Abstract

Overcasts seen from below seldom are uniform, unchanging cloud shields, yet little is known about their short-term photometric variability (periods ≤2 h). Visible-wavelength spectra of daytime and twilight overcast skies measured at 30-s intervals reveal unexpected temporal variability in horizontal illuminance E_v and zenith luminance L_v. Fourier analysis of these time series shows peak fluctuations at periods of 2–40 min. Factors such as cloud type and optical depth, presence of fog or snow, and instrument field of view can affect overcast brightness variability. Surprisingly, under some circumstances overcast twilight E_v exceeds clear-sky E_v at the same Sun elevation.

© 2005 Optical Society of America

Colors of the daytime overcast sky

Raymond L. Lee and Javier Hernández-Andrés
Appl. Opt. 44(27) 5712-5722 (2005)

Observed brightness distributions in overcast skies

Raymond L. Lee, Jr., and David E. Devan
Appl. Opt. 47(34) H116-H127 (2008)

Measuring overcast colors with all-sky imaging

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

Measuring and modeling twilight’s Belt of Venus

Raymond L. Lee
Appl. Opt. 54(4) B194-B203 (2015)

Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies

Ramón Hegedüs, Susanne Åkesson, and Gábor Horváth
J. Opt. Soc. Am. A 24(8) 2347-2356 (2007)