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

Abstract

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 .

Infrared Sky Surveys

“…After several stops and starts, the instrument was completed in 1965 and a near-infrared survey of the northern Galactic plane was begun that summer. Freeman Hall (1966) reported initial measurements of several extended sources such as the Crab nebula but left ITTFL shortly thereafter to complete his PhD on the effects of cirrus clouds on the 8-13 µm sky radiance (e.g., Hall 1968). …”

On the Transport of Polarized Infrared Signals Through the Terrestrial Atmosphere

“…Usually, but not always, the crystals tend to be oriented with their long axes randomly distributed within the horizontal plane. Although Hall, [1968] provides some evidence that the axes of the prisms or plates are oriented at random, the existence of pronounced optical effects suggests that at least on occasions preferential orientation of the crystals occurs, with one of their axes being predominantly vertical.…”

Degree of linear polarization in spectral radiances from water-viewing infrared radiometers

“…Infrared atmospheric emission by itself is usually unpolarized. Although infrared atmospheric radiance can become partially polarized by aerosols, ice crystals, and water drops, the net degree of polarization rarely exceeds 5% for wavelengths longer than approximately 3.5 m. 2,3,6 Multiple scattering and broad particle size distributions tend to obliterate most polarization signatures except in optically thin clouds such as thin cirrus. Such weakly polarized atmospheric radiance changes the results in Section 3 almost imperceptibly except at the shortest wavelengths, where the effect is no greater than that of reasonable changes to the aerosol model.…”

“…Water surfaces and clouds were some of the first partially polarized infrared sources discussed in the literature. [2][3][4][5] Egan and his colleagues 4 investigated infrared polarization with a polarimeter and computer calculations similar to those presented here. They measured infrared polarization for water surfaces and provided excellent insight into how the emitted and reflected radiances combine to produce the net polarization.…”

“…Polarization adds a dimension to passive infrared sensing that can yield information about a source or its environment beyond that conveyed by brightness and spectral content. As sophisticated infrared sensors have become more widely available, infrared polarization 1 has evolved from its largely astronomical roots into fields as diverse as environmental remote sensing, [2][3][4][5][6][7] military surveillance, 8 -15 and machine vision. 16,17 For example, in both ocean remote sensing and military surveillance, Fourier-transform infrared spectrometers are being used increasingly as passive emission spectroradiometers, sometimes without sufficient consideration of the instrument polarization sensitivity or the scene polarization state.…”