Abstract
Fractional cloud area is derived from spectral images collected by the airborne visible‐infrared imaging spectrometer (AVIRIS). AVIRIS covers the spectral region from 0.4 to 2.5 μm with 224 channels and has a ground instantaneous field of view of 20×20 m from an altitude of 20 km. The derivation is made by ratioing radiances near the 0.94‐ and the 1.14‐μm water vapor band centers against those in the intermediate atmospheric window regions. The derivation makes use of the facts that (1) the reflectances of most ground targets vary approximately linearly with wavelength in the 0.94‐ and the 1.14‐μm water vapor band absorption regions, and (2) the peak absorptions of the water vapor band over cloudy areas are smaller than those over nearby clear surface areas because of the rapidly decreasing atmospheric water vapor concentration with height. The band ratioing technique effectively discriminates among clouds and surface areas having similar reflectance values. Such discrimination is not possible using standard radiance thresholding techniques. It is expected that the use of water vapor channels in the near‐infrared region in future satellites will improve the ability to determine cloud cover over land.
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