Skip to 0 minutes and 7 secondsSo 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. This demonstration that we're going to do now is the kind of demonstration we do in our laboratory, but you can make versions of this experiment yourself, and you can even make a very much simpler version of it as you'll see a bit later. The version I'm going to do today involves a large glass demijohn jar, and a bike pump, and a rubber bung. So this is a kit that you can get your hands on yourself if you wanted to.

Skip to 0 minutes and 43 secondsGo to your local home brew store and a local bike store, and you'll be able to find these pieces of kit. 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. What I'm going to do, is I'm going to pump air into the bottle to increase the pressure.

Skip to 1 minute and 14 secondsAnd 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. Keep it all nice and tight so nothing leaks out.

Skip to 1 minute and 50 secondsIt's all nice. Going to pump now, and about 20 strokes of the pump. Should be enough. [PUMP THUMPING]

Skip to 2 minutes and 3 secondsFeel the pressure starting to build up now.

Skip to 2 minutes and 10 secondsAnd now we're going to release it.

Skip to 2 minutes and 14 secondsAnd there's our cloud.

Clouds in a lab

In this video Pete demonstrates how changes in pressure result in cloud formation, using a demijohn bottle, a bicycle pump and ethanol.

Pete pumps air into the sealed jar, increasing the pressure to the point at which the rubber bung is ready to pop from the neck of the demijohn. When the rubber bung is removed, the pressure in the jar drops rapidly and the vapour in the jar condenses to form a cloud.

How do you think the state of the atmosphere in the room you are 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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This video is from the free online course:

Come Rain or Shine: Understanding the Weather

University of Reading