You might know that clouds are made of water, but why does it only rain sometimes, and not whenever there are clouds in the sky? This is because the water particles are usually too small to fall to the ground while they are part of a cloud. This experiment will model what is happening when clouds finally start to rain.
People have been speculating about rain and the water cycle for a long time. There were lots of ideas about where rivers, rain, and oceans came from in many cultures all over the world. The ancient Greeks thought that all land floated on water, while the ancient Hindus described the process of evaporation and precipitation in some of their religious songs. By 1580 CE, an engineer and potter from France named Bernard Palissy is credited with the idea that all rivers can be sourced from rain and snow alone, without water having to come from underground.
You should see some of the water stay in the jar, until the cotton balls can’t hold anymore and it starts to pour through the holes in the bottom of the cup. This is a good model of what is happening in a cloud. In clouds, tiny particles of water vapor start to condense around dust or dirt particles. As these droplets move around, they get bigger as they bump into more particles, kind of like raindrops on a windshield. When there are enough drops that are big enough, they start to fall to the ground as rain, snow, or hail, depending on the conditions. Obviously, the cotton balls aren’t made of water vapor, but they act as a good model, or example of what is actually going on.
Be sure to clean up when you are done. Pour the water down the drain, throw away the cotton balls and cup, and put the other materials back where they belong.
Try putting your hand on the top of the cup and sealing it while the water is “raining” out of the clouds. What happens? Why is that happening? Is that the same or different than what happens to real clouds? Why?
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.
ESS (Earth and Space Science) 2: Earth’s Systems