As our final activity in our March 12 physics-behind-the-weather extravaganza, we made clouds in a bottle, pretty much as shown in this video. This experiment shows that to form clouds we need three things: humid air, a change in temperature/pressure which brings the air below the dew point, and condensation nuclei, which are provided by particles in the smoke we waft into the bottle. Without these, water droplets tend not to form; see the Wikipedia article on supersaturation.
After many minutes of squeezing to see the clouds form and releasing to see the clouds disappear, they tended to stop forming. I think that must be due to the particles being driven into the liquid water rather than floating in the air.
This was a relatively quick (~15 minutes) activity, but I was surprised at how the kids had problems getting the smoke into the bottle. They didn't think about how the bottle had to exhale before it could inhale the smoke. I didn't trust the kids with matches, so I was quite busy servicing the kids as they each tried to get smoke multiple times. My advice to teachers is to have several adults help if you do this with a larger number of kids.
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