How Do Clouds Form? Make a Cloud in a Jar

Clouds are made of water, but a cloud isn’t a puff of steam or smoke. Making one in a jar shows what’s really going on, and it takes about five minutes.

What you need

Two identical clear glass jars (canning jars work best)
Something to sit on top to hold ice: the jar’s metal lid turned upside down
Ice cubes
Just-boiled water from a kettle (grown-up only)
Optional, grown-up only: a match or birthday candle you light and blow out

The method

A grown-up pours about 3 cm of hot water into each jar. Swirl to warm the glass.
Lay a lid upside down on each jar and put ice on top.
Watch the air near the top. A thin mist forms, swirling inside the jar.
For a better cloud, a grown-up lights a match, blows it out and drops it in one jar through the lid opening, then quickly covers it again. The smoky jar fills with a thicker cloud, and the clean jar fills with less.

Looking against a dark background makes the cloud easier to see.

What’s happening

The hot water makes warm, moist air (water vapour) in the jar. The ice-cooled lid chills the air at the top. When warm moist air cools, it can’t hold as much vapour, so some condenses into tiny droplets of liquid water. Millions of droplets together make a visible cloud.

Why does smoke help? Droplets need something to form on. Tiny specks, such as dust, smoke and salt, give water vapour a surface to collect on. Real clouds form the same way: warm moist air rises, cools and condenses on specks of dust and sea salt high in the atmosphere.

Where’s the cooling in nature?

Air rises when it’s heated by the ground, pushed up a mountain or lifted over a cold front. Higher up it’s colder and the pressure is lower, so rising air cools. That’s when clouds form.

Questions kids ask

Why doesn’t the cloud fall? Cloud droplets are so tiny that moving air keeps them up. When they merge into drops big enough, they fall as rain.

Is a cloud steam? Steam is invisible water vapour. The visible white stuff is liquid droplets, which is also what you see above a kettle.

Why the smoke jar? It’s a fair test: same setup with one change, the specks.

Safety

Hot water and matches are for grown-ups only. Use jars without cracks and keep a tray under them.

The full activity is How does a cloud form? in Earth and Sky at Home. Classrooms can go on to Where does rain come from? Sign up for three lessons free.