Some unusual clouds, like lenticulars, are familiar. Others, like Kelvin Helmholtz Waves, you may never see in your lifetime. But how do these remarkable natural shapes form?
As children, we all played that game where we would lie on the grass and find various shapes in the sky. Depending on the time of day, wind speed, moisture, and climate, we can see works of art, a collection of farmyard animals, or even a story.
What creates the strangest shapes and patterns we see? Let’s take a look at some of the rarest cloud formations. Some of them seem to tell an entire novel.
Mammatus clouds
Mammatus clouds are cotton-ball-like clouds that form at the base of cumulonimbus, cirrus, altocumulus, and even volcanic ash clouds.
The name derives from the Latin word for breast, “mamma”. Usually, they indicate approaching bad weather, particularly thunderstorms. While many believe that mammatus clouds presage tornadoes, that is a myth.
Their puffy shape comes from the turbulent conditions in cumulonimbus clouds. A large cluster of mammatus may stretch hundreds of kilometers and can last for several hours. Each pocket is around half a kilometre long and one kilometre in diameter.
Mammatus clouds follow a thunderstorm over northern Nebraska. Photo: Menno van der Haven
Surprisingly, scientists are not entirely sure how they form. Meteorologists do agree that they include sinking cold air and the presence of ice crystals. If more ice crystals are present, the mammatus last longer, as evaporation takes longer. This is contrary to how clouds usually form, by rising air.
In the early 20th century, German scientists determined that mammatus clouds appear more often in summer than winter, that temperature and wind influence their formation, and that they can appear in clouds other than cumulonimbus.
Fallstreak hole
Sometimes called “hole punch clouds” or “sky punch clouds”, fallstreak holes are very rare. You can find these large holes in cirrocumulus and altocumulus clouds.
One popular myth states that UFOs make the holes as they descend from the upper atmosphere. In fact, the cirrocumulus and altocumulus clouds contain supercooled water that has not yet turned into ice crystals. The water stays suspended in the atmosphere until ice crystals eventually form and the surrounding water droplets evaporate. The ice crystals then fall in a streak, creating a hole that can grow up to 50km wide. These usually occur in stable weather.
A fallstreak hole over Victoria, Canada. Photo: Shutterstock
Scientists point out that our own technology helps form these holes. Planes can trigger the freezing of the supercooled water. When a plane flies through, the air passes over its wings, momentarily freezing the water droplets.
Nacreous clouds
Nacreous clouds are beautiful, rare cloud formations found in the circumpolar Arctic, Antarctica, and some parts of the UK. These stratospheric clouds are often mistaken for cirrus clouds. Most of the time, they don’t stand out. However, they change dramatically before dawn and after sunset. The sky then becomes a kaleidoscope of thin, iridescent streaks. Some mistake these displays for the aurora borealis.
Their appearance comes from a complex combination of high altitude, low stratospheric temperatures (-85°C), and the diffraction of light. They form at high altitudes, ranging from 15,000m to 25,000m, and contain smaller ice particles that scatter sunlight when the sun is one to six degrees below the horizon. They form in cold, dry polar vortexes.
Nacreous clouds. Photo: Pixabay
Despite their beauty, these clouds indicate climate change. Their frequent appearance can cause our ozone layer to break down. The clouds are composed of water, sulphuric acid, and nitric acid. The acids react with the O3 in our ozone layer, leading to ozone holes.
Scientists have determined that man-made chlorofluorocarbons cause these harmful elements to enter the stratosphere. So these spectacular yet damaging displays are often our doing.
Mother-of-pearl or nacreous cloud in Norway. Photo: Uwe Michael Neumann
Kelvin Helmholtz Waves
Kelvin Helmholtz Waves are perhaps the rarest cloud formation of all. Rumored to be the inspiration for Van Gogh’s masterpiece “Starry Night”, they are incredibly distinctive. They are mainly associated with cirrus, altocumulus, and stratus clouds over 5,000m.
Kelvin Helmholtz Waves are named after British mathematician/physicist William Thomson, first Baron Kelvin, and German physicist Hermann von Helmholtz.
Kelvin Helmholtz clouds. Photo: Red Buffalo Studios
They are also known as billow or fluctus clouds. They form mostly on windy days where there is a dramatic difference in air densities. The air above is moving faster than the air below. This temperature inversion, when warm air flows over cool air, creates a wave-like shape in the clouds.
Physicists call this surreal, otherworldly sight an instability. It describes a difference in velocity between two fluids. As the air is very unstable, planes will experience turbulence flying through these clouds.
Kelvin Helmholtz Waves on Jupiter. Photo: NASA
Kelvin Helmholtz Waves are not exclusive to our planet. They have been observed on Jupiter, Saturn, and even the surface of the Sun.
Lenticular Clouds
Lenticular clouds are the most familiar. These towering lens-shaped clouds envelope or cap the tops of mountains. They sometimes look like UFOs, and some UFO sightings trace back to them.
Lenticular clouds in Greenland. Photo: Pixabay
Lenticular clouds occur when air and moisture hit an obstacle. Stable air flows over mountains, creating waves on the mountain’s downwind side. If there is a lot of moisture in the air and the temperature is low enough, condensation forms these clouds.
In rare instances, you can glimpse iridescence on the clouds’ curves. While the instability can cause airplane turbulence, adventurous paragliders seek them out, soaring higher on the updrafts.
