Asperatus, arcus, mammatus: strange names for unusual clouds

Scientific names like arcus, altocumulus and mammatus, may sound bizarre, but they are the scientific names of clouds of rare and surreal shapes. Some can be really disturbing, so disturbing that superstitious people may think they are harbingers of the Apocalypse!

The asperatus (Latin name for brutal) is a very rare type of cloud. It was proposed to the International Cloud Atlas of the World Meteorological Organization in 2009 as a hitherto uncharted formation.

Mammatus or mamma is a term that refers (in meteorology) to clouds of circular shape at the base of convective clouds like altocumulus and cumulonimbus pockets. Their color is normally blue-gray but they can take on gold-  or reddish tones when the setting sun illuminates them. They are most often associated with thunderstorms.

The arcus is a type of low cloud in the shape of a roll. They lay on a horizontal plane appearing during a storm and are located at the immediate front line of precipitation arc.


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The formation of cloud is determined by some rules of physics, such as the Kelvin-Helmholtz instability law. This law states that a wave motion is created when two thermally stable fluids are superimposed and move at different speeds at their contact surface. The effect studied in the nineteenth century by the physicists Lord Kelvin and Hermann Ludwig von Helmholtz showed that the difference in speed will result in a cross- border turbulent flow.

Nacreous clouds form in the stratosphere at an altitude between 15,000 and 25,000 meters. Nacreous clouds are rare and mostly form during the winter months near the poles. They were described by astronomer Robert Leslie in 1885. Involved in the formation of holes in the ozone layer, they support the chemical reactions that produce molecules of chlorinated compounds. These molecules serve as a catalyst for the reaction destroying ozone molecules.

An altocumulus lenticularis or lenticular cloud is a type of altocumulus. They are stationary and take the form of wings of airplane. They are formed downstream from the mountaintops and signal the presence of a downward wind. Depending on natural conditions, there is often a stack of several layers of this cloud. It is popular with lovers of gliding because it shows the presence of a strong and stable descending wind.

Really amazing, don’t you think?


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