If you look at big fluffy clouds in the sky, you may notice that they’re only fluffy on the top, while the bottoms are pretty flat. That’s because the cloud isn’t just floating through the sky. It’s actually riding on top of a heavier layer of air!
Often we look at air (or as much of it as we can because it’s invisible) and we think that it’s the same all through the sky. Air actually changes a lot. There are heavier parts and lighter parts. Oxygen gas and nitrogen gas, two of the many gases that make up air, are actually heavier than the water vapor in clouds! It may seem weird to think about, but water is actually made up of lighter stuff than the gas in the air! It floats just like whipped cream does on hot chocolate. Only, imagine that the hot chocolate is invisible. You’d get something that looks a lot like a cloud: fluffy on top, flat on the bottom.
When water is a liquid though, the pieces of water get packed together very tightly and, together, they become heavier than the gases in the air. That’s when we get rain!
ExplorationLighter than Air!
If you’re having a hard time believing that water can be lighter than air, there’s a pretty easy experiment you can do to see for yourself! All you need is a cup of water and a dry, sunny day. In the morning, fill the cup with water and make sure there are no leaks in the cup. Then leave the cup outside all day. Then come back and check on it.
What happened to the level of water in the cup? If there is less water, where do you think it went? You checked for leaks in the cup, so it didn’t leak! Where did the water go?
Sources & links
Williams, Jack. “Understanding Air Density and Its Effects.” 17 May 2005. USAToday.com. 12 December 2009. "atmosphere." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 12 Dec. 2009. Dobson, John. “Clouds.” Quanta-Gaia. 1995. 12 Dec. 2009.