Explainer
Emily Winterburn investigates the history of NLC observations
Noctilucent clouds (NLCs) are the highest clouds in the atmosphere and we can see them around astronomical twilight in a summer’s night sky. The name comes from Latin, meaning ‘night-shining clouds’, and they occur when the Sun is below the horizon and the clouds are still in sunlight. Although no one as yet fully understands why, this appears to be a relatively new phenomenon first seen in 1885 and with an increased frequency in recent years.
First sightings
While these clouds can be a spectacular sight, curiously no one appears to have recorded observing them before 1885. Two years earlier, in 1883 the volcano Krakatoa erupted, destroying land, creating tsunamis and causing around 36,000 deaths. The after-effects of the eruption were felt for years and included vivid red sunsets, which were seen across the world as the ash from the volcano drifted and filled the skies. Prepped by the after-effects of this volcano to look more closely at atmospheric phenomena in the night sky, a number of observers began to notice these curious, shining clouds and naturally assumed these, like the deep red sunsets, must be linked to the volcanic eruption.
Thomas Backhouse, an astronomer and meteorologist based in Sunderland, was interested in cloud formation. He had studied lots of different types of cloud, and in 1884 began to notice what he considered to be a new type of cloud which he noted was rather unusual in only being visible after sunset. He kept watching, even after the effects from the volcano began to fade, and in 1885 published a paper on his findings.
At around the same time over at Armagh Observatory in Ireland, the observatory’s director, Thomas Romney Robinson, was possibly observing the same thing. He never published his findings but he did note the appearance of “luminous clouds” in his meteorological record books over several decades. These observations were not often in the summer, however, which suggests that they might relate to some other phenomenon.
At first, following Backhouse’s publication, it was widely supposed that the clouds were a result of the volcanic ash from Krakatoa, but the clouds persisted even after the volcanic dust settled and the link was eventually disproved in 1926.
Back in the 1880s, astronomers and meteorologists continued to observe and study these new clouds. At the Berlin Observatory in Germany, Otto
Jesse took a particular interest, taking the first
photographs in 1887 and he appears to have even coined the name we still use today: ‘noctilucent cloud’. Studying his earlier observations, he concluded that these NLCs first appeared in 1885 and were not visible before this; he then organised a programme of systematic photographs to be taken so further investigations could be made of what and where these clouds were.
NLCs are in fact ice crystals and dust found in the mesosphere (in the upper atmosphere at an altitude of around 80km) that reflect sunlight, but not enough to make them visible during daylight hours. Recent studies suggest the phenomenon may be exacerbated by climate change and the increased concentrations of atmospheric methane. Dust from events such as volcano eruptions and Space Shuttle launches can also make them more likely to occur, showing that Backhouse’s initial thoughts weren’t quite as wrong as his successors believed.
In 1972, NASA’s OGO 6 satellite (the Orbiting Geophysical Observatory) became the first instrument to detect NLCs from space. Today, it is the AIM (Aeronomy of Ice in the Mesosphere) satellite, launched in 2007 that provides us with our most in-depth studies of these clouds. This satellite is dedicated to the study of NLCs and its work over the past 14 years has allowed scientists to forecast when and where NLCs are most likely to be visible.
Other atmospheres
It would seem that NLCs are not unique to our planet. In 2019, the Curiosity rover on Mars was reported to have seen noctilucent clouds in the Red Planet’s sky. More recent observations by the Curiosity rover suggest some of these clouds might be made of frozen carbon dioxide rather than water ice, which may help scientists determine their altitude. Who knows, perhaps the next breakthroughs in research into NLCs will come not from our planet, but from settlers on Mars?