Sylver clouds are the highest in Earth's atmosphere.
Photo: Shutterstock.
From December 17 to 19 of last year, residents of Krasnoyarsk Krai, Irkutsk Region, and Magadan observed a peculiar glow in the sky. The phenomenon most resembled silver clouds – the highest clouds in Earth's atmosphere. But they can typically only be seen in summer! Have summer and winter swapped places?
Amateur photographs spread across the internet, catching the attention of Oleg Ugolnikov, a senior researcher at the Institute of Space Research of the Russian Academy of Sciences.
We called Oleg Stanislavovich just in time. Just recently, another rare atmospheric phenomenon, nacreous clouds, was sighted in St. Petersburg. These clouds are usually visible beyond the Arctic Circle and appear even less frequently as they drift so far south. Has something changed in the atmosphere? We are about to find out.
What are silver clouds? Why do thousands of people hunt for them in summer?
On June 12, 1885, privat-docent of Moscow University Witold Tserski looked up at the bright summer night sky and couldn't believe his eyes. Above the northern horizon hung incredibly bright, pearly-hued fibrous formations. Clouds? Tserski understood perfectly: such a phenomenon was impossible. The sun was far below the horizon and could not illuminate clouds known at that time. Yet, they resembled, as the song goes, "white-horse foals." That means they were high up… about 80 kilometers or more. At that time, science had a vague understanding of the atmosphere. It was believed that above 30 kilometers, the air was so thin that solid or liquid particles could not exist. But here they were.
Another unexpected event occurred over 20 years later, when abnormally bright "silver" shone over Russia in late June – early July 1908. The news of a celestial body exploding in the atmosphere over the Siberian taiga (later dubbed the Tunguska event) reached St. Petersburg later; moreover, bright clouds appeared before the Tunguska phenomenon! So, are silver clouds, in some sense, cosmic?
It was hypothesized, later confirmed in 2001, that "noctilucent clouds" consist of ice crystals, similar to some ordinary clouds, such as cirrus clouds. However, silver clouds are located much higher, at altitudes of 75 to 90 km. From the perspective of an average person, that's already space (though formally, the boundary is marked at about 100 km). If we were to ascend to that height, the sky would be dark, and the Earth would appear as a large sphere with a blue band of atmosphere along its edge.
What is needed for their formation? The presence of small dust particles and remnants of meteors can aid in their formation. This is precisely why bright clouds appeared on summer nights in 1908. According to modern understanding, the Tunguska body was a fragment of a comet, and dust particles settled in the atmosphere even before the impact! Water vapor, which forms ice, is also essential. But most importantly, there must be cold. Real, "mesospheric" cold, minus 125 degrees or lower. At such altitudes, there is very low pressure and a minimal concentration of water vapor, and only at these temperatures can ice form.
Water molecules "freeze" onto cosmic dust particles, crystallizing into ice. The sun illuminates it along the Earth's surface. The illuminated clouds create a strong contrast against the dark sky. That's the recipe for a summer magical spectacle.
And now for the most important part. The "cosmic cold," those minus 120-130 degrees, does not always occur at such altitudes (see, by the way). Surprisingly, it is coldest there not in winter, but in summer. This is precisely why silver clouds are a summer phenomenon. In winter, the mesosphere simply isn't cold enough for ice crystals to form.
And suddenly – there they are, in December photos from Siberia!
Perhaps silver clouds had been seen somewhere, at some time in December before. But no evidence could be found. So, these observations are the first documented cases in history. But are they really silver, mesospheric clouds?
At one time, Oleg Ugolnikov developed an original method: how to determine the height of silver clouds from a single photograph. He applied it to the Siberian photos: it turned out that the mysterious formations hovered at an altitude of 70 km. This is about 10 km lower than the "silver" seen in summer; nevertheless, it is still in the mesosphere. Clarity emerged after analyzing satellite temperature measurements in this region of the atmosphere: during those days, the temperature sharply dropped precisely at that altitude! This means that real silver clouds were observed over Siberia. Just in winter.
Siberians saw "summer" clouds for the first time in history – in winter
Photo: Shutterstock.
The reason for such a natural anomaly may also seem surprising: it all started, on the contrary, with a sharp rise in temperature over Eurasia, but at lower altitudes – around 40 km. Scientists call this "sudden stratospheric warming" (the English abbreviation SSW). A cold polar stratospheric vortex, formed over the pole, shifted to the western hemisphere, replaced by warm air masses. Indeed, as satellite data revealed, during those December days, the stratosphere was 50 degrees warmer than normal. Moreover, all changes occurred within a few days. At higher altitudes, however, the temperature dropped significantly: to -115 - 120 degrees. The pole of cold was at an altitude of 68-70 km – and it was there that the silver clouds appeared.
The attentive reader may notice: minus 115-120 is close, but perhaps not cold enough for ice formation. As Oleg Ugolnikov believes, this was sufficient because the upward air currents from the warm stratosphere transported a large amount of water vapor into the mesosphere. The more vapor there is, the higher the temperature required for freezing.
- So, is global warming on the march? Summer clouds in winter, in Siberia?
- I wouldn't directly link all of this to global warming. All this time we have been saying that it has warmed in the stratosphere, that is, at altitudes of 10-50 km. However, warming at those heights and in the troposphere (the layer of air where we live and breathe) have nothing in common. Moreover, there are signs of an inverse relationship. If the upper layers (stratosphere) warm up, the lower layers (troposphere) cool down. Perhaps this does not operate on global, planetary scales. But it certainly worked back then, in December, and now, in January. Here we are, a warm January. And just before this warming, the stratosphere, on the contrary, cooled down. It is very cold up there now, - says Oleg Ugolnikov.
Yes, it is difficult to understand without preparation. We are accustomed to think broadly and "swing swords": the Earth is warming, the climate is deteriorating! But in reality, the atmosphere is a layered cake, and in this "cake," complex, paradoxical interactions occur.
- In your research, you write: planetary waves warmed the stratosphere and "pushed back" Arctic air. What are these waves?
- Waves of various amplitudes and scales propagate in the atmosphere. Planetary waves arise from the difference in linear movement speeds caused by the Earth's rotation at different latitudes. These are precisely related to temperature changes in the stratosphere on a large scale, as happened in December 2024. There are also acoustic-gravity waves, somewhat akin to ocean waves. Their sources are inhomogeneities in air movement in the lower layers: atmospheric fronts, thunderstorms. If ocean waves intensify towards the shore, atmospheric waves grow in amplitude where air density decreases. In other words, as they reach higher altitudes, the waves become stronger. This occurs in summer in the mesosphere, which also contributes to the formation of silver clouds. The same thing happened over Siberia in December, throwing water vapor up to 70 km.
- Still, when observing silver clouds, whether in winter or summer, can we draw any conclusions about global warming?
- Indirectly, yes. Greenhouse gases contribute to cooling in the mesosphere. More accurately, they "lower" the mesosphere. But this is not the only factor influencing the appearance of silver clouds. The amount of water vapor is very important – we mentioned this. It is precisely because of this that clouds have become brighter and more frequent in recent years and decades. However, last summer