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Create a cool cloud inside a jar using these instructions.

Safety: This activity is appropriate for ages 8+ with adult assistance, as it uses matches. Don’t do this activity if there is a total fire ban. Make sure you have fire safety equipment nearby.

First aid: If you burn yourself, run the burn under cold water for 20 minutes.

You will need

• Glass jar
• Balloon
• Scissors
• Warm water
• Matches
• Rubber band

What to do

1. Cut the neck off the balloon.
2. Carefully stretch the balloon over the mouth of the jar. You’ll need to do this quite quickly, so if you find it tricky, practice it a few times.
3. Take the balloon off.
4. Pour about one centimetre of warm water into the jar.
5. Go outside, and find an area clear of leaves and other flammable things.
6. You’ll need to do the next few steps quite quickly. First, light a match and let it burn for a bit.
7. Blow out the match, and then hold it in the jar for a few seconds to put some smoke in the jar.
8. Drop the match into the jar.
9. Stretch the balloon over the jar.
10. Put a rubber band around the neck of the jar to hold the balloon in place.
11. Hold the jar in the sunlight, and look through the jar at a dark shadow.
12. With your other hand, gently push and pull on the balloon. Do you notice a cloud in the jar?

What’s happening?

When you boil water, it turns into gas called steam. But not all gaseous water is hot. There is always at least a little bit of water in the air we breathe. This water is called humidity. On a very humid, 32 degree day, the air can be up to 3% water. That can be more water than clouds hold! So why are clouds up there, and not down here near the ground?

The main reason is temperature. Down at ground level the air can be cool, but the higher you go, the colder it gets. At jet cruising altitude, it’s typically around –55 degrees. As air gets colder, it can hold less gaseous water. If there’s too much water in the air, some of it has to turn into liquid water or solid ice. Tiny droplets or crystals form – these are what makes a cloud!

That still doesn’t explain why there’s a cloud in this activity, when there doesn’t seem to be any cooling going on. But actually, there is cooling! When you pull on the balloon, you stretch the air inside the jar. This stretching cools the air inside the jar, making a cloud form. If you push the balloon down, you squeeze the air inside the jar and that heats it up. The extra heat means the air can take more water and the cloud evaporates.

Why do we add a match to the mixture? It’s actually for the smoke, not the match itself. Smoke is made up of tiny particles of soot. These particles help the gaseous water in the air liquefy and turn into clouds. Tiny airborne particles such as smoke or dust are typically needed to act as ‘seeds’ in order for water to form clouds, even in the sky.

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