Almost always, clouds form in the troposphere, the lowest level of the atmosphere where we live and where all weather takes place. The troposphere easily has the most water vapor compared to any other layer of the atmosphere. In fact, the other layers have so little water vapor that it is incredibly difficult for clouds to form anywhere above the troposphere. However, on rare occasions, special mechanisms in the atmosphere can force water vapor to exist in above average quantities in the stratosphere or mesosphere.
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).
The second type of upper atmosphere clouds is called noctilucent clouds. These clouds form in the mesosphere, the layer right above the stratosphere. These clouds are known to be made of tiny ice particles, and are even thinner than nacreous clouds. Like nacreous clouds, they are still not completely understood. But, are plausible explanations as to how they get their water vapor. One explanation is that trapped water vapor in small meteors escapes into the mesosphere, where most meteors burn up. Another explanation is that the water vapor comes from chemical changes in methane over time. However these clouds form, one thing is certain: they are truly a sight to behold. These clouds are only seen right after the sun is over the horizon, when most of the sky is dark. The clouds glow as light from the sun strikes their ice particles from beyond the horizon. Sometimes, the clouds are so brilliant that they are mistaken for the aurora borealis. The only areas one can find these clouds are between the latitudes of about 50 to 65 degrees (north or south) during the summer when the mesosphere is coldest. The mesosphere must be quite cold for noctilucent clouds because they only form below -120 degrees Celsius. An example of one of these clouds is included in the thumbnail image of the article, while a diagram of atmospheric layers is included at the bottom (credit: NIWA).
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