David Alvarado photographed polar stratospheric clouds, or as they are called mother-of-pearl clouds, that appeared in the sky over Peru on January 18, 2018.
These clouds, unlike their tropospheric counterparts that we can see in the sky every day, form at altitudes of 15 to 25 km in the cold regions of the stratosphere (temperatures below –78 °). And they rarely appear. Perhaps, in the entire history of atmospheric physics, nacreous clouds have been observed only about a hundred times. Even the highest clouds, called silvery, we can observe more often than mother-of-pearl. It is clear that observation of nacreous clouds, as well as noctilucent ones, requires at least clarifications in tropospheric clouds.
Nacreous clouds, if we are lucky, we can observe, either in the evening immediately after sunset, or shortly before the appearance of the daylight. They are usually fully illuminated by the Sun for 20 minutes after sunset or before sunrise. These light and transparent clouds cannot be confused with anything else: while the lower, tropospheric clouds are still in the earth's shadow and stand out in dark silhouettes against the background of dawn, the high stratospheric mother-of-pearl clouds, due to their higher height above the earth, are already illuminated by the Sun and visible in the sky, painted in bright pearlescent colors. This color range is given by small crystals of water and nitric acid of approximately the same size, which make up the cloud and refract the sun's rays.
In the sky, the cloudy elements of these clouds are visible in the form of "lenticular" ("lenticular") forms, similar to Altocumulus lenticularis. At the same time, a wavy structure can be traced. The nacreous clouds differ in color and characteristic shape from the higher noctilucent clouds that form at heights of 75 km. Therefore, it is impossible to confuse them. In addition, noctilucent clouds are visible only in summer, and nacreous clouds appear mainly in winter.
Scientific research on these clouds is very important for a better understanding of the processes taking place in the stratosphere. After all, the stratosphere plays an important role in our life. First, it contains the ozone layer, which protects us from the harmful effects of solar radiation. Second, the influence of dynamic processes in the stratosphere also affects tropospheric dynamics. There are a number of scientific studies of this effect, which show that circulation processes in the stratosphere with a certain delay affect tropospheric circulation, which may serve as a key to the creation of more accurate methods for long-term forecasting of weather anomalies. And studies of nacreous clouds allow scientists to unravel the riddles of such processes occurring at high altitudes as condensation of water vapor and the conditions for its existence in the stratosphere. It is also possible to determine the nature and speed of air movements at altitudes from 20 to 30 km.
According to their chemical composition, nacreous clouds are divided into three types: Ia, Ib, II. Type I clouds contain nitric acid and water. So, type Ia consists of crystals of nitric acid and water. Type Ib includes supercooled droplets of sulfuric acid. But type II consists exclusively of water crystals.
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The air in the stratosphere is very dry, so clouds usually do not form in it. But in winter, the temperature of the stratosphere sometimes drops to such values that clouds begin to form in it. As we noted above, nacreous clouds form at temperatures below –78 °. Such temperatures are observed in the lower stratosphere in winter. In Antarctica, temperatures sometimes even drop below –88 ° C often result in type II stratospheric clouds. In the Arctic, such low temperatures are rare.
As we noted above, stratospheric polar clouds are composed of small crystals of water and nitric acid. And the chemical reactions taking place in these clouds are the result of the transformation of the composition of the stratosphere. Chlorine, which enters the stratosphere mainly from industrial centers located on the earth's surface, begins to react with ozone, which leads to the depletion of the latter. Thus, these beautiful clouds are involved in a chain of events that lead to the depletion of the ozone layer.