Polar stratospheric clouds appeared in the southern Argentinian city of Uhuaia in recent days, surprising residents | NATIONAL WEATHER SERVICE/DISTRIBUTION
Southern Argentina from Argentina and Patagonia had one very cold month of july and with Markings on thermometers near 20C below zero some daysas observed in the city of Maquinchao in the province of Rio Negro, where the Thermometers read 19.9ºC. The southernmost month of the continent was also marked by plenty of snowfalla phenomenon that made tourists happy and brought a lot Inconvenience to local people and transportation.
This is on the surface, because at the top of the atmosphere it was even colder in southern South America at nearpolar latitudes, leading to another phenomenon we’ve never seen here in central Argentina’s midlatitudes. Uruguay and CentralSouth from Brazil. Stratospheric clouds appeared in Patagonia.
In the last few days, residents of the province of Tierra del Fuego, the southernmost province of Argentina, have been able to observe iridescent (colored) clouds in the sky, a very attractive visual phenomenon, but one that poses a threat to the environment because it affects the ozone layer, which affects the National Weather Service of Argentina (SMN) labeled it “dangerous” although these clouds do not cause acid rain nor pose a hazard to people on the surface.
The very special conditions for their formation make nacre clouds a rare phenomenon at high latitudes. Scandinavia, Iceland and Northern Canada are popular places to see them. Sightings here in the southern hemisphere are even rarer because there is so little land to the south other than Antarctica.
Therefore, images of this type of cloud cover are very common in northern European countries and rare here in South America. When they do appear, they do not miss attention and are a spectacle in the sky. The high frequency of cloud cover and unstable weather associated with the lowpressure belt around Antarctica is another complicating factor in observing these clouds.
Pearlescent clouds in Rio Grande, Tierra del Fuego, Argentina in July 2013 | GERARDO CONNON/NASA
But it’s not the first time they’ve appeared or been registered. 2013, also in the second half of July, Stratospheric clouds provided stunning images in Patagonia. They were recorded by Gerardo Connon and published on the NASA website. At that time, the clouds could be seen in the city of Rio Grande, also in the province of Tierra del Fuego.
How stratospheric clouds form
Due to the extreme dryness, clouds do not normally form in the stratosphere. However, during winter at high latitudes, the stratospheric temperature sometimes becomes low enough to encourage cloud formation. The colorful appearance of these clouds is due to the fact that they contain small, similarly sized particles, each of which deflects sunlight in a similar way.
Because of their high altitude, clouds typically remain fully illuminated on the ground for about twenty minutes after sunset, resulting in a spectacular twilight sky appearance. They are best seen before sunrise and after sunset, when the sun is between about 1° and 6° below the horizon.
Called “mother of pearl” by the Scandinavians because of its iridescent appearance, these clouds are composed of mixtures of natural water and nitric acid trihydrate (alphaNAT or aNAT). The chemical reactions taking place on the surface of these clouds result in a remarkable change in stratospheric composition.
The hue changes depending on the height of the sun in relation to the viewer. The iridescence increases when it is several degrees below the horizon, i.e. at dawn or dusk. In the case of polar stratospheric clouds, which are composed of nitric acid and water, they appear more opaque.
Clouds that consist only of ice are called nacreous. Those that form above the ice sublimation point (direct transition from solid to vapor phase) are the result of the condensation of nitric acid and water. If they are below that, they are mostly made of ice.
Chlorine, which enters the stratosphere primarily from industrial sources, is converted from relatively unreactive forms to other forms that are highly reactive with ozone. Polar stratospheric clouds only occur at high latitudes in winter or near winter, when temperatures in the lower and middle stratosphere fall below about 78 °C.
In winter, a vortex forms at the poles. This vortex is a columnar or spiral air mass with cyclonic rotation. As a result, when this vortex forms, the region becomes “isolated” and concentrates very cold air throughout the area. This favors condensation over the troposphere and allows these clouds to form.
Why did the clouds appear?
In order to analyze why clouds of this type have appeared just now, it is necessary not to examine what is happening in the lower atmosphere, in the troposphere, a region we usually work with for weather forecasting. It is necessary to look for higher places where nacre (ice) or nitric acid clouds are formed.
The data is revealing. Note the graph above, which shows the temperature development at a pressure level of 10 hPa in 2021 and 2022 between 65 °S and 90 °S. Note that at the moment the temperature is well below the historical average at 10 hPa, so with altitude temperatures that favor these clouds.
Effects on ozone
Science has proven that polar stratospheric clouds are linked to ozone depletion. The first major article on the subject, entitled “The Hole Story,” written by Gabrielle Walker, appeared in the March 25, 2000 issue of New Scientist magazine.
Scientists recently found that out Polar stratospheric clouds, long known to play an important role in ozone depletion in Antarctica, are also occurring with increasing frequency in the Arctic..
Such highaltitude clouds, which form only at very low temperatures, contribute to ozone destruction in two ways: they provide a surface that converts benign forms of chlorine into reactive, ozonedepleting forms, and they remove nitrogen compounds that inhibit chlorine’s destructive effects mitigate . . .
In recent years, the atmosphere over the Arctic has been cooler than usual, and polar stratospheric clouds have persisted well into the spring months. As a result, ozone levels have dropped. In 2020, a rare and large ozone hole opened up in the Arctic region.