Cumulus clouds dot the afternoon sky. Image credit: Melynda Huskey, Flickr //CC BY-NC 2.0

Clouds are incredible. Their endless shapes can add beauty to a sunny afternoon or terror to a day marked by tragedy. When you look at how diverse these billowing formations of atmospheric water are, it’s easy to forget that they’re just that—atmospheric water. Even so, there’s much more to clouds than meets the eye. Here are 15 interesting tidbits about these mainstays of everyday life.

1. THEY'RE NOT WEIGHTLESS.

Clouds look like they weigh little more than a tuft of cotton, but they’re heavier than they look. Your average cumulus (fair weather) cloud can weigh more than a million pounds, and a vivacious thunderstorm can pack billions (if not trillions) of pounds of water in one tiny part of the sky. Yet, all of that weight seems effortlessly suspended in the air. It’s both a little unsettling and, at the same time, awesome to think about.

2. CIRRUS CLOUDS ARE MADE OF ICE.

Wispy cirrus clouds fill the sky near sunset. Image credit: Dennis Mersereau

While most clouds we see are made up of tiny liquid water droplets, there is one common type of cloud that’s made of ice: cirrus. These clouds are collections of ice crystals that form in the upper levels of the atmosphere when water vapor deposits onto tiny particles like dust or smoke. Strong winds then shred these clouds apart, giving them their iconic wispy appearance.

3. VIRGA DOES A DISAPPEARING ACT BEFORE IT REACHES THE GROUND.

Virga falls from the clouds at sunset. Image credit: Bryce Bradford, Flickr // CC-BY-NC-ND 2.0

Another phenomenon that’s often mistaken for a cirrus cloud is something called “virga,” or precipitation that evaporates before reaching the ground. The great thing about virga is that it’s both cool to look at and won’t ruin your day; it’s an indication that the lower and middle levels of the atmosphere are very dry—usually too dry to rain or snow.

4. CONTRAILS BEGIN WITH HOT, MOIST JET EXHAUST.

Contrails lingering in the sky on a day with high upper-level humidity. Image credit: Mark Robinson, Flickr // CC BY-NC 2.0

While most clouds form from natural processes, some can occur as a result of human activities. The best example of this is a condensation trail, commonly known as a contrail for short. Contrails form from an airplane’s hot, moist jet exhaust condensing in the extremely cold air of the upper atmosphere. These cirrus clouds can immediately dissipate or linger for hours depending on how much moisture is present.

5. FEAR THE SUPERCELL.

The rotating updraft of a supercell looms over the horizon. Image credit: Niccolò Ubalducci, Flickr // CC BY-NC-ND 2.0

Most thunderstorms are uneventful, but a tiny percentage of them can grow strong enough that they rage for hours and produce unimaginable horror. These storms, known as supercells, are characterized by a rotating updraft that powers them like an engine. In addition to their enormous hail and monstrous tornadoes, supercells are known for their incredible appearance. The most striking part of a supercell is the rotating updraft, which looks like a column that stretches from the horizon to the heavens.

6. ANVIL CLOUDS ARE THE BEAUTIFUL RESULT OF A COLLISION.

An overshooting top towers over an anvil in an intense thunderstorm in Kansas in June 2009. Image credit: Jeff Slater, Flickr // CC BY-NC 2.0

One of the most impressive sights that fills the sky near a thunderstorm is a thin, flat cloud that covers an area miles around like an umbrella. This is known as an anvil cloud, and it occurs when a thunderstorm’s updraft hits the tropopause, usually the point at which air is neutrally buoyant and it can no longer rise on its own. The air hits this layer like a ceiling, spreading out in all directions and forming this beautiful feature.

7. IF YOU SEE AN OVERSHOOTING TOP, TAKE COVER.

Sometimes, though, an updraft will be so strong that some of the rising air shoots straight through the tropopause and continues soaring hundreds (if not thousands) of feet above the top of the thunderstorm. This creates an overshooting top, a cloud that looks like a dome on an intense thunderstorm. If you see an overshooting top on an approaching storm, it’s a good idea to take shelter, because it’s going to be a doozy.

8. SHELF CLOUDS APPEAR IN SPRING AND SUMMER.

A shelf cloud precedes a thunderstorm in Sydney, Australia. Image credit: Andrea Schaffer, Flickr // CC BY 2.0

Shelf clouds are a common sight in an afternoon thunderstorm during the spring or summer. These formations roll across the horizon like a shelf or a wedge suspended just above the surface, immediately preceding heavy rain and wind. Shelf clouds form as a result of rain-cooled air descending from a thunderstorm and hugging the ground like a bubble. This creates an outflow boundary, which acts like a mini cold front scooping up warm air ahead of it. The shelf cloud forms at the ridge of the pool of cold air, creating a striking scene.

