a clear, two liter soda bottle with its cap
water
paper
a match or lighter

WARNING! This experiment uses fire. Be sure you get permission. Be very careful and use common sense.

Before we make our cloud, we need to be sure we can see it. Take a clear, two liter soda bottle and remove the label. Be sure you saved the top and be sure that it fits tightly.

Now for the experiment. What are clouds made of? Water, right? So the first thing we need to do is put a little water into the soda bottle. It does not take much. Just enough to cover the bottom of the bottle. Next, we need to get the water to change into water vapor. An easy way to do that is to put the cap on the bottle and shake it a bit. This covers the inside of the bottle with water. The more surface that is wet, the faster some of the water will evaporate, adding water vapor to the air.

Once you have lots of water vapor in the bottle, look carefully to see if you can see a cloud forming. Do you see one? No? Why not? The water vapor is invisible. To see a cloud, we need to change the water vapor back into liquid water. How can we get the water vapor to condense back into a liquid? One way to do that is with a sudden drop in air pressure. Squeeze the bottle very hard and then quickly release it. Did you get a cloud? Probably not. Why?

Squeezing the bottle raises the pressure and the temperature. This allows more water vapor to evaporate. Quickly releasing the bottle lowers the pressure and cools the air. That cooling causes some of the water vapor to condense back into liquid water. Then why don't we see a cloud in the bottle? The water vapor needs something to condense onto. That could be tiny piece of something like dust, but in our experiment we will use some particles of smoke. Do this over the sink, so we don't start a fire. Cut a long, thin strip of paper. It should be thin enough so that it easily fits into the mouth of the soda bottle. Remove the cap from the bottle. Carefully use the lighter to set the end of the paper strip on fire. Give it a second to burn and then blow out the flame. When it is out, you will see smoke rising from the burned paper. Quickly stick the end of the paper into the bottle, so that a little smoke gets inside. Remove the paper and put the cap on the bottle tightly. You will not see any smoke in the bottle, but some of the particles are still in there.

Once again, squeeze the bottle very hard and quickly release it. Watch carefully and you should see a thin fog appear in the bottle. Squeeze it again and release. The cloud gets thicker. After you do this several times, you will have a very nice cloud in your bottle.

Notice that when you squeeze the bottle, the cloud vanishes. Squeezing raises the temperature and the water droplets quickly change back into water vapor. The increase in temperature also helps more water vapor form from the water on the sides of the bottle, so as you do the experiment several times, you get more and more cloud. If you look carefully, you might even be able to see the silver lining. Have a good week.