On the phenomenon of the blue sun

Nellie Wullenweber,Christian Von Savigny,Alexei Rozanov,Anna Lange

doi:10.5194/cp-17-969-2021

Nellie Wullenweber, Christian Von Savigny + Show 2 more

Open Access

https://doi.org/10.5194/cp-17-969-2021

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Abstract

Abstract. This study examines the cause of the blue colour of the sun as observed after the eruption of Krakatao in 1883 as well as other volcanic eruptions or massive forest fire events. Aerosol particles, e.g. volcanic ash or products of biomass burning, are believed to be able to modify the spectral distribution of transmitted solar radiation making it appear blue or green to a human observer. Previous studies already showed that narrow aerosol particle size distributions with radii on the order of about 500 nm can lead to anomalous scattering; i.e. scattering cross sections increasing with increasing wavelength in the visible spectral range. In this work we treat the effect of Rayleigh scattering on the shape of the transmitted solar spectrum correctly employing radiative transfer (RT) simulations with the SCIATRAN RT-model. The colour associated with solar transmission spectra is determined based on the CIE (International Commission on Illumination) colour matching functions and CIE chromaticity values. It is shown that a blue sun can be simulated for aerosol optical depths (at 550 nm) of about τ=0.5 (or higher) if Rayleigh scattering is taken into account. Without considering Rayleigh scattering – as in most of the previous studies – a blue sun is in principle produced with aerosol optical depths as low as about τ=0.1 (at 550 nm) if the aerosol particle size distribution is chosen to maximize anomalous scattering in the visible spectral range. It is demonstrated that Rayleigh scattering – as expected – has a strong impact on the transmission spectrum, particularly at low solar-elevation angles, and needs to be considered for a correct determination of the perceived colour of the sun. We also test the hypothesis that the blue sun after the eruption of Krakatao was caused by large abundances of water vapour in the atmosphere, as proposed in earlier studies. In addition, we present a case study on a particularly noteworthy blue sun event in the past, i.e. the one related to the large Canadian forest fires in September 1950.

Highlights

This work has been inspired by various reports on sightings of the sun appearing blue
Previous studies already showed that narrow aerosol particle size distributions with radii on the order of about 500 nm can lead to anomalous scattering; i.e. scattering cross sections increasing with increasing wavelength in the visible spectral range
It is noticeable that a lower refractive index leads to a larger particle radius range with stronger anomalous scattering, making the blue sun phenomenon more probable

Summary

Introduction

This work has been inspired by various reports on sightings of the sun appearing blue. Gelbke, 1951; Wilson, 1951; Penndorf, 1953), widespread observations of a blue sun have been made following the eruption of the Krakatao volcano in 1883 Witnesses described the sun rising in a “splendid green”, and turning “bright blue” near the zenith later during the day in the aftermath of the Krakatao eruption as noted in Symons et al (1888). Noteworthy is the book by Kiessling (1888) that provides a comprehensive compilation of unusual atmospheric optical effects associated with the Krakatao eruption and during the 2000 years before, including several reports on green and blue suns. Historical reports on an unusually coloured sun or moon can provide important independent information on exceptional natural events, e.g. volcanic eruptions Blue suns and moons are discussed in standard textbooks on atmospheric optics (e.g. Bohren and Huffman, 1998, Sect. 4.4; Van de Hulst, 1957, Sect. 20.23)

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