I don't know if you've noticed this, but sometimes, when you go outside, there are these big fluffy white things in the sky. We call them clouds.   And even though we think of them as being gaseous, clouds are actually made of condensed water vapor -- so, they’re actually made of droplets of liquid water, or sometimes ice, stuck to tiny bits of dust and other particles.   And clouds can take different forms, depending on factors like altitude and temperature and humidity.   Since there are countless combinations of these conditions, each of which can create different kinds of clouds, meteorologists had to create an extensive classification system for them.   There are only ten major categories of clouds, based mostly on altitude. But get into subcategories, and there are hundreds more.   Hidden among the poofy cumulus clouds and the wispy cirrus clouds, there are the rarer types.   And some of them are just... strange.    Take lenticular clouds, which look like pancakes stacked into a lens shape -- or maybe like a flying saucer, if you squint.    These clouds usually form when air hits a vertical barrier, like a mountain peak.    The barrier creates circular waves of air around it, generating an effect that’s kind of like throwing a pebble in a pond.   If the rippling air is the right temperature, and there’s enough moisture in it for a cloud to form, you get these wispy sky-pancakes.    But lenticular clouds don’t have the monopoly on shapes that look like delicious food.   Morning glory clouds, which look like long strings of rolling dough, can be 1000 kilometers long and up to two kilometers high, hovering just a few hundred meters above the ground.   These clouds are a specific type of roll cloud, which form low, rolling, horizontal tubes that tumble across the landscape -- as a cloud disperses on its trailing end, it keeps forming at the front.   They’re also incredibly rare -- so unusual that there’s only one place on Earth where meteorologists can predict that they’ll form, in the northern part of Queensland, Australia.   And even then, they only appear a few times a year, during the Australian spring.   This part of Queensland, called the Gulf of Carpentaria, seems to have a particular arrangement of land and ocean that allows morning glory clouds to form.    Namely, it’s where eastern and western sea winds collide, forcing the air in the middle up and into a bank of clouds, essentially dividing them into rows. As the rows of air cool overnight, they form cylinders.    But there’s still a lot we still don’t understand about morning glory clouds -- there have only been a few studies on them, because they’re too rare to have a widespread effect on the weather.    Polar stratospheric clouds, though? They have been studied a lot, because they can be bad.   At first glance, they seem mostly harmless, and actually very pretty: a calm-looking cloud that shimmers like a flat rainbow.    But it turns out that they’re partly to blame for that pesky hole in the ozone layer over Antarctica.   Polar stratospheric clouds form way up in the stratosphere -- 15 to 20 kilometers up, where it is super duper dry.    So if clouds are going to form from what little water vapor is in the air, they need some extenuating circumstances.   In this case, that means that it needs to be below minus 78 degrees Celsius. Even in the arctic, it doesn’t get that cold very often, so polar stratospheric clouds mostly form by the south pole.   If the cloud that forms contains certain substances -- like nitric acid and sulfuric acid -- then it can become a catalyst for a chain reaction that leads to the destruction of ozone -- the molecular oxygen that blankets our atmosphere and helps absorb a lot of the sun’s ultraviolet radiation.    But that’s not the clouds' fault -- it’s humans that put too much of those substances in the atmosphere in the first place.    So how can you tell if the polar stratospheric cloud you’re looking at is the bad kind or not?    I mean, if you happen to be in Antarctica right now? Which would be awesome!   Well, check the brightness of its rainbow shimmer. When these clouds are only composed of ice crystals, they diffract, or separate, the light better, making them more iridescent.    But if the rainbow is dim, that cloud is ever so slowly doing its part to make us more vulnerable to the sun’s radiation. So feel free to shake your fist at the sky in anger.   Thank you for watching this SciShow Dose. And thank you especially to our latest President of Space, Tab for a Cause! One of our subscribers on Subbable, and also a browser extension that donates money to charity every time you open a new tab in your web browser. And it doesn't cost you a thing. Click on the link in the description to start raising money for Water.org, Human Rights Watch, Save the Children, and more.    And if you want to keep learning about how remarkable the world is even though you were thinking "Oh, that's just a cloud. That's not that interesting." But it turns out they're SUPER interesting, you can go to YouTube.com/SciShow and subscribe.