Unusual Clouds of the Upper Atmosphere
The first of the two strange upper atmosphere clouds are called nacreous clouds, also known as mother-of-pearl clouds due to their vibrant colors. These clouds typically form in the polar regions of the stratosphere, the layer of the atmosphere just above the troposphere. Clouds are usually not found in the stratosphere because it is warmer there than at the top of the troposphere. The warmth of the stratosphere prevents air from rising past the troposphere due to convection, which is why clouds usually can’t form there. Nacreous clouds are the exception. Although the exact way they form is not completely understood, there are two major theories as to how they come about. The first method is by lingering updrafts of powerful storms. Storms have a section where they draw up moist, warm air called an updraft, and occasionally they can be so powerful that they drive water vapor into the lower stratosphere. If the temperature is cold enough in the lower stratosphere (about -85 degrees Celsius), spherical ice crystals can form and condense into nacreous clouds. The second method is through lifting from tall mountain chains. When air travels up tall mountain chains, sometimes water vapor from the lower atmosphere can find its way into the stratosphere via complex physical processes. Once again, if it is cold enough, nacreous clouds can form. Since the minimum temperature required for these clouds to form is so low, the only places one can see these clouds are near the poles. Further adding to the rarity of their sightings, the rainbow colors of nacreous clouds can only be seen when the sun is very low in the sky. Otherwise, nacreous clouds strongly resemble wispy cirrus clouds. An example of nacreous clouds at sunset is given below (photo credit: Albert de Nijs).
These bizarre, scarce clouds are beautiful atmospheric phenomena that represent the complexity of the atmosphere. Complicated interactions in the upper atmosphere happen often, but it is rare for the average person to see such a clear product of them. Upper atmosphere clouds are truly fascinating formations that add to the allure of the sky.
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© 2019 Weather Forecaster Cole Bristow
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