Remove the lid and add some chalk powder. Immediately cover the jar with the balloon and put a rubber band around the neck of the jar to keep the balloon firmly stretched. Press the balloon down with your fist to compress the air. Hold it like this for about twenty seconds. Remove the balloon.
Clouds form when air containing vaporised water cools.
When you compress the air in your jar, the air becomes warmer and absorbs more vaporised water. When you remove the balloon cover, the air cools and some of the vaporised water condenses on the chalk dust to form a cloud. Air is the mixture of gases that we breathe. It surrounds the Earth to a height of about 300 miles. It is often called the atmosphere.