The Relative Sensitivities of Polyethylene Shielded Net Radiometers for Short and Long Wave Radiation

Abstract

Many commercially available net radiometers using polyethylene shields to protect their sensing surfaces have 5% of these surfaces covered with a highly reflective white paint. The purpose of this paint is to reduce the sensitivities of these instruments to short wave radiation and make them equal to a supposedly lower long wave radiation sensitivity. Theoretical considerations, however, yield no conclusive evidence for such a differential transmission of short and long wave radiation by thin films of polyethylene. Furthermore, rederivations of the net long wave radiation equations for the calibration chambers of J. P. Funk [J. Sci. Instrum. 36, 267 (1959); J. Geophys. Res. 67, 2753 (1962)] and L. J. Fritschen [J. Appl. Meteor. 2, 165 (1963); 4, 528 (1965)], who pioneered in the construction and evaluation of these instruments, indicate that certain approximations made in the original derivations completely account for the original 5% discrepancy. New experiments verify these findings and demonstrate the equality of short and long wave calibration constants for polyethylene shielded net radiometers without the customary 5% areal covering of white paint.