For most people on Earth, clouds have been a part of life since their earliest memories. We lay on hillsides imagining dragons and great spaceships in the billowing pillows of water vapor floating a few miles above our heads. Clouds bring the rains, block the sun and are a direct connection to our water cycle and weather patterns. Even in elementary school, we are taught to memorize different types of clouds and their shapes.
By a certain age in life, however, you reach a point where you assume that you’ve seen all the different cloud varieties on our planet. In other words, it’s hard for those puffy companions in the sky to surprise up. Even the most tumultuous thunderstorm, packed with its angry cumulonimbus clouds, eventually seems like yesterday’s news.
However, would you believe that there are certain rare types of clouds that the vast majority of human beings will never see? In fact, you’ve probably never even heard of them, since they occur in such a far-flung part of the planet, and require such specific conditions to exist! Yes, nacreous and noctilucent clouds are some of the most elusive cloud formations on Earth, but also two of the most fascinating!
What are Nacreous Clouds?
In the early hours before dawn, or in the first hours after the sun descends, if you are at an extreme enough latitudes, you may have the rare and wondrous opportunity to see nacreous clouds gleaming 10-15 miles up—higher than even the most massive storm clouds! These thin, wispy clouds don’t appear to move, and have an incredible iridescence that has earned them the nickname “mother of pearl” clouds. At first glance, they seem impossible, as they seem to glimmer and roil quietly, like a dream, but they are very real!
Primarily seen in the skies above Alaska, northern Canada, parts of Scandinavia, Russia, Iceland, Antarctica, and even the northern tip of the UK, these polar stratospheric clouds (PSCs) are rare both because of their transience, their extreme location and the weather conditions required for them to form. There are low-level clouds in the troposphere that have similar iridescence, but it is nowhere near as bright or long-lasting as the high-flying nacreous clouds. These blazingly bright formations are found in the bitterly cold space of the lower stratosphere, and form when the temperature at those heights falls below -78 degrees Celsius (roughly -110 degrees Fahrenheit). Any moisture in the air at that temperature will form tiny ice crystals of supercooled liquid.
Due to their incredible height in the sky, the rays of the sun, in the hours after sunset, will still strike the clouds and those crystals will refract the light, which is where the brilliant iridescence comes from. So, whether you’re approaching the North Pole or the South Pole, if the temperature falls to a truly brutal depth, there’s a good chance that you will be able to witness these remarkable and unforgettable clouds, making you one of the few to ever see them!
What are Noctilucent Clouds?
If you thought that nacreous clouds were a rarity, they’re nothing compared to noctilucent clouds, which are found in Earth’s upper atmosphere, in a region called the mesosphere. These are the highest clouds on Earth, sometimes lingering more than 50 miles above the surface of the planet—three times higher than nacreous clouds! Nacreous clouds have a shimmering iridescence caused by lingering rays of sunlight, and the same is true of noctilucent clouds, but they are usually seen only at night, when the sun is well below the horizon during local summer in those extreme latitudes. In fact, the name “noctilucent” means night-shining, when roughly translated from Latin. You can recognize these clouds because of their rippling appearance, like slowly moving waves that are only lit from below, on one edge of the crest, seemingly bathed in moonlight (but it’s actually caused by weak sunlight!).
These clouds primarily appear in the summer months between the 50 and 70 degree latitude lines both north and south of the equator. In other words, you once again need to be approaching either the North or South Pole to witness these majestic natural wonders. At more than 50 miles high, these clouds can catch the light from the sun, and may last all night thanks to the sun’s constant, meager illumination. These clouds are not nearly as bright or vibrant as nacreous clouds, but their presence in otherwise dark skies creates just as mesmerizing of an effect.
For those who don’t know, normal tropospheric clouds form when moisture attaches to other particles or molecules in the atmosphere, but in the mesosphere, where so few other particles exist, these clouds can spontaneously form from water vapor itself, or even through weak interactions with dust particles. These rare particles may come from micrometeors entering the atmosphere, particulate matter from volcanoes, or manmade microparticles that have drifted up from Earth, such as CFCs (chlorofluorocarbons), and haven’t dissipated, despite strict environmental regulations in recent decades.
The Dark Side of the Clouds
Seeing these clouds is something that few people could ever forget, but they have been witnessed in increasing numbers in recent decades. The first sighting of a noctilucent cloud was less than 150 years ago, while nacreous clouds have been documented for far longer, but since their formation can often be caused by particulate matter in the higher layers of the atmosphere, human effects are boosting their frequency.
In fact, scientists and researchers pay close attention to the appearance of these polar cloud formations, because they can indicate changes that are going on in the upper atmosphere. These parts of our planet are highly sensitive to changes in molecular consistency, so a rash of noctilucent clouds may indicate an unusually high percentage of unwanted particulate matter in the air.
For example, an increased presence of methane in this atmosphere means that more of that methane will dissociate in reaction with ozone, with water vapor as a byproduct, which can lead to ice crystal formation. Additionally, the more CFCs in the atmosphere, the more the stratosphere shrinks, further dropping the temperature of the mesosphere, and making the formation of noctilucent clouds more likely. Basically, the more noctilucent clouds we see—despite their beauty—the more manmade harm we’re likely doing to the environment.
A Final Word
Nacreous and noctilucent clouds are wonderful reminders of how expansive our natural world is, from the deepest oceans to the highest points of our atmosphere, everything is connected. A volcano that erupts in Hawaii can affect the color of the clouds high over Iceland. So, if you ever find yourself in the most remote parts of the planet, look up…. you just might catch something truly special.
- Atmospheric Optics
- The Conversation
- Project PoSSUM
- ZME Science: not exactly rocket science