The Effect of the Long-wave Reflectivity of Natural Surfaces on Surface Temperature Measurements Using Radiometers

Abstract

This paper deals with the influence exerted by natural surfaces with non-black body characteristics on surface temperature measurements using the radiation method. The difference between the true surface temperature and the temperature measured with a radiometer in the 8- to 15-μ region (the “radiation temperature”), is computed for smooth and rough water surfaces, and generally found to be below 1C. Spectra of vertical long-wave sky radiation were used for the case of specular reflection of a smooth water surface. The values for hemispherical long-wave sky radiation, necessary in the computation for rough water surfaces, were derived from the vertical spectra by a method developed for this purpose. The computations were verified empirically by measurements taken from a helicopter. The effect of the spectral distribution of reflectivity and long-wave sky radiation is also discussed. It is shown that for the measurement of water surface temperature a radiometer with an 8 to 11- or 12-μ bandpass would be more satisfactory. Furthermore, some “gray” reflectivities (infrared albedo) of natural surfaces for which no spectral reflectivity curves were available to date, have been determined by measurements. The difference between true surface temperature and radiation temperature is computed for gray reflectivities of 2.5, 5.0 and 7.5 per cent.