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(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.