15 Billowing Facts About Clouds

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.

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15 Podcasts That Will Make You Feel Smarter
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It's easy to feel overwhelmed by all the podcast options out there, but narrowing down your choices to the titles that will teach you something while you listen is a good place to start. If you're interested in learning more about philosophy, science, linguistics, or history, here are podcasts to add to your queue.

1. THE HABITAT

The Habitat is the closest you can get to listening to a podcast recorded on Mars. At the start of the series, five strangers enter a dome in a remote part of Hawaii meant to simulate a future Mars habitat. Every part of their lives over the next year, from the food they eat to the spacesuits they wear when they step outside, is designed to mimic the conditions astronauts will face if they ever reach the red planet. The experiment was a way for NASA to test plans for a manned mission to Mars without leaving Earth. The podcast, which is produced by Gimlet media and hosted by science writer Lynn Levy, ends up unfolding like a season of the Real World with a science fiction twist.

2. STUFF YOU SHOULD KNOW

Can’t pick a topic to educate yourself on? Stuff You Should Know from How Stuff Works is the podcast for you. In past episodes, hosts Chuck Bryant and Josh Clark (both writers at How Stuff Works) have discussed narwhals, Frida Kahlo, LSD, Pompeii, hoarding, and Ponzi schemes. And with three episodes released a week, you won’t go long without learning about a new subject.

3. THE ALLUSIONIST

Language nerds will find a kindred spirit in Helen Zaltzman. In each episode of her Radiotopia podcast The Allusionist, the former student of Latin, French, and Old English guides listeners through the exciting world of linguistics. Past topics include swearing, small talk, and the differences between British and American English.

4. PHILOSOPHIZE THIS!

Listening to all of Philosophize This! is cheaper than taking a philosophy class—and likely more entertaining. In each episode, host Stephen West covers different thinkers and ideas from philosophy history in an approachable and informative way. The show proceeds in chronological order, starting with the pre-Socratic era and leading up most recently to Jacques Derrida.

5. MORE PERFECT

In 2016, Radiolab, one of the most popular and well-established educational podcasts out there, launched a show called More Perfect. Led by Radiolab host Jad Abumrad, each episode visits a different Supreme Court case or event that helped shape the highest court in the land. Because of that, the podcast ends up being about a lot more than just the Supreme Court, exploring topics like police brutality, gender equality, and free speech online.

6. SLOW BURN

The Watergate scandal was such a important chapter in American history that it has its own suffix—but when asked to summarize the events, many people may draw a blank. Slow Burn, a podcast from Slate, gives listeners a refresher. In eight episodes, host Leon Neyfakh tells the story of the Nixon’s demise as it unfolded, all while asking whether or not citizens would be able to recognize a Watergate-sized scandal if it happened today.

7. LETTERS FROM WAR

Instead of using a broad scope to examine World War II, the Washington Post podcast Letters From War focuses on hundreds of letters exchanged by four brothers fighting in the Pacific during the period. Living U.S. military veterans tell the sibling's story while reflecting on their own experiences with war.

8. LEVAR BURTON READS

Just because you’re a grown-up doesn’t mean you have to miss out on the soothing sound of LeVar Burton’s voice reading to you. The former host of Reading Rainbow now hosts LeVar Burton Reads, a podcast from Stitcher aimed at adults. In each episode, he picks a different piece of short fiction to narrate: Just settle into a comfortable spot and listen to him tell stories by authors like Haruki Murakami, Octavia Butler, and Ursula K. Le Guin.

9. BRAINS ON!

Brains On! is an educational podcast for young audiences, but adults have something to gain from listening as well. Every week, host Molly Bloom is joined by a new kid co-host who helps her explore a different topic. Tune in for answers to questions like "What makes paint stick?" and "How do animals breathe underwater?"

10. SCIENCE VS

There’s a lot of misinformation out there—if you’re determined to sort out fact from fiction, it can be hard to know where to start. The team of “friendly fact checkers” at the Science Vs podcast from Gimlet is here to help. GMOs, meditation, birth control, Bigfoot—these are just a few of the topics that are touched upon in the weekly show. The goal of each episode is to replace any preconceived notions you have with hard science.

11. FLASH FORWARD

No one knows for sure what the future holds, but Flash Forward lays out the more interesting possibilities. Some of the potential futures that host and producer Rose Eveleth explores are more probable than others (a future where no one knows which news sources to trust isn’t hard to imagine; one where space pirates drag a second moon into orbit perhaps is), but each one is built on real science.

12. HIDDEN BRAIN

What motivates the everyday choices we make? That’s the question Shankar Vedantam tries to answer on the NPR podcast Hidden Brain. The show looks at how various unconscious patterns shape our lives, like what we wear and who we choose to spend time with.

13. PART-TIME GENIUS

The fact that it’s hosted by Mental Floss founders Will Pearson and Mangesh Hattikudur isn’t the only reason we love Part-Time Genius. The podcast from How Stuff Works wades into topics you didn’t know you were curious about, like the origins of Nickelodeon and the hidden secrets at the Vatican. Each episode will leave you feeling educated and entertained at the same time.

14. ASTRONOMY CAST

It’s a big universe out there—if you want to learn as much about it as possible, start with Astronomy Cast. Fraser Cain, publisher of the popular site Universe Today, and Dr. Pamela L. Gay, director of the virtual research facility CosmoQuest, host the podcast. They cover a wide range of topics, from the animals we’ve sent to orbit to the color of the universe.

