Standard Article

Atmospheric Optics

  1. Craig F. Bohren

Published Online: 15 JUL 2005

DOI: 10.1002/3527600434.eap295.pub2

Encyclopedia of Applied Physics

Encyclopedia of Applied Physics

How to Cite

Bohren, C. F. 2005. Atmospheric Optics. Encyclopedia of Applied Physics. .

Author Information

  1. Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania, USA

Publication History

  1. Published Online: 15 JUL 2005


Colors of the sky and colored displays in the sky are mostly a consequence of selective scattering by molecules or particles, absorption usually being irrelevant. Molecular scattering selective by wavelength—incident sunlight of some wavelengths being scattered more than others—but the same in any direction at all wavelengths gives rise to the blue of the sky and the red of sunsets and sunrises. Scattering by particles selective by direction—different in different directions at a given wavelength—gives rise to rainbows, coronas, iridescent clouds, the glory, sun dogs, halos, and other ice-crystal displays. The size distribution of these particles and their shapes determine what is observed, water droplets and ice crystals, for example, resulting in distinct displays.

To understand the variation and color and brightness of the sky as well as the brightness of clouds requires coming to grips with multiple scattering: scatterers in an ensemble are illuminated by incident sunlight and by the scattered light from each other. The optical properties of an ensemble are not necessarily those of its individual members.

Mirages are a consequence of the spatial variation of coherent scattering (refraction) by air molecules, whereas the green flash owes its existence to both coherent scattering by molecules and incoherent scattering by molecules and particles.


  • sky colors;
  • mirages;
  • green flash;
  • coronas;
  • rainbows;
  • the glory;
  • sun dogs;
  • halos;
  • visibility