A NATURAL PHENOMENON: NACREOUS CLOUDS

Nacreous Clouds: Stunningly Beautiful, Surprisingly Destructive

Some natural phenomena are rare and beautiful, like red diamonds, the Northern Lights and double rainbows. But others, like nacreous clouds, are rare, beautiful and destructive.

Behind their iridescent, rainbow-colored façade, these clouds damage on atmosphere.

According to the Australian Antarctic division, an arm of the Australian government, nacreous clouds are a type of “polar stratospheric cloud.” They’re a main factor in the formation of ozone holes in the Arctic and Antarctic.

How Nacreous Clouds Form

Nacreous clouds develop in high latitudes – 60 to 90 degrees – of the Northern and Southern hemispheres.  They are most likely to form in winter.They form 13-19 miles up in the lower part of the stratosphere.

Nacreous clouds form in the stratosphere, at 70,000 feet or above. This is also where the ozone layer reside Clouds generally don't form in the stratosphere. Typically, it’s far too dry there for ice crystals or supercooled water droplets to develop.

But nacreous clouds are different. They're a mixture of supercooled water, ice crystals and nitric acid.

During winter, when there’s  little sunlight in the polar regions, the stratospheric polar vortex strengthens and locks out warmer air. This creates extraordinarily cold temperatures in the stratosphere. At this phase, water vapor in the stratosphere changes to a supercooled liquid or ice crystals.

At this point, the atmosphere is still unaffected by the clouds, but atmospheric chemistry will change that.

Destructive force

As pretty as they may look, nacreous clouds have a darker side too. These clouds enhance the breakdown of the Earth’s ozone layer, a vital part of our atmosphere that provides protection from the sun’s harmful ultraviolet rays. The ice crystals in the clouds encourage a chemical reaction between the ozone layer, which is made up of a specific type of molecular oxygen (O₃), and gases such as chlorine and bromine. In fact, it is estimated that just one atom of chlorine in the stratosphere can destroy over 100,000 ozone molecules. This means chlorine acts as an agent to accelerate the breakdown of ozone.

How did these chemicals get into the stratosphere? In a word: Humans.

The presence of these ozone-destroying gases in the stratosphere is a problem of our own making. The prime reason for their presence is our use of chlorofluorocarbons (CFCs) in products such as refrigerators and aerosol cans. While usage of CFCs has been significantly reduced, it is estimated that it may take another 50-100 years before the effects of CFCs in the atmosphere is reduced.

The nacreous clouds provide an ideal surface for chemical reactions that involve CFC-derived compounds. As these compounds interact with nacreous clouds, chlorine gas is released.

As soon as sunlight comes into contact with a chemical compound, the molecule absorbs energy and enters an unstable state, initiating a photochemical reaction.

This is where the atmosphere is affected: Chlorine gas kills the naturally occurring ozone in the stratosphere.

While the hole in the ozone layer is still general at certain times of the year, due to the reduction of CFC output in the 1980s and early 1990s, stratospheric ozone concentrations have been on the rise.

So while they may be beautiful, nacreous clouds will be harming the ozone layer well into the future.