absolute measurements of solar radiation. I have added an Annex containing a simple theory of the “edge effect” in the pyrheliometer. The main content of the annex coincides practically with the theory I gave already in 1914, but I repeat it here with some minor corrections together with a discussion of the factors influencing the effect and also of the results of practical experiments.

The important result is derived, that the edge correction is to some extent dependent on the internal convection within the pyrheliometer, and may from this reason vary between about 1.3 and 3.8 percent of the measured radiation. In the new constructions, now available in Sweden, this error is eliminated.