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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

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

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  1. B. A. Kimball, S. B. Idso, and J. K. Aase, "A model of thermal radiation from partly cloudy and overcast skies," Water Resour. Res. 18, 931-936 (1982).
  2. Harshvardhan, W. Ridgway, V. Ramaswamy, S. M. Freidenreich, and M. Batey, "Spectral characteristics of solar near-infrared absorption in cloudy atmospheres," J. Geophys. Res. 103, 28793-28799 (1998). [CrossRef]
  3. C. Erlick, J. E. Frederick, V. K. Saxena, and B. N. Wenny, "Atmospheric transmission in the ultraviolet and visible: aerosols in cloudy atmospheres," J. Geophys. Res. 103, 31541-31555 (1998). [CrossRef]
  4. W. E. K. Middleton, "The color of the overcast sky," J. Opt. Soc. Am. 44, 793-798 (1954).
  5. V. Hisdal, "Spectral distribution of global and diffuse solar radiation in Ny-Ålesund, Spitsbergen," Polar Res. 5, 1-27 (1987).
  6. J. Hernández-Andrés, R. L. Lee, Jr., J. Romero, and J. L. Nieves, "Color and spectral analysis of daylight in southern Europe," J. Opt. Soc. Am. A 18, 1325-1335 (2001).
  7. E. M. Feigelson, Radiation in a Cloudy Atmosphere (Reidel, Dordrecht, 1984), pp. 52-62, 164-169.
  8. S. Nann and C. Riordan, "Solar spectral irradiance under clear and cloudy skies: measurements and a semiempirical model," J. Appl. Meteorol. 30, 447-462 (1991). [CrossRef]
  9. D. A. Siegel, T. K. Westberry, and J. C. Ohlmann, "Cloud color and ocean radiant heating," J. Climate 12, 1101-1116 (1999). [CrossRef]
  10. Spatial Distribution of Daylight--CIE Standard Overcast Sky and Clear Sky, CIE Standard S 003/E-1996 (Commission Internationale de l'Eclairage, Vienna, 1996), p. 3.
  11. G. Wyszecki and W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), pp. 144-145.
  12. Sometimes this spectral assumption is explicit, as in A. J. Preetham, P. Shirley, and B. Smits, "A practical analytic model for daylight," in SIGGRAPH 99 Conference Proceedings, A. Rockwood, ed. (Association for Computing Machinery, New York, 1999), pp. 91-100.
  13. T. S. Glickman, ed., Glossary of Meteorology, 2nd ed. (American Meteorological Society, Boston, 2000), p. 550.
  14. Although the CIE 1976 UCS diagram is itself perceptually isotropic, note that in order to show as much detail as possible, we make the ordinate and abscissa scales differ in Fig. and later chromaticity diagrams.
  15. PR-650 spectroradiometer from Photo Research, Inc., 9731 Topanga Canyon Place, Chatsworth, Calif. 91311. According to Photo Research, at specified radiance levels a properly calibrated PR-650 measures luminance and radiance accurate to within +/-4%, has a spectral accuracy of +/-2 nm, and its CIE 1931 colorimetric errors are x<0.001, y<0.001 for a 2856 K blackbody (CIE standard illuminant A).
  16. R. L. Lee, Jr., "Twilight and daytime colors of the clear sky," Appl. Opt. 33, 4629-4638, 4959 (1994). Gamut g^ ranges from 0 for constant chromaticity to 1 for the spectrum locus, and thus represents the fraction of the CIE diagram that a given chromaticity curve spans.
  17. R. L. Lee, Jr. and J. Hernández-Andrés, "Measuring and modeling twilight's purple light," Appl. Opt. 42, 445-457 (2003).
  18. Reference , pp. 306-310.
  19. Reference , pp. 306-310. Here we follow convention and set the JND equal to the semimajor axis length of the MacAdam color-matching ellipse at the given chromaticity.
  20. D. B. Judd, D. L. MacAdam, and G. Wyszecki, "Spectral distribution of typical daylight as a function of correlated color temperature," J. Opt. Soc. Am. 54, 1031-1040 (1964).
  21. J. Hernández-Andrés, R. L. Lee, Jr., and J. Romero, "Calculating correlated color temperatures across the entire gamut of daylight and skylight chromaticities," Appl. Opt. 38, 5703-5709 (1999).
  22. Olympus Camedia E-10 User's Manual (Olympus Optical Co., Ltd., Tokyo, 2000), p. 102.
  23. Reference , pp. 224-225.
  24. Our experience is that cloud tops occur approximately where radiosonde relative humidity falls below 96% as altitude z increases. Using this admittedly imperfect criterion, for 24 different overcasts our mean cloud-top z(top)=1700 m above sea level, median z(top)=1445 m, and the z(top) standard deviation =961 m.
  25. R. Sekuler and R. Blake, Perception (Knopf, New York, 1985), pp. 189-192.
  26. B. Sierk, S. Solomon, J. S. Daniel, R. W. Portmann, S. I. Gutman, A. O. Langford, C. S. Eubank, E. G. Dutton, and K. H. Holub, "Field measurements of water vapor continuum absorption in the visible and near-infrared," J. Geophys. Res. 109 (part 8), D08307 (2004). [CrossRef]
  27. C. F. Bohren and A. B. Fraser, "Green thunderstorms," Bull. Am. Meteorol. Soc. 74, 2185-2193 (1993). [CrossRef]
  28. C. F. Bohren, "Multiple scattering of light and some of its observable consequences," Am. J. Phys. 55, 524-533 (1987). Our Eq. is derived from Bohren's Eq. (15) for T as a function of tau, and we use Bohren's value of g=0.85 for visible-wavelength scattering by cloud droplets. [CrossRef]
  29. MODTRAN uses a plane-parallel atmosphere to calculate multiple-scattering contributions to daylight and skylight, and when h0< or =0° the model produces nonphysical spectra in the visible.
  30. C. F. Bohren and A. B. Fraser, "Colors of the sky," Phys. Teach. 23, 267-272 (1985).
  31. Based on radiosonde data from nearby Dulles International Airport (code IAD), we estimated cloud Deltaz=0.2 km and 1.0 km on 2-6-03 and 2-17-03, respectively. Perhaps surprisingly, using rlambda for green grass in the 2-6-03 wintertime landscape was not unrealistic (this choice aids comparison with Middleton's results in Fig. ). Even though we do not know the actual mean Owings rlambda spectrum 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 rlambda.

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