Purpose:

To understand how clouds form as atmospheric pressure decreases.
 

Equipment:

A large jar containing a small amount of water, a stopper connected to a bicycle air pump and a match.
 

Procedure:

Experiment 1. Put a layer of warm water in the bottom of the jar. Pump air into the bottle through the stopper to increase the pressure in the jar. Release the stopper. You should be able to see a weak cloud form in the jar.

Experiment 2. Put a layer of warm water in the bottom of the jar. Tie a match to a string, light the match and dangly the lighted match on a string in the bottle to put smoke into the jar. Then, as before, pump air into the bottle through the stopper to increase the pressure in the jar. Release the stopper. You should be able to see a stronger cloud form in the jar.

Explanation:

The decrease of pressure that occurs in the jar when the stopper is removed leads to decreased pressure and temperature. The water vapor in the air condenses because of the lower pressure and temperature. The smoke particles help the vapor condense by providing surfaces on which the vapor molecules can aggregate.