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Polar Stratospheric Clouds And Its Dark Secret



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Thankfully, ozone is not the only molecule the chlorine atom reacts with. They also interact with other chemicals in the atmosphere to form two very stable compounds —hydrochloric acid and chlorine nitrate. These two compounds have locked up the majority of chlorine in the stratosphere, keeping them out of harm’s way.

But during the long winter months in the polar regions where there is little sunlight, temperature becomes so frigid in the stratosphere that clouds begin to form despite the air being so thin and dry. These nacreous clouds, formed out of frozen crystals of water, nitric acid and sometimes sulphuric acid, provide an ideal surface upon which chemical reactions occur that release the free chlorine atoms back into the atmosphere. The presence of sunlight is essential to the equation, so this happens only during spring when sunlight returns to the poles, and ultraviolet light break the bonds between the chlorine atoms. The process can only stop once the nacreous clouds are destroyed by air flows from lower latitudes.

Such chemical reactions could not take place anywhere else in the atmosphere. This is why the ozone hole is more prominent in the polar regions than elsewhere. Also, nacreous clouds are more commonly observed in the southern hemisphere. Consequently, the ozone layer is more depleted over the south pole, compared with the north pole.

Also read: Upside Down RainbowsandFire Rainbows


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Polar stratospheric clouds

nacreous clouds


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Sources: NASA / Ozone Hole Watch / Atoptics / The Guardian
