Let’s take a look at the science of clouds!
Pockets of warm air near the Earth’s surface naturally float up through the atmosphere like hot air balloons. As an air pocket rises, it expands and cools down. This causes the water molecules within it to group together, creating droplets large enough to see. When a lot of these droplets gather in one place, they form a cloud. We usually observe clouds on a very large scale, but we can also create a cloud in a bottle by imitating the pressure & temperature differences found in nature.


Diagram of how a cloud forms. Credit: North Carolina State University

This experiment begins with a small amount of rubbing alcohol in a 2-liter bottle. There are several ways to do this demonstration. We use rubbing alcohol because it evaporates at a lower temperature than water, giving us a more impressive tabletop cloud!
First, we pressurize the bottle using an air pump.  Next, we release the pressure, causing the air inside the bottle to expand and cool (much like a pocket of warm air does as it rises through Earth’s atmosphere).  Just as cooling, expanding air causes water vapor to condense in nature, the cooling, expanding air in the bottle allows the rubbing alcohol to condense into a visible cloud of droplets.
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This cloud looks very similar to the ones you see in the sky, but there are some important differences, as you can clearly see.

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