15. SCIENCE OF HAPPINESS

The Science of Happiness podcast from PRI is here to improve your life, one 20-minute episode at a time. Science has proven that adopting certain practices, like mindfulness and gratitude, can make us happier—as does letting go of less unhealthy patterns like grudges and stressful thinking. With award-winning professor Dacher Keltner as your host, you can learn how to incorporate these science-backed strategies for happiness into your own life.

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How Does Catnip Work?
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If you have a cat, you probably keep a supply of catnip at home. Many cats are irresistibly drawn to the herb, and respond excitedly to its scent, rubbing against it, rolling around on the floor, and otherwise going nuts. There are few things that can get felines quite as riled up as a whiff of catnip—not even the most delicious treats. But why does catnip, as opposed to any other plant, have such a profound effect on our feline friends?

Catnip, or Nepeta cataria, is a member of the mint family. It contains a compound called nepetalactone, which is what causes the characteristic catnip reaction. Contrary to what you might expect, the reaction isn’t pheromone related—even though pheromones are the smelly chemicals we usually associate with a change in behavior. While pheromones bind to a set of specialized receptors in what’s known as a vomeronasal organ, located in the roof of a cat's mouth (which is why they sometimes open their mouths to detect pheromones), nepetalactone binds to olfactory receptors at the olfactory epithelium, or the tissue that lines the mucus membranes inside a cat’s nose and is linked to smell.

Scientists know the basics of the chemical structure of nepetalactone, but how it causes excitement in cats is less clear. “We don’t know the full mechanisms of how the binding of these compounds to the receptors in the nose ultimately changes their behavior,” as Bruce Kornreich, associate director of the Cornell Feline Health Center, tells Mental Floss. Sadly, sticking a bunch of cats in an MRI machine with catnip and analyzing their brain activity isn’t really feasible, either from a practical or a financial standpoint, so it’s hard to determine which parts of a cat’s brain are reacting to the chemical as they frolic and play.

Though it may look like they’re getting high, catnip doesn’t appear to be harmful or addictive to cats. The euphoric period only lasts for a short time before cats become temporarily immune to its charms, meaning that it’s hard for them to overdo it.

“Cats do seem to limit themselves," Michael Topper, president of the American Veterinary Medical Association, tells Mental Floss. "Their stimulation lasts for about 10 minutes, then it sort of goes away.” While you may not want to turn your house into a greenhouse for catnip and let your feline friend run loose, it’s a useful way to keep indoor cats—whose environment isn’t always the most thrilling—stimulated and happy. (If you need proof of just how much cats love this herb, we suggest checking out Cats on Catnip, a new book of photography from professional cat photographer Andrew Martilla featuring dozens of images of cats playing around with catnip.)

That said, not all cats respond to catnip. According to Topper, an estimated 70 percent of cats react to catnip, and it appears to have a genetic basis. Topper compares it to the genetic variation that causes some individuals to smell asparagus pee while others don’t. Even if a cat will eventually love the smell of catnip, it doesn’t come out of the womb yearning for a sniff. Young kittens don’t show any behavioral response to it, and may not develop one until several months after birth [PDF].

But some researchers contend that more cats may respond to catnip than we actually realize. In one 2017 study, a group of researchers in Mexico examined how cats might subtly respond to catnip in ways that aren’t always as obvious as rolling around on the floor with their tongue hanging out. It found that 80 percent of cats responded to catnip in a passive way, showing decreased motor activity and sitting the “sphinx” position, an indicator of a relaxed state.

There are also other plants that have similar effects on cats, some of which may appeal to a wider variety of felines than regular old catnip. In a 2017 study in the journal BMC Veterinary Research, researchers tested feline responses to not just catnip, but several other plants containing compounds similar in structure to nepetalactone, like valerian root, Tatarian honeysuckle, and silver vine. They found that 94 percent of cats responded to at least one of the plants, if not more than one. The majority of the cats that didn’t respond to catnip itself did respond to silver vine, suggesting that plant might be a potential alternative for cats that seem immune to catnip’s charms.

Despite the name, domestic cats aren’t the only species that love catnip. Many other feline species enjoy it, too, including lions and jaguars, though tigers are largely indifferent to it. The scent of the plant also attracts butterflies. (However, no matter what you’ve heard, humans can’t get high off it. When made into a tea, though, it reportedly has mild sedative effects.)

The reason Nepeta cataria releases nepetalactone doesn’t necessarily have to do with giving your cat a buzz. The fact that it gives cats that little charge of euphoria may be purely coincidental. The chemical is an insect repellant that the plant emits as a defense mechanism against pests like aphids. According to the American Chemical Society, nepetalactone attracts wasps and other insect predators that eat aphids, calling in protective reinforcements when the plant is in aphid-related distress. That it brings all the cats to the yard is just a side effect.

Because of this, catnip may have even more uses in the future beyond sending cats into a delighted frenzy. Rutgers University has spent more than a decade breeding a more potent version of catnip, called CR9, which produces more nepetalactone. It’s not just a matter of selling better cat toys; since catnip releases the compound to ward off insects, it’s also a great mosquito repellant, one that scientists hope can one day be adapted for human use. In that case, you might be as excited about catnip as your cat is.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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