25-03-2025
How Does A Cloud Stay In The Sky?
Have you ever wondered how a cloud stays in the sky? In a recent conversation with my friend and colleague Alan Sealls, the AMS President-Elect told me that he pondered this very question, and it sparked his interest in becoming a meteorologist. Since I have been writing about important but heavy topics in recent weeks, let's 'lighten' it up and explore a little meteorology 101. Further reading, however, will reveal that clouds are not as 'light' as you may have thought.
What is a cloud anyhow? The Glossary of the American Meteorological Society defined a cloud as, 'A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface.' The shape, altitude, and precipitation potential of clouds depend on various meteorological conditions, but that's not the focus of this conversation. How does these collections of water droplets and/or ice remain suspended in the air?
Even though an innocent-looking cumulus cloud may look as light as a cotton ball, it can be heavier than you may think. A U.S. Geological Survey website cited columnist Cecil Adams who wrote, 'A modest-size cloud, one kilometer in diameter and 100 meters thick, has a mass equivalent to one B-747 jumbo jet.' How can a cloud weigh so much? Clouds are comprised of droplets or ice particles so that means they have mass, density and weight.
Let's explore something called cloud density. For arguments sake, I will focus on a cumulus cloud. In a typical cloud, there are a certain number of cloud particles in a given volume. A USGS website gave an estimate that a typical cumulus cloud may contain 1 billion cubic meters. If density is considered 0.5 grams of cloud droplets within that given volume, the mass is 500,000 kilograms (1.1 million pounds or 551 tons). I arrived at that number by doing a simple math problem. Mass is equal to density multiplied by volume.
So how does something that heavy float? Believe it or note, the dry air beneath the cloud is denser than clouds above it. At the same temperature and pressure, moist air is less dense than dry air. This is where I have to introduce Avogadro's law. Dry air is made up of nitrogen and oxygen. Moist air contains water. The molecular mass of nitrogen is greater than water, which means the less dense water replaces denser nitrogen in moist air. If you want to see how differences in density work, pour a sugary soda in a glass and then add a sugarless soda on top of it. I used to do stuff like that at food buffets all of the time.
Can you imagine how much a massive cumulonimbus cloud weighs? Do the math yourself on your lunch break. Happy Cloud-Gazing Day.
