Cloudspotting: make a cloud in a jar!

As the sky brightens throughout the morning, you’ll probably get a good view of the clouds. These enormous masses of air and water vapour form for quite specific reasons, and they aren’t all built the same. You can even use clouds to do some basic weather forecasting. Mel Strong is a weather expert from the Bradbury Science Museum in Los Alamos, New Mexico, and he ran Phil Sansom through the basics. Get ready, because it’s kitchen science time...

Mel - So today we're going to try to create a cloud in a jar. I want you to find a clear jar or a glass, and preferably something that has a lid. The lid doesn't have to be tight fitting - a piece of paper could serve as a lid for this. You're going to want to fill this jar a quarter to halfway full of warm water. Go ahead and put whatever lid you have on it and just set it aside. You're also going to need to get a little sandwich baggie full of ice cubes, and you'll need a match.

Phil - Okay, so I've got my jar and it's got the warm water in, and I've got my sandwich bag and I'm putting some ice cubes in it, and I've got my match. What do I do?

Mel - Okay, so the lid has been on your jar and as it's been sitting there, there's been water evaporating. Do you see anything on the sides of the jar right now?

Phil - Yeah, there's a bit of condensation.

Mel - What's happening is we have a lot of water vapour that's inside that jar. And the sides of the jar are relatively cool, and there's water that's condensating onto the sides of the jar. But that's not a cloud. What we're forming here is dew. To get a cloud, we don't want to cool the surface, we want to cool the air. So here's what we're going to do: I'm gonna explain it first, and I want you to do it kind of all in one motion.

Phil - Okay.

Mel - We're going to take the lid off the jar. We're going to light a match. We're going to blow out the match and immediately throw it into the jar, and then we're going to replace the lid with the bag of ice.

Phil - This is with the warm water in the jar?

Mel - Yes, right.

Phil - Here it goes. All right, I've done it. The ice is over it.

Mel - See if you see anything happening underneath the bag of ice.

菲尔,我做!空气become cloudy!

Mel - Yeah, so we are forming a cloud. Now, if you just peel the bag of ice back a little bit, the cloud might come out.

Phil - It's not coming out as a big puffy cloud, but it's coming out as cloudy vapour.

Mel - So we have cooled the air with ice. In nature, generally what happens is air is cool because it is lifted upwards into the atmosphere. When water condensates, it needs a surface to condensate onto. In nature, those surfaces are little tiny microscopic pieces of dust and salt: condensation nuclei. We added condensation nuclei with our match that added some smoke. When we put the ice over the top of the jar, we then cooled the air inside the jar such that condensation would occur on those little tiny smoke particles that we added.

Phil - Oh, so just like the water was condensing on the cool sides of the glass before, now it was condensing in the cool air, but on the little bits of smoke that came from the match.

Mel - That's right.

Phil - And that's really how clouds form?

Mel - That's really how clouds form.

Phil - Okay. If that's how clouds are made, how come you get different kinds of clouds?

Mel - Right. Almost all clouds are formed by air rising, and there's really two ways that air can rise. There's a whole class of clouds that we call cumuliform clouds, and these are the puffy, sort of cotton-looking clouds. When you see one of these, you know that air is rising because you have little pockets of air that are warmer than the air around it. The other type of clouds are more like blankets. We call these stratiform clouds, and in this case what's happening is the entire air above our heads, the entire sky you see is being uplifted slowly. They're pretty boring to look at. It makes a grey sky.

Phil - Okay, I've got all this new cloud knowledge. Is there anything I can do with this? Can I start to predict weather patterns?

梅尔——实际上是一个可预见的爵士ies of clouds that you can get that we often refer to as a winter storm. You're going to start out with these high streaky ice clouds, so the sky's going to look streaky. And then you're going to notice that there's a ring around the sun, and maybe another half day later the sign is a fuzzy ball, perhaps a half day after that, the sun is not visible. Then we have rain and the rain will last maybe 24 hours or so. During this segment, what's happening is the warm pool of air is passing over our heads. This is called the warm front. Perhaps the next day you'll notice that the temperatures plummet and the sky seems to sort of break up, and the rain seems to have stopped. But what you'll notice is the types of clouds have changed to these little individual puffy clouds. And usually in a well-developed storm, these will turn into tall towering puffy clouds that we call cumulonimbus. And you'll see pockets of rain; generally we call this portion of the storm ‘scattered showers’. And this will happen for perhaps another couple of days. And then finally you'll be in this pool of relatively cold, dry air. So back to your question about weather prediction with clouds: if you see those initial steps, the ring around the sun, the fuzzy sun, you can pretty much bet that it's probably going to be raining in about 24 hours or so.

Phil - Okay. I can predict the weather!

Mel - Yes, you can. And you can imagine that if you were a farmer back several hundred years ago, this is how you would predict the weather.