Potential depictions of noctilucent clouds in William Ascroft’s sketches in 1884

Abstract Noctilucent clouds (NLCs) are the highest clouds in the Earth’s atmosphere and occur at about 83 km altitude in the polar summer mesopause region. According to the current understanding, the first known observations of NLCs took place in the northern hemisphere in summer 1885. It is generally assumed that these first reports are related to the eruption of the Indonesian volcano Krakatoa in August 1883. So far, no convincing evidence exists for NLC occurrences before June 1885. In this article, we suggest that twilight sketches by British artist William Ascroft created in August 1884 show depictions of NLCs. The solar zenith angle at the time when the sketches were made, excludes the possibility of ordinary cirrus clouds. It is not known whether these potential NLCs were enhanced by the Krakatoa eruption – e.g. by the H 2 O the eruption injected into the middle atmosphere – or whether the clouds would also have been visible without the eruption. However, a volcanic impact is a valid possibility. To this end we carried out simulations with the global circulation model HAMMONIA to study the transport of a tropical volcanic H 2 O plume in the stratosphere and mesosphere. The simulations show that a H 2 O transport time to the northern polar summer mesopause as short as 1 year is possible, if the H 2 O is injected at altitudes of 40 km or higher. In summary, we cannot conclude with certainty that Ascroft’s cloud sketches in August 1884 indeed depict NLCs, but in combination with available eye witness reports on clouds in summer 1884 this is a clear possibility.