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'Letterbox' Sunset & Receding Twilight Arch.
The stratospheric aerosols from the Hunga-Tonga eruption on 15th January (that have given rise to spectacularly coloured sunrises and sunsets in Mutare for the past two weeks) are moving to the west resulting in less colourful sunrises and sunsets in which the twilight arches recede more quickly and the colours are concentrated closer to the horizon. On Friday evening a ‘letterbox’ sunset (in which the setting sun appears briefly between low cloud and the horizon) was followed half an hour later by a rapidly receding strongly coloured twilight arch as shown in a x120 speed time-lapse video posted on YouTube at https://youtu.be/fMRy2INIII8 The strange undulations in the highest cloud visible in the accompanying sunset image are due to the effect of gravity waves on the volcanic aerosols in the stratosphere. More information at https://eos.org/research-spotlights/clouds-visible-twilight-reveal-mysteries-gravity-waves