Clouds in the Air: Why Are They There?

This experiment demonstrates why there are clouds in the sky. This process involves air, water vapor, particles we call condensation nuclei, and air pressure...the cloud comes later!

Learning Objectives

• Students explore the relationship among air pressure, temperature, and volume.
• Students learn that moisture, cooling temperature, and condensation nuclei are needed for clouds to form.

Materials

• A 1-liter clear plastic bottle
• A rubber stopper with a small hole in it
• A tire pump to attach to the rubber stopper
• Water or rubbing alcohol
• Safety glasses (enough for everyone doing this experiment)

Directions

1. Put on your safety glasses and have your assistant do the same.
2. Add approximately 1/2 teaspoon of rubbing alcohol to the inside of the bottle so that it coats the sides. (You can do this experiment with ¼ cup of water, too.)
3. Plug the rubber stopper into the bottle and pump 10 times, making sure to have your assistant hold the stopper so it doesn’t pop off the bottle. Notice how clear the air is in the bottle after you do this.
4. Now, have your assistant pull the rubber stopper off the bottle. Notice what the air looks like now. A cloud has formed.
5. Put the plug back onto the bottle with the cloud still in it, and pump about 5 times. What happens to the cloud? It disappears! Why?
6. Connect the model with the atmosphere by asking: What is needed for water vapor to condense so that water droplets or ice crystals appear in a cloud? (condensation nuclei, cooling temperature and low pressure) and What does the sky look like where you are right now? Is it clear or cloudy? How did that happen?

Background

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.