Short-term variability of overcast brightness

Overcasts seen from below seldom are uniform, unchanging cloud shields, yet little is known about their short-term photometric variability (periods < or = 2 h). Visible-wavelength spectra of daytime and twilight overcast skies measured at 30-s intervals reveal unexpected temporal variability in horizontal illuminance E_nu and zenith luminance L_nu. 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_nu exceeds clear-sky E_nu at the same Sun elevation.