1.9

## Come Rain or Shine

Skip to 0 minutes and 7 secondsIn 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 secondsSo 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 secondsAnd as the pressure in the bottle increases, the temperature increases. And so that's encouraging the evaporation of the liquid in the bottle.

Skip to 1 minute and 8 secondsSo 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 secondsGood, 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 secondsThis time when I release a pressure, you shouldn't see much of a cloud forming.

Skip to 1 minute and 58 secondsThere'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 33 secondsLet's see if we get a cloud this time when I release the air.

Skip to 2 minutes and 45 secondsThis time we've got a really good cloud. So it needed the water and the cloud condensation nuclei.

Skip to 2 minutes and 58 secondsOK, 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.

# Clouds in a bottle

Sylvia shows you how you can recreate Pete’s demonstration from the previous Step using a plastic drinks bottle and a thermometer.

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.