Observation of a blue sun over new Mexico, U.S.A., on 19 April 1991

A blue sun was observed through a dissipating cloud during a NASA DC-8 mission flight and a color photograph was taken. Measurements with an optical particle counter yield a size distribution which can be approximated by a bimodel lognormal distribution. The modal diameters of both modes are significantly larger than the background aerosol measured in the cloud-free troposphere on the same flight. Calculated transmission of sunlight through the cloud resulted in a slightly higher extinction in the red than in the blue. The long path of sight through the cloud resulted in a transmission of 10 −4–10 −6, which enhances the color effect of the cloud particles. The observation and interpretation agree with reports on colored suns in the literature.