We’ve seen those giant, puffy clouds that look like cotton balls in the sky. With clouds like that floating high above our heads it seems like they’re almost weightless. Would you be able to guess how much a cloud like that weighs? How would you even begin to measure that weight? Well, it starts with understanding what a cloud is made of. Clouds are composed of billions of water droplets. Those water droplets make about half a gram of water per cubic meter of cloud. This gives us the density of the cloud.
After the density is found, the overall size of the cloud must be determined. Peggy LeMone, who led a lot of the cloud weighing research at the US National Centre for Atmospheric Research, calculated that the average cumulus is about a kilometre across and roughly has the shape of a cube, so it’s as tall as it is wide. Using these measurements, we can determine that the average cloud has a volume of one billion cubic metres. Multiplying this by the density gives us a cloud that’s 500,000kg or 1.1 million pounds!
How can a cloud stay afloat if it’s that heavy? Well, it’s because of that density value we mentioned earlier. The density of a cubic meter of cloud is actually lower than the density of a cubic meter of dry air. This allows the cloud to float because it’s lighter than the dry air surrounding it. It’s like an ice cube floating in a glass of water. The ice cube has a lower density than the surrounding water so it floats!