This experiment is as much about air pressure as it is about clouds! When you pump a lot of air into the bottle, you create high pressure. The air crowds together and warms, and there are no clouds. The skies are clear!
When you remove the stopper, you lower the pressure by letting out much of the air you pumped in. The air expands and cools. Any vapor in the air cools, too, causing it to condense on particles and form small droplets that make a visible cloud. What was once invisible becomes visible. It’s not magic, but it’s definitely magical!
In this experiment, we used rubbing alcohol because it evaporates faster than water. As a result, we get a very visible cloud quickly. But we could use water, too, just like it happens in the atmosphere. When air pressure is high, there is more air above you so it is heavier, denser, and sinking, and the skies are clear. Sorry, there are no clouds on high-pressure days, but there is a lot of sunshine!
For more resources about clouds, explore our Clouds Teaching Box.