Noctilucent clouds
Noctilucent clouds are thin, high-altitude clouds that form at altitudes of about 85 kilometers in the mesosphere, primarily at very high latitudes beyond 50° from the equator. They are most visible during twilight, when they appear to glow against the darkened sky, hence the name "noctilucent," which means "night-shining" in German. These clouds are composed of tiny ice crystals that require extremely cold temperatures to form, typically below –120° Celsius, making them more likely to occur in the summer months of polar regions. They were first scientifically reported after the 1883 eruption of Krakatoa, although their persistence beyond this event prompted further study.
Despite ongoing research, the exact mechanisms behind their formation and relationship to broader climate phenomena remain unclear. Notably, there are indications that noctilucent clouds have become increasingly common, and they have recently been observed at lower latitudes than traditionally expected. Some theories suggest that their presence may correlate with global warming and the cooling of the upper atmosphere, as well as potential contributions from greenhouse gases. Advances in technology now allow scientists to monitor these clouds even when they are not visible, leading to new insights into their nature and potential connections to climate change.
Subject Terms
Noctilucent clouds
Definition
At very high latitudes, usually farther than 50° from the equator, very thin, extremely high-altitude clouds sometimes form during the summer. These clouds are very difficult to observe directly from Earth. They are easiest to see at the end of twilight, when darkness has settled on the surface of the Earth but sunlight is still shining at very high altitudes: The ultra-high-altitude clouds appear to shine after dusk, so they are called noctilucent clouds (German for “night-shining” clouds).
Noctilucent clouds form in the mesosphere, an upper layer of Earth’s atmosphere, at an altitude of about 85 kilometers. At that altitude, the air is exceedingly thin and cold. Dust and tiny ice crystals form the clouds, but the ice crystals require exceedingly cold temperatures, typically colder than –120° Celsius, which occur during the summer months in the polar regions (when surface temperatures are warmest). As a result, the technical name for noctilucent clouds is polar mesospheric clouds.
The first known reports of noctilucent clouds followed the 1883 explosive eruption of Krakatoa, when the clouds were extensively studied by Jesse Otto. Initially, it was believed that the clouds were simply a by-product of the volcanic eruption. However, the clouds persisted long after the ash from the eruption had dissipated. Otto and subsequent researchers were able to determine very little about the clouds. They are too high for balloon studies and have prompted satellite missions to study them, including the Aeronomy of Ice in the Mesosphere (AIM) mission.
Though little is known about noctilucent clouds, scientists have observed that they are somewhat more frequent after meteor showers. Some rocket exhausts, such as those of the Space Shuttle main engines, have also been implicated in the formation of noctilucent clouds. Despite over a century of research, the nature and actual cause of noctilucent clouds is still not known.
Significance for Climate Change
Because noctilucent clouds were not reported until 1883, some have speculated that they did not exist before then, but that claim is unsubstantiated. Many subtle phenomena were first studied scientifically in the nineteenth century, but that does not mean that they did not exist beforehand. Noctilucent clouds have also been more commonly reported over the course of the twentieth century, leading to claims that these clouds are becoming more common. That claim, too, is disputed. Noctilucent clouds have been more widely publicized over the course of the last century, so there may have simply been more people looking for them or recognizing and reporting them when they saw them. Concerted and in-depth scientific investigations of noctilucent clouds have been fairly recent, so there is little long-term data.
Not only have noctilucent clouds been observed more frequently in high latitudes, but they have also been observed at lower latitudes than before, with reports of noctilucent clouds from as far south as Colorado. The apparent increase in noctilucent cloud reports appears to correlate with the reported increases in global temperatures over the last century. Since noctilucent clouds require extremely cold mesospheric temperatures, sighting these clouds at lower latitudes may suggest a cooling of the mesosphere. This has led to claims that noctilucent clouds are in some manner related to global warming, which may also relate to cooling of the upper atmosphere. However, these claims are difficult to substantiate, because the nature of the noctilucent clouds is still not understood, nor is the mechanism by which these clouds form.
Some theoretical models indicate that certain greenhouse gases, such as methane, may provide hydrogen to the mesosphere that can oxidize to form water from which the clouds can form. One of the mysteries about noctilucent clouds is the source of the water in the mesosphere, one of the driest parts of Earth’s atmosphere. An increase in methane could be one explanation. However, other sources for the water in the mesosphere could also result in noctilucent clouds. Major volcanic eruptions or impacts from icy bodies from space could also impart water to the mesosphere, but impacts should occur at fairly consistent rates, and there have been no major volcanic eruptions of the order of Krakatoa in the last century.
In recent years, it has been discovered that noctilucent clouds could be monitored from the ground by radar even when they were not visible. The ice crystals forming the clouds seem to act as attractors for iron and sodium ions in Earth’s thermosphere. These metal ions are believed to come from meteoroids burning up in Earth’s upper atmosphere. Satellite detection of noctilucent clouds is also now possible. These new methods of study may answer questions about the origins of the clouds and any possible link between them and global climate change.
Bibliography
Dalin, P., et al. “Ground-Based Observations of Noctilucent Clouds with a Northern Hemisphere Network of Automatic Digital Cameras.” Journal of Atmospheric and Solar-Terrestrial Physics 70, nos. 11/12 (August 2008): 1460-1472.
Hooper, David. "Noctilucent Clouds: A Complete Guide to the Rare 'Night-Shining' Clouds." Space.com, 5 June 2024, www.space.com/noctilucent-clouds. Accessed 20 Dec. 2024.
Petersen, Carolyn Collins. “Noctilucent Clouds from Rocket Exhaust.” Sky and Telescope 106, no. 3 (September 2003): 26.
Schroder, Wilfried. “Otto Jesse and the Investigation of Noctilucent Clouds 115 Years Ago.” Bulletin of the American Meteorological Society 82, no. 11 (November 2001): 2457-2468.
Stevens, Michael H. “Heavenly Harbingers.” Smithsonian 32, no. 2 (November 2001): 20.
Witze, Alexandra. “Enigmatic Clouds Illuminated.” Nature 927, no. 7172 (2007): 927.