Polar Stratospheric Clouds And Its Dark Secret

Sometimes deep into the winter, after sunset or before dawn, sheets of unbelievably bright and vividly colored clouds can be seen forming against the partially dark twilight sky. This rare type of cloud formations are known as “polar stratospheric clouds” or “nacreous clouds”, and they can only be seen from high latitude regions such as Iceland, Alaska, Northern Canada, the Scandinavian countries and Antarctica. The Scandinavians call them “mother of pearls” because of their spectacular iridescent colors. Described as “one of the most beautiful of all cloud formations,” nacreous clouds are also the most destructive to our atmosphere. Their presence encourages a chemical reaction that breaks down the ozone layer, which is an essential shield protecting us from the sun's harmful rays.

Nacreous clouds develop at very high altitudes, within the lower stratosphere at 70,000 feet or above. For comparison, some of the highest clouds in the troposphere have a ceiling height of about 40,000 feet.

Clouds generally do not form in the stratosphere because there is not enough moisture. But nacreous clouds are different. They are not entirely composed of water droplets, but a mixture of naturally occurring water and nitric acid that comes from industrial sources.

Decades ago, we started using substances such as chlorofluorocarbons (CFCs) in aerosols and refrigeration. These chemicals have been phased out, but they are so stable they persist to this date. CFCs take several years to rise through the troposphere until they reach the stratosphere where they begin to break down by ultraviolet light producing free chlorine atoms. Like any free radicle, chloride ions are very reactive and they go on a rampage attacking and destroying the ozone layer.