9. MAMMATUS CLOUDS MEAN A WILD RIDE.

Mammatus clouds produced by a nearby thunderstorm. Image credit: David Putz/Connie Sieh, Flickr // CC BY-NC-ND 2.0

If you ever encounter mammatus clouds, chances are you just experienced horrible weather or you’re about to go through a wild ride pretty soon. These numerous, bulbous protrusions hanging high in the sky beneath a deck of clouds look like the mammary glands of a cow or human, hence their name. These clouds are thought to form due to intense turbulence produced by the strong thunderstorm, leading to their smooth, bubbly appearance.

10. ROLL CLOUDS FORM THE LEAD EDGE OF A BOUNDARY YOU CAN'T SEE.

A roll cloud lumbers over Canyon, Texas. Image credit: Kenneth Cole Schneider, Flickr // CC-BY-NC-ND 2.0

Roll clouds are similar to shelf clouds, forming along the leading edge of a boundary like a sea breeze or cold front. Unlike shelf clouds, though, these formations aren’t attached to a neighboring deck of clouds, unspooling across the sky like a thick rope. They’re both unnerving and beautiful, but like almost every other cloud mentioned here, also completely harmless.

11. IRIDESCENCE IS STUNNING BUT RARE.

Cloud iridescence around the thin edges of a cumulus cloud. Image credit: Mike Lewinsky, Flickr // CC BY 2.0

Every once in a while, you might be able to look up at the clouds near the sun and see an abrupt smattering of colors mixed together like the sun reflecting off of an oily sheen on a puddle. This is called “iridescence,” and it’s somewhat rare. Cloud iridescence occurs when sunlight diffracts through water droplets or ice crystals in very thin clouds.

12. A SKY FULL OF IRIDESCENCE GIVES YOU NACREOUS CLOUDS.

Nacreous clouds over Oslo, Norway, in 2008. Image credit: Eirik Newth, Flickr // CC BY-NC 2.0

An even rarer sight is a deck of nacreous clouds, which is pretty much an entire sky full of iridescent clouds. Nacreous clouds are technically called “polar stratospheric clouds,” as they occur in the stratosphere (tens of thousands of feet above the cruising altitude for jets) and are most commonly seen near the poles as they require extremely cold temperatures to form.

13. NOCTILUCENT CLOUDS ARE THE HIGHEST IN OUR ATMOSPHERE.

Noctilucent clouds after sunset. Image credit: Jan Erik Paulsen, Flickr // CC BY 2.0

A distant cousin to the nacreous cloud is the noctilucent cloud, which are thin, wispy clouds that occur in the mesosphere dozens of miles above Earth’s surface. These clouds are the highest that form in our atmosphere, and they reflect a beautiful blue hue as they appear to glow against the dark night sky. These clouds are most common near the Arctic/Antarctic Circles, including parts of northern Canada, Scandinavia, and Russia. Rocket launches can also produce these vivid formations.

14. HALOS NEED THE ICE CRYSTALS IN CIRRUS CLOUDS TO FORM.

A halo around the Moon. Image credit: Nico Nieuwstraten, Flickr // CC BY-NC-ND 2.0

A halo is a spectacular sight that occurs when sunlight or moonlight scatters through the ice crystals that make up a thin layer of cirrus clouds covering the part of the sky directly between the observer and the celestial body. Most halos completely encircle the Sun or the Moon, but depending on the shape or size of the ice crystals, the halos can be partial, inverted, or appear on different sides of the sky.

15. DIAMOND DUST ONLY APPEARS IN THE EXTREME COLD.

Diamond dust is extremely hard to photograph—the Sun creates a sun dog (a type of rainbow-tinted halo) in the diamond dust close to the ground in this picture. Image credit: Peter von Bagh, Flickr // CC BY 2.0

Fog is simply a stratus cloud that forms at the surface. Freezing fog is fog that forms when temperatures are below freezing, consisting of supercooled water droplets that don’t have a nucleus to allow them to freeze into ice crystals. Diamond dust, on the other hand, is fog that forms into ice crystals instead of water vapor. This rare event occurs when the air is so cold (usually below 0°F) that water vapor deposits onto tiny particles in the air, creating suspended ice crystals that float around like snow. Visibility usually doesn’t drop much during diamond dust events, leading to a phenomenon that looks like light snow falling on a brilliantly clear day.