Abstract

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

Measuring overcast colors with all-sky imaging

Raymond L. Lee, Jr.
Appl. Opt. 47(34) H106-H115 (2008)

The Color of the Overcast Sky

W. E. Knowles Middleton
J. Opt. Soc. Am. 44(10) 793-798 (1954)

Tropospheric haze and colors of the clear daytime sky

Raymond L. Lee
Appl. Opt. 54(4) B232-B240 (2015)

Short-term variability of overcast brightness

Raymond L. Lee and Javier Hernández-Andrés
Appl. Opt. 44(27) 5704-5711 (2005)

Twilight and daytime colors of the clear sky

Raymond L. Lee
Appl. Opt. 33(21) 4629-4638 (1994)