This is the second of two papers dealing with the effects of volcanic debris from the eruption of El Chichon on light from the sunlit sky. The polarization of skylight was considered in the first of the two, whereas this one is devoted to skylight intensity. It is shown here that the magnitude of the skylight intensity is modified very significantly from its clear sky value by the volcanic cloud, as is its change with solar depression angle during twilight and its distribution over the sky during the day. Emphasis is on measurements at a wavelength of 0.07 μm. Generally the volcanic cloud produces a diminution of zenith intensity during twilight with a considerable enhancement of intensity over the sky throughout the main part of the day. The solar aureole is not as sharp as it is in normally clear conditions, but the volcanic cloud causes a very diffuse type of aureole which covers a large portion of the sky. The preferential scattering of the longer wavelengths of sunlight, which is made evident by brilliant red and yellow colors in the sunrise period, causes a pronounced change of longwave/shortwave color ratios during twilight from their values in clear atmospheric conditions. The combination of intensity data shown here with polarization data in the previous paper should give a relatively complete picture of the effects of volcanic debris on solar radiation in the atmosphere and be useful in the verification of radiative transfer models of atmospheric turbidity.
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