Noctlilucent, or “night-shining” cloud season is around the solstice for most places, but it’s a little later in the first two weeks of August, when darkness returns, for noctilucent cloud season at high latitudes! “Noctilucent clouds gleam in late-summer evenings as if they’re lit from within, but it’s sunlight that makes them shine. They look brilliant in the evening sky because they are so high that everything lower is in shadow while they’re still in sunshine” (Helfferich, 1989). Their origins are mysterious, but the University of Alaska Fairbanks Geophysical Institute (UAFGI) has been studying them since the 1960’s. In this blog post we add a few modern updates (and recent gorgeous photos) to a redux of three excellent short pieces from UAFGI’s Alaska Science Forum: “Unusual Events in the Sky,” written by T. Neil Davis in 1978; “Clouds that Glow at Night,” written by Larry Gedney in 1982; and “Bad News Beautifully Packaged,” written by Carla Helfferich in 1989.
Clouds that Glow at Night
Noctilucent clouds are the highest-altitude clouds. They occur in the “mesosphere”, the coldest-temperature layer of the upper atmosphere.
Noctilucent (night-shining) clouds ride in the sky above 99.9 percent of the atmosphere and over 40 miles above the highest clouds associated with weather. At an average altitude of 50 miles (80 km), they actually skirt the lowest fringes of the aurora, and are above the height at which meteors are observed….
Noctilucent clouds are usually seen in the western sky 10 to 20 degrees above the horizon after the sun has set. During this time, the local area is in darkness and nearby clouds appear dark against a lighter sky. Noctilucent clouds, however, are high enough above the Earth that they are still illuminated by the sun and appear bright against a darker sky. They are a beautiful, wispy blue-white and usually display a wave-type structure that changes quickly with time. They are so tenuous that starlight shines through them without diminution. (Gedney, 1982).
Dr. Liz started to get curious about the communication gaps between what the expert amateur observers are noting and what the scientists are investigating. Modern-day citizen scientists report that the clouds are more broadly and brightly seen in the pre-dawn than post-sunset sky, which may also be a new clue for scientists. (K. Roylance, Aurora Borealis Washington State).
What are they made of and why are they there? These questions have never been answered to anyone’s total satisfaction, but some rocketborne observations have provided clues. The first of these studies was made in Sweden in 1962. A Nike-Cajun rocket with a payload designed to trap particles of a cloud and return them to earth was fired into a noctilucent display and successfully recovered.
Under an electron microscope, the surfaces on which the particles were captured revealed millions of minute motes of dust as small as 0.05 microns in diameter (a micron is one-thousandth of a millimeter, a millimeter is about half the thickness of pencil lead)….
The picture which therefore emerges is that noctilucent clouds are meteor dust particles covered with ice. Knowing what they are, however, in no way explains why they behave as they do. It would be expected that meteoritic particles would be evenly distributed in the earth’s upper atmosphere. Why, then, are noctilucent displays localized; why do they occur only occasionally; why only during the summer months; and, why only at the higher latitudes? These questions about the rare and beautiful spectacle remain to be answered. (Gedney, 1982).
Dr. Cora Randall, who is Principal Investigator on the Cloud Imaging and Particle Size (CIPS) experiment on the NASA Aeronomy of Ice in the Mesosphere (AIM) satellite mission, provided us with an update on these questions. We now know the seasonality is determined by atmospheric circulation that leads to the polar summer mesosphere being the coldest place on Earth. Only when it’s extremely cold (< 150 K, which is -189.67 F and -123.15 C) and there is water vapor can the conditions occur, so they can be quite localized.
Bad News Beautifully Packaged
Noctilucent clouds are considered relatively new to Western science—the first official report was written by a German scientist in 1885.
Soon other observers documented their appearance over Great Britain, Europe, and Russia. They were considered rare over North America until researchers at the Geophysical Institute began studying them over Fairbanks in 1962. [University of Alaska Fairbanks] graduate student Ben Fogle made them his special field of study, and helped establish that it had been only a lack of organized observations at high latitudes in the Western Hemisphere that had made them seem rare here.
