The Effect of Cirrus Clouds on 8-13-micro Infrared Sky Radiance

An experimental investigation of ir sky radiance and radiance fluctuations in the 8-13-micro atmospheric window is reported. Measurements were made with ground-based, filtered bolometer detector radiometers under clear sky and cirrus overcast conditions. Sky radiance was measured very close to the limb of the sun to permit detection of the solar aureole caused by forward scattering by cirrus ice crystals. Polarized sky radiance was found at large zenith angles and is attributed to scattering by cirrus of thermal emission from the earth. The radiance due to tropospheric water vapor is predicted by means of a radiation chart. Measurements of clear sky radiance exceeded that predicted by the chart in all but one case. The radiance of visible cirrus greatly exceeds the radiation chart prediction. Diffraction about cirrus cloud particles leads to a prediction of a solar aureole of a size that corresponds to the measured aureole. It is concluded that even a cirrus haze, which is quite difficult for an unaided, observer to detect, can cause an excess zenith radiance of 0.1 mW cm(-2)sr(-1), which increases to twice this value at a zenith angle of 60 degrees . Even thin but visible cirrus clouds can easily produce an excess zenith radiance of 1 mW cm(-2)sr(-1), which increases by a factor 1.4 at a zenith angle of 60 degrees .