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Short-term variability of overcast brightness.

Raymond L Lee1, Javier Hernández-Andrés

  • 1Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, USA. raylee@usna.edu

Applied Optics
|October 6, 2005
PubMed
Summary

Overcast skies exhibit short-term photometric variability, with fluctuations occurring every 2-40 minutes. Surprisingly, twilight overcast illuminance can exceed clear-sky levels at the same solar elevation.

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Area of Science:

  • Atmospheric optics
  • Cloud physics
  • Radiative transfer

Background:

  • Overcast cloud shields are often perceived as uniform but exhibit poorly understood short-term photometric variability.
  • Previous research has limited understanding of cloud brightness changes over periods less than or equal to 2 hours.

Purpose of the Study:

  • To investigate the short-term photometric variability of overcast skies during daytime and twilight.
  • To identify the characteristic periods and influencing factors of cloud brightness fluctuations.

Main Methods:

  • Measurements of visible-wavelength spectra of overcast skies at 30-second intervals.
  • Analysis of temporal variability in horizontal illuminance (E(v)) and zenith luminance (L(v)).
  • Application of Fourier analysis to time series data to identify dominant fluctuation periods.

Main Results:

  • Unexpected temporal variability was observed in both horizontal illuminance and zenith luminance.
  • Fourier analysis revealed peak fluctuations in the period range of 2–40 minutes.
  • Factors like cloud type, optical depth, fog, snow, and instrument field of view influence variability.
  • Overcast twilight illuminance was found to exceed clear-sky illuminance under certain conditions.

Conclusions:

  • Overcast skies display significant short-term photometric variability not previously well-characterized.
  • Cloud brightness fluctuations are influenced by a combination of atmospheric and instrumental factors.
  • The phenomenon of twilight overcast illuminance surpassing clear-sky levels warrants further investigation.

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