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Applied Optics

Applied Optics


  • Vol. 44, Iss. 27 — Sep. 20, 2005
  • pp: 5712–5722

Colors of the daytime overcast sky

Raymond L. Lee, Jr. and Javier Hernández-Andrés  »View Author Affiliations

Applied Optics, Vol. 44, Issue 27, pp. 5712-5722 (2005)

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Time-series measurements of daylight (skylight plus direct sunlight) spectra beneath overcast skies reveal an unexpectedly wide gamut of pastel colors. Analyses of these spectra indicate that at visible wavelengths, overcasts are far from spectrally neutral transmitters of the daylight incident on their tops. Colorimetric analyses show that overcasts make daylight bluer and that the amount of bluing increases with cloud optical depth. Simulations using the radiative-transfer model MODTRAN4 help explain the observed bluing: multiple scattering within optically thick clouds greatly enhances spectrally selective absorption by water droplets. However, other factors affecting overcast colors seen from below range from minimal (cloud-top heights) to moot (surface colors).

© 2005 Optical Society of America

OCIS Codes
(010.1290) Atmospheric and oceanic optics : Atmospheric optics
(290.1090) Scattering : Aerosol and cloud effects
(330.1710) Vision, color, and visual optics : Color, measurement
(330.1730) Vision, color, and visual optics : Colorimetry

Original Manuscript: December 13, 2004
Revised Manuscript: May 12, 2005
Manuscript Accepted: May 19, 2005
Published: September 20, 2005

Raymond L. Lee and Javier Hernández-Andrés, "Colors of the daytime overcast sky," Appl. Opt. 44, 5712-5722 (2005)

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  31. Based on radiosonde data from nearby Dulles International Airport (code IAD), we estimated cloud Δz= 0.2 km and 1.0 km on 2-6-03 and 2-17-03, respectively. Perhaps surprisingly, using rλ for green grass in the 2-6-03 wintertime landscape was not unrealistic (this choice aids comparison with Middleton’s results in Fig. 18). Even though we do not know the actual mean Owings rλspectrum on 2-6-03, MODTRAN predicted negligible differences in overcast chromaticities when we tried several different materials (e.g., tree bark, a mixture of dead and living vegetation) for the snow-free surface’s rλ.

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