1981 Volume 32 Issue 2 Pages 79-88
Atmospheric turbidity is one of the important factors in monitoring climatic change but the accuracy of the present observation by the hand-operated type of sunphotometer may not be sufficient. The design feature of a new sunphotometer that measures atmospheric turbidity at discrete wavelengths is described. This sunphotometer was designed to measure solar intensity at seven wavelengths with less than 1-3% error by using interference filters and Si photodiode. Its specifications satisfy the WMO recommendations.
In order to derive accurate atmospheric turbidity, great care in calibration is required. In this paper, errors due to various temporal variations in atmospheric optical thickness are calculated and it is shown that the linear Langley plot in a larger airmass region is not always an index to good conditions for calibration. A method is proposed to estimate atmospheric optical thickness (τ) from the Langley plot when it changes as τ=A+Bm or τ=A+Bm2, where m is the optical airmass and A, B are constant.