Some Measurements of the Attenuation of Solar Radiation During BOMEX

Measurements of the upward and downward flux of solar radiation in two broad wavelength bands defined by glass filters were made at height between about 300 m and 12,000 m in the neighborhood of Barbados during the BOMEX experiment in the summer of 1969. These were examined on selected occasions, apparently cloudless, to determine the attenuation—absorption and upward scattering—that could not be attributed to atmospheric gases and must be considered to be caused by particles. Total particle absorption between the lowest and highest flight levels was found to be of order 5%, and upward scatter of order 1% of the incident radiation. Unexplained attenuation above the highest level was of order 2%. The attenuation attributed to particles was of approximately the same magnitude as the geometrical cross section deduced by direct particle sampling on the same flights. Unexplained attenuation above the highest flight level was not found in a series of similar flights in northern temperate latitudes. It is attributed to cirrus cloud in the high tropical troposphere not otherwise observed.