During the following flurry of research here and elsewhere, a good bit of information was amassed about noctilucent clouds. Sometimes they covered huge areas; four of the 50-odd displays seen during 1963 covered more than a million square kilometers of sky. They could last for hours, though portions of them could fade away in a few minutes. Often they provided wonderful photographs, and were recorded not just near Fairbanks but from Homer, Gustavus, and virtually all over the north.
Interesting though all these things were, they amounted to mere curiosities. The noctilucent clouds seemed to be just clouds, great streaky patches of ordinary water vapor, frozen because of their chilly altitude.
Recently an international team of investigators suggested that noctilucent clouds are becoming brighter and more common because more water exists 82 kilometers above the earth….
The new research predicts there will be bigger and better cloud displays because human activity is putting more methane into the atmosphere.
Methane moves up through the atmosphere. Above about 30 kilometers, it breaks down and recombines with other atoms and molecules; water is one of the products. The amount of methane is increasing. Air bubbles trapped in ancient polar ice show much lower concentrations of it before the industrial age. When the scientists ran calculations on the effect of increased methane, given what they knew about its behavior and the conditions high above the earth, back came the result: more and brighter noctilucent clouds (Helfferich, 1989).
In other words, it’s possible that the rise of noctilucent clouds is a product of climate change. Dr. Cora Randall gave us an update.
This is still an active area of research, in that scientists have not quantified the long-term (climate) effects of both increasing methane and increasing carbon dioxide emissions. The text above explains that increasing methane could lead to more noctilucent clouds because methane is a source of water vapor. Not stated above is that increasing carbon dioxide emissions could also lead to more noctilucent clouds. This is because, contrary to how increasing carbon dioxide warms the atmosphere near the surface, increasing carbon dioxide actually cools the mesosphere. But to date the observations have not been good enough for us to determine the temperature trends accurately enough to draw definitive conclusions.
Unusual Events in the Sky
Scientists aren’t the only ones watching for these phenomena. High-latitude photographers often capture the beautiful wisps of noctilucent clouds. In the 1970’s, UAFGI collected sightings of unusual sky phenomena (Davis, 1978). As with Aurorasaurus, some of the most important criteria to report were:
- Time of observation
- Duration of the event
- Location in the sky
- Size of the object
- Condition of the sky
- Brightness of the object
- Internal structure
Read the post for evergreen tips on these eight quantities.
Noctilucent clouds are a ripe topic for citizen scientist inquiry and one that we are interested in at Aurorasaurus. Many observations are reported on Facebook, on Twitter, by some astronomical societies, and on spaceweather.com. Unlike auroras, the exciting times when an “outbreak” goes to a lower latitude are often localized to a specific longitude and latitude on a given night. The next night, they may have drifted and appear in a different place for no obvious reason. For example, when clouds appear at low latitude in the UK, that probably means they are not also low in Canada that same night. Looking for patterns in reported observations and with meteorological observations may yield clues as to their predictability. Observer reports can also be compared to AIM satellite “daily daisies” available a few days after the events, though the two may have different detectability thresholds.
Whether it’s aurora or noctilucent clouds, citizen scientists and scientists working together can make new discoveries and solve some of the mysteries of our changing world.
Aurorasaurus is grateful to all of these Alaska Science Forum writers, as well as to Dr. Cora Randall and Lynda McGilvary, who contributed updates, and to Chandresh Kedhambadi and Paul Zizka for sharing their beautiful photos. We especially want to recognize the late T. Neil Davis, who, along with his work on noctilucent clouds, studied aurora and wrote The Aurora Watcher’s Handbook (1992). While aurora science has advanced in the last few decades, the book contains excellent information and much of it is still useful to aurora chasers today.
- “Measurements of Luminous Night Clouds in Norway 1933 and 1934” by early aurora scientist Carl Størmer
- NASA AIM satellite mission
- NASA Heliophysics Citizen Science
- NASA Citizen Science seed funding is available for interested researchers
- Press release on the Supersoaker rocket