## Does cloud weigh? How Much?

Have clouds  any weight at all ? How can they, if they are floating in the air like a balloon filled with helium? If one tie a helium balloon to a kitchen scale it won’t register any weight, so why should a cloud?

Since air has weight it must also have density, which is the weight for a chosen volume, such as a cubic inch or cubic meter. If clouds are made up of particles, then they must have weight and density. The key to why clouds float is that the density of the same volume of cloud material is less than the density of the same amount of dry air. Just as oil floats on water because it is less dense, clouds float on air because the moist air in clouds is less dense than dry air.

A good sized cumulonimbus cloud, or thunderhead, might be ten kilometers tall, with a base ten kilometers in diameter with a volume of about 1 cubic kilometer (km) located about 2 km above the ground. In other words, it is a cube about 1 km on each side. The National Oceanic and Atmospheric Administration (NOAA) provides some estimates of air and cloud density and weight. NOAA found that dry air has a density of about 1.007 kilograms/cubic meter (kg/m3) and the density of the actual cloud droplets is about 1.003 kg/m3. In the final calculations, the 1 km3 cumulus cloud weighs a whopping 2.211 billion pounds (1.003 billion kilograms)! However, remember that air also has mass, so the cloud floats because the weight of the same volume of dry air is even more, about 2.220 billion pounds (1.007 billion kilograms). So, it is the lesser density of the cloud that allows it to float on the dryer and more-dense air.

To put it another way, a modest-size cloud, one kilometer in diameter and 100 meters thick, has a mass equivalent to one 747. And they let these things just float around up there! Why, if one fell on us, it would . . . it would . . . well, it would get real foggy, that’s what. Because of course weight isn’t the same as mass–a cloud put on a scale wouldn’t weigh anything. This fine but crucial distinction gets you off the hook in the argument you’re having with your coworker.