OBSERVATION OF A BLUE SUN OVER NEW-MEXICO, USA, 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 appro ximated by a bimodel lognormal distribution. The modal diameters of bo th modes are significantly larger than the background aerosol measured in the cloud-free troposphere on the same flight. Calculated transmis sion of sunlight through the cloud resulted in a slightly higher extin ction 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 co lor effect of the cloud particles. The observation and interpretation agree with reports on colored suns in the literature.