Wispy cirrus clouds or cartoonishly fluffy cumulus clouds can be cool enough on their own. But occasionally, you’ll witness a formation that looks eerily like a UFO, a breaking ocean wave, or a white version of the smoke monster from Lost. Those and other unusual clouds have names, too. Read on to find out what they are and why they happen (and what they might mean for your afternoon picnic plans).
1. Lenticular Cloud
True to their name, lenticular clouds are lens-shaped, and they’re also often compared to UFOs or stacks of pancakes. When high winds encounter a tall structure—like a mountain or even a building—the air is sometimes diverted up and over it. That air cools as it rises, and if it contains enough moisture, it will condense into a flat cloud formation at the crest of the wave. Though a lenticular cloud often looks like it’s hovering stock-still right above a mountain peak, it’s actually in constant motion: The air warms up and dries out as it progresses downward, and the wind steadily refills the cloud with newly condensed air from the other side.
When a straight-shooting wind runs into an obstacle, it won’t immediately return to its straight path once it clears the hurdle. “You take your grandma's Cadillac and drive it over a speed bump, and after that it goes up and down for a while,” cloud physicist Patrick Chuang explained to WIRED. In other words, the air will continue to move in waves on the other side of the obstacle. Lenticular clouds can form at the crest of each wave, which explains why you might see them suspended above nothing.
2. Fallstreak Hole
The reason a fallstreak hole is sometimes called a “hole punch cloud” doesn’t need much clarification: It looks like someone used a massive hole-puncher on a cloud. In order to create one, you first need a layer of cirrocumulus (high-altitude) or altocumulus (medium-altitude) clouds that contain “supercooled” droplets. Though these droplets are at below-freezing temperatures, the lack of anything solid for them to form ice around keeps them in a liquid state. When something solid is introduced—often ice crystals that fall from an airplane flying overhead—all the supercooled droplets around it freeze and fall, leaving a hole in their wake.
3. Kelvin-Helmholtz Cloud
It’s not a coincidence that Kelvin-Helmholtz clouds—named for Lord Kelvin and Hermann Von Helmholtz, who studied the physics that causes them—resemble certain ocean waves: They both happen when the top layer of a substance is moving at a faster rate than the bottom layer. When the upper cloud layer is warmer than the lower layer, it’s also less dense, and it can move along more quickly than the colder, denser cloud below. The instability created at the boundary causes the top edge of the lower layer to rise as it moves forward until it curls over, much like a wave breaks. If you happen to overhear the pilot mention Kelvin-Helmholtz clouds during a flight, expect some turbulence.
4. Roll Cloud
Roll clouds are long, low, tubular arcus clouds that can extend for hundreds of miles. Like Kelvin-Helmholtz clouds, they occur when there’s warm air atop cooler air—known as an inversion—and indicate instability. In this case, it’s often because of a thunderstorm. Roll clouds are exceedingly rare, and the only place that produces them with any consistency is Australia’s Cape York, where they’re called “Morning Glory clouds” and don’t necessarily coincide with storms. As for why they’re so consistent there, it’s still a bit of a mystery. But scientists believe it has to do with the routine collision of the sea breeze moving east across the cape from the Gulf of Carpentaria and the sea breeze coming west from the Coral Sea.
5. Anvil Cloud
When a thunderstorm develops, air currents called updrafts rise, cool, and form clouds. But there’s a point in the atmosphere between the troposphere and the stratosphere—called the tropopause—where air stops cooling with altitude. If an updraft is strong enough to get there, it won’t be able to rise any higher and will instead start moving laterally, forming a flat-topped cloud called a cumulonimbus incus, or anvil cloud (incus means “anvil” in Latin). Occasionally, an updraft will be strong enough to rise past the tropopause and into the stratosphere, in which case a cloud protrusion known as an “overshooting top” will appear above the anvil’s flat surface. If you see this, brace yourself for an especially formidable storm.
6. Asperitas Cloud
In 2008, Cloud Appreciation Society founder Gavin Pretor-Pinney suggested that the World Meteorological Organization add a new designation to its International Cloud Atlas to describe tumultuous, billowing cloud waves that lack an obvious pattern. Almost a decade later, the WMO obliged, tweaking Pretor-Pinney’s proposed name—asperatus, Latin for “roughened”—to the noun form of the word: asperitas, or “roughness.” They form when stable, lateral cloud stretches at middle to low altitudes get disrupted from more than one direction, often by a brewing storm.
“Imagine cannonballing in a pool. Once you hit the water, waves—or a splash—radiate outward in circles,” meteorologist Matthew Cappucci wrote for The Washington Post. “That’s analogous to local pockets of air moving up or down in the atmosphere and inducing gravity waves, as is common in the vicinity of thunderstorms. If multiple people leaped in the pool simultaneously, however, turbulent wavelike motions would sculpt the surface of the water into chaos.”
7. Nacreous Cloud
If the Northern Lights don’t show up during your trip to the Arctic, nacreous clouds, also known as mother-of-pearl clouds, are probably the next best thing. They’re a type of polar stratospheric cloud, so named because they form in the stratosphere when temperatures are well below freezing. For nacreous clouds, that means about -180°F or colder, so they’re mostly confined to Arctic and Antarctic regions. The ice crystals in nacreous clouds diffract light waves, creating a dazzling display of colors across the sky.
Beautiful to look at, but bad news for the environment. Research has shown that polar stratospheric clouds activate the chlorine in compounds like chlorofluorocarbon—a human-made greenhouse gas that was used in refrigerants, aerosols, and cleaning solutions. That chlorine destroys the ozone layer, which is part of the stratosphere.
8. Mammatus Cloud
Like so many other entries on this list, mammatus clouds—from the Latin word mamma, meaning “breast” or “udder”—require an unstable atmosphere and often appear around stormy weather. The details of why they form are still up for debate, but some scientists believe it involves sublimation: when ice turns directly into water vapor, without first stopping at the liquid stage. According to this theory, ice crystals from a cloud become vapor, which causes the surrounding air to get colder (and therefore denser) and start to sink in pockets. Though they’re most commonly seen beneath a storm cloud following severe weather, they can also form from fair-weather clouds like cirrus and altocumulus.