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Upwelling natural light from the sea during daylight may originate from reflection from the sea bottom, scattering from within the sea, or reflection from the sea surface. Spectra of upwelling light were photographed from an aircraft at various places over the sea off the eastern coast of the United States. Subsequent calibration of the spectroscopic plates resulted in data which gave spectral irradiances on the underside of a flat, horizontal surface over the water. Curves of spectral irradiances were drawn for various locations and for different sea and atmospheric conditions. All curves were low in the red and rose gradually toward the blue; for example, over water off North Carolina near the Gulf Stream the irradiance was 4.0 μw×cm−2×mμ−1 at 700 mμ and 18.6 μw×cm−2×mμ−1 at 430 mμ. This result was typical of deep, clear water in clear weather at midday. Closer to shore in shallow and less clear water, irradiances peaked at about 450 mμ and decreased at shorter wavelengths. The curves showed evidence of the green color of this water. The measured spectral radiances of deep water were higher in the red than theoretical values which neglect surface reflections.
Stamm et al. (Sun,) studied this question.