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Skip to 0 minutes and 7 seconds In the atmosphere, there are lots of places where the air is cooling, and that might be because the air is rising. And as it rises, the pressure falls and so the air cools. And sometimes that can lead to cloud forming. So we’re going to show you a few ways that you can demonstrate that. Here I’ve got a lemonade bottle and a bike pump. The lemonade bottle’s got some liquid in it. And at the moment, the air in the bottle is at atmospheric pressure. So if I squeeze a bottle it’s very squidgy because there’s the air outside bottle is able to get in. And the air inside bottle’s able to get out.

Skip to 0 minutes and 42 seconds So I’m going to increase the pressure in the bottle by pumping in extra air with the bike pump.

Skip to 0 minutes and 51 seconds And as the pressure in the bottle increases, the temperature increases. And so that’s encouraging the evaporation of the liquid in the bottle. There we go. So the temperature in the bottle fell very suddenly as I let the air out, and so very suddenly it was colder. There was more condensation going on than evaporation, and so a cloud formed. However, I cheated a bit when I made that cloud in the bottle. It’s actually not water in this bottle. It’s surgical spirit. Let’s see what happens if we try again, this time using warm water. So I’ve got some warm water here, which I’m going to pour in a little bit into the bottle.

Skip to 1 minute and 37 seconds Good, swirl it around a lot so that a lot of the surface of the bottle is wetted, and then try and make a cloud.

Skip to 1 minute and 45 seconds This time when I release a pressure, you shouldn’t see much of a cloud forming.

Skip to 1 minute and 53 seconds There’s hardly anything there. And the reason that is, is because there weren’t any what are called cloud condensation nuclei in the bottle. But this time, I’m going to introduce some cloud condensation nuclei into the bottle by blowing some smoke from a match into the bottle.

Skip to 2 minutes and 29 seconds Let’s see if we get a cloud this time when I release the air. This time we’ve got a really good cloud. So it needed the water and the cloud condensation nuclei. OK, I’m going to do the same thing again now. But this time, we’ve got a thermometer pointing at the bottle. So hopefully you should see that as the pressure in the bottle increases, the temperature goes up. And then as the pressure in the bottle falls when we let the extra air out very suddenly, you should see the temperature falling as well.

Skip to 3 minutes and 38 seconds So we’re down in our laboratory here at the University of Reading Meteorology Department, and we’re going to do a demonstration of how pressure changes resulting cloud formation. The version I’m going to do today involves a large glass demijohn jar, and a bike pump, and a rubber bung.Now in the demijohn here, I’ve got small amount of liquid at the bottom of the jar. And the liquid that I’m using is actually ethanol, so it’s actually alcohol. The reason I’m using this rather than water is because it’s got much lower condensation point. So it’ll form cloud much easier, and it’s a much more exciting visual demonstration, which is what we’re aiming at here.

Skip to 4 minutes and 20 seconds What I’m going to do, is I’m going to pump air into the bottle to increase the pressure. And then at some point the pressure is going to build up so much, that the rubber bung is going to want to pop out the top. Just like a champagne cork, pretty much. So that’s going to happen when the pressure’s increased so much that the jar can’t take any more air. When it pops out, the pressure is going to decrease quite rapidly. And at that point we’ll see the vapour that’s within the jar condense almost instantaneously and form a cloud. So let’s go for it. So I’m going to attach the pump here onto the valve.

Skip to 4 minutes and 56 seconds Keep it all nice and tight so nothing leaks out.

Skip to 5 minutes and 2 seconds It’s all nice. Going to pump now, and about 20 strokes of the pump. Should be enough. [PUMP THUMPING] Feel the pressure starting to build up now. And now we’re going to release it. And there’s our cloud.

Clouds in a bottle

In this video Sylvia demonstrates how changes in pressure result in cloud formation, using a plastic bottle and a bicycle pump.

The bike pump increases the pressure, and therefore the temperature in the bottle and encourages evaporation. Once this pressure is released, the temperature falls and condensation forms. Cloud forms in the bottle when there is more condensation than evaporation.

This demonstrates what also happens in the atmosphere. When the pressure falls and the rate of condensation is higher than the rate of evaporation, cloud droplets form.

Then Pete recreates this demonstration using a demijohn bottle in one of the University of Reading’s laboratories.

How do you think the state of the atmosphere in the room you’re in – the temperature and humidity – might affect the cloud that forms? We look forward to hearing your thoughts in the Comments area within this Step.

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Come Rain or Shine: Understanding the Weather

University of Reading

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