09 Aug 2023
 | 09 Aug 2023

Explaining the green volcanic sunsets after the 1883 eruption of Krakatoa

Christian von Savigny, Anna Lange, Christoph Hoffmann, and Alexei Rozanov

Abstract. Volcanic sunsets are usually associated with extended and enhanced reddish colours typically complemented by purple colours at higher elevations. However, many eyewitnesses reported remarkably clear and distinct green twilight colours after the eruption of Krakatoa (Sunda Strait, Indonesia) on August 27, 1883. To our best knowledge, no earlier studies exist providing an explanation for this unusual phenomenon. In the current work we employ simulations with the SCIATRAN radiative transfer model to investigate the processes leading to green volcanic sunsets. Green sunsets can be simulated based on plausible assumptions by anomalous scattering on stratospheric sulfate aerosols. We investigate the sensitivity of the twilight colours to relevant parameters such as aerosol optical depth, the parameters of the particle size distribution and the amount of ozone. The main requirements for the occurrence of green twilights are a sufficiently large aerosol optical depth combined with particle radii of about 500 – 700 nm (assuming stratospheric sulfate aerosols) and a preferably narrow aerosol particle size distribution. The occurrence of green twilights after historic eruptions provides important constraints on the particle size of volcanic aerosols.

Christian von Savigny et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • EC1: 'Comment on egusphere-2023-1669', Timothy Garrett, 26 Aug 2023
  • RC1: 'Comment on egusphere-2023-1669', Filip Vanhellemont, 29 Sep 2023
  • RC2: 'Comment on egusphere-2023-1669', Anonymous Referee #2, 04 Oct 2023

Christian von Savigny et al.

Christian von Savigny et al.


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Short summary
It is well know that volcanic eruptions strongly affect the colours of the twilight sky. Typically, volcanic eruptions lead to enhanced reddish and violet twilight colours. In rare cases, however, volcanic eruptions can also lead to green sunsets. This study provides an explanation for the occurrence of these unusual green sunsets, based simulations with a radiative transfer model. Green volcanic sunsets require specific aerosol sizes to occur.