I get excited when I see the outside temperature drop below 0°F (-18°C). This is not because I enjoy cold weather. It is because when the outside temperature gets this cold, I can conduct a particular experiment that I think is quite beautiful. The experiment is easy to carry out: boil some water and throw the hot water into the frigid air. Check it out below:
Isn’t that cool? Throwing very hot water into very cold air results in the rapid formation of a cloud.
This simple experiment brings several questions to my mind. First, what is going on here? I’ve thought about this experiment a bit, and I’m still not completely sure what is happening. My guess is that the hot water has a very high vapor pressure, which causes a lot of the hot liquid molecules to vaporize into the cold air. The vaporized water molecules quickly condense, then freeze (or do they just sublime?) into tiny particles of ice to form an ice cloud. I’d be interested to hear your thoughts on what you think might be happening.
I’ve also wondered about the factors involved in cloud formation. I’ve conducted this experiment at outside temperatures ranging from 5°F (-15°C) to -4°F (-20°C). While I haven’t performed in-depth experiments, the cloud formation seems to work better at colder air temperatures. I think an analysis of cloud formation at various outside air temperatures would be a great winter project for students. The temperature rarely drops below 0°F (-18°C) where I live so unfortunately I haven’t had my students look at this.
A second set of experiments involves looking at the temperature of the water thrown into the air. As seen in the video, water close to its boiling temperature forms a nice cloud. However, I have tried using hot water from the tap (about 104°F or 40°C), but water at this temperature does not form a cloud. Is there a threshold temperature above which the water must be to form a visible cloud? And does this threshold temperature change with the temperature of the outside air?
I’ve conducted this experiment as a demonstration for my chemistry classes when the weather cooperates. (Be sure to do a trial run before demonstrating this to your classes! I once tried this experiment in front of my class when the outside temperature was too warm for cloud formation. I am sure my students wondered why their professor decided to take them outside on a cold day so he could throw hot water into the air for no reason at all). If you happen to try this demonstration for your classes or if you try any of the suggested experiments above, let me know how things work out. I would love to hear from you.