If you catch a glimpse of an absolutely beautiful rainbow-coloured cloud over the grey winter sky around the poles, then you probably got introduced Nacreous clouds. They are also known as the ‘Mother of Pearl Clouds’ because of their resemblance to the substance nacre i.e., mother of pearl, gleaming in the sunlight. But don’t be fooled by the stunning beauty of these magnificent clouds. They are dangerous to the atmosphere. Let us find out more about them.&nbsp;&nbsp;



How are Nacreous clouds formed?&nbsp; These spectacular clouds form during the winter in the polar region’s stratosphere, a layer of the atmosphere at a height of 15,000 to 25,000 metres above the Earth’s surface. Interestingly, they rarely form in the stratosphere, which is dry, and clouds require moisture to form. So, what leads to the creation of Nacreous clouds here? Well, one possible explanation could be that recent storms have pushed moisture high into the atmosphere in this layer. Temperature also influences their formation. These clouds only form below -78°C! Only high altitudes experience such temperatures in the winter. At this temperature, any moisture in the air turns into ice crystals. The is the reason behind the rainbow-colored appearance of these clouds. Actually, the light of the sun is refracted by the ice crystals within these clouds, creating a rainbow effect. Nacreous clouds are usually visible only after sunset or shortly before dawn, a period known as ‘nautical twilight’.&nbsp;



Why are they dangerous for the atmosphere?&nbsp; Nacreous clouds have a sinister side to them. They contribute to the depletion of the Earth’s ozone layer which shields the Earth from the sun’s harmful ultraviolet rays.&nbsp;&nbsp;



But how do nacreous clouds harm the ozone layer, you might wonder? The ice crystals in the clouds, which contribute to their beauty, are at fault. The ice crystals promote a reaction between a molecular oxygen in the ozone layer and other gases in the stratosphere such as chlorine and bromine. The ice crystals act like tiny nuclei on which the ozone breakdown by chlorine or bromine occurs. Each atom of chlorine in the stratosphere is estimated to destroy over 100,000 ozone molecules! This phenomenon is more common during the winter and in the Antarctic regions. This is why the ozone layer over the South Pole is more depleted compared to the North Pole.&nbsp;



Greenhouse gases are the problem here!&nbsp; The culprits behind the depletion of the ozone layer happen to be us, humans. Our activities lead to the presence of ozone-destroying gases in the stratosphere. Yes, we are talking about the ones that lead to the generation of of greenhouse gases. So, what greenhouse gas in particular do you think is the root cause of it? Here’s a clue: It is used in refrigerators and aerosol cans but was banned in 1987. And the answer is chlorofluorocarbons (CFCs). Now you might wonder why a gas banned back in the late 80s is still causing trouble? This is because the effects of CFCs take 50–100 years to disappear from the atmosphere! Till then, they will continue producing Nacreous clouds and damaging the ozone layer!&nbsp;&nbsp;



Although these clouds are frequently seen in Antarctica, they can now be seen in Britain too. This is a major cause of concern because even though CFCs are not being released into the atmosphere anymore, other greenhouse gases are produced every day, making the atmosphere around us hotter and that above us colder.&nbsp;&nbsp;

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