What are noctilucent clouds and how to observe a rare natural phenomenon

What are noctilucent clouds?

Noctilucent clouds form in the upper atmosphere - the mesosphere - on

an altitude of 80 km above the Earth's surface.They are believed to be composed of ice crystals that form on small dust particles from meteors. They can only form at incredibly low temperatures and when water is available to form ice crystals.

How are they formed?

Why are these clouds that need so lowtemperatures formed in summer? All because of the dynamics of the atmosphere. In fact, at this altitude in the mesosphere in summer near the poles the lowest temperatures of the year.

Roskosmos

This is how it works:in summer, the air near the ground heats up and rises. As atmospheric pressure decreases with altitude, the rising air expands. When this happens, it also cools down. This, along with other processes in the upper atmosphere, raises the air even higher, forcing it to cool even more. As a result, the temperature in the mesosphere can drop to -134 ° C.

Because clouds are so sensitive to atmospheric temperatures, they give scientists more information about the wind circulation causing these temperatures.

When were noctilucent clouds first discovered?

Noctilucent clouds are known to have beendiscovered in 1885, two years after the eruption of a volcano in Indonesia - Krakatoa - in 1883. It remains unclear whether their appearance had anything to do with the volcanic eruption or whether their discovery was due to the fact that more people watched spectacular sunsets caused by volcanic debris in the atmosphere.

Today noctilucent clouds are known to be caused bynot only volcanic activity, although dust and water vapor can enter the upper atmosphere as a result of eruptions and contribute to their formation. And yet, scientists at the time assumed that the clouds were another manifestation of volcanic ash. However, after the ash fell out of the atmosphere, the noctilucent clouds did not disappear. Finally, the theory that the clouds are composed of volcanic dust was refuted by Malsev in 1926.

In the years since their discovery, the clouds have beenscrutinized by Otto Jesse of Germany, who first photographed them in 1887. In addition, he was the first to come up with the term "noctilucent clouds".

Systematic photographic observations ofclouds were organized in 1887 by Wilhelm Foerster, and after - by the staff of the Berlin Observatory. It was then that the height of the clouds was first determined using triangulation. This project ended in 1896.

In the decades since Otto's deathJesse in 1901, research into the nature of noctilucent clouds made little progress. Wegener's hypothesis that they are composed of water ice later proved to be correct. Research was limited to ground-based observations. Scientists knew very little about the mesosphere until the 1960s, when humans began to actively explore and explore space. Then it turned out that the appearance of clouds coincided with very low temperatures in the mesosphere.

Later, the Solar Mesosphere Explorer satellite applied ona map of cloud distribution between 1981 and 1986 using an ultraviolet spectrometer. The first physical confirmation that water ice is indeed the main component of noctilucent clouds was obtained with the HALOE instrument on the upper atmosphere satellite in 2001.

In 2001, the Swedish satellite Odin conducted spectral analysis of clouds and compiled daily global maps, which revealed large patterns in their distribution.

Satellite AIM (Aeronomy of Ice in the Mesosphere),which was launched on April 25, 2007, was the first whose work consisted only of the study of noctilucent clouds. The first data appeared already a month later. The images taken by the satellite show cloud shapes similar to the shapes of tropospheric clouds, which hints at a similarity in their dynamics.

Research published in the journal Geophysical Research Letters in June 2009 suggests that the noctilucent clouds observed after the 1908 Tunguska event indicate that the impact was caused by a comet.

What do noctilucent clouds look like?

Noctilucent clouds are usually colorless orpale blue, although other colors are occasionally seen, including red and green. The characteristic blue color arises from absorption by ozone in the path of sunlight illuminating the silvery cloud. They may look like featureless stripes, but they often exhibit patterns - stripes, undulating waves and swirls.

How and where to observe noctilucent clouds?

Clouds can be observed only in summer in the Northhemispheres - they are usually seen in June-July at geographic latitudes from 45 to 70 degrees. Moreover, most often the phenomenon is observed from mid-June to mid-July at latitudes from 55 to 65 degrees.

Due to their high-rise position, silveryclouds glow only at night, scattering sunlight that hits them from below the horizon. During the day, even against the background of a clear blue sky, these clouds are not visible - for this they are too thin. Only deep twilight and night darkness make them visible to a ground observer.

According to astronomers, notice noctilucent cloudsit is possible every year, but they do not always reach high brightness. At the same time, it is best to observe rare clouds around midnight above the northwestern horizon.

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