Have you ever wondered how much a cloud weighs? Even though a cloud seems to float in air, both the air and the cloud have mass and weight. Clouds float in the sky because they are less dense than air, yet it turns out they weigh a lot. How much? About a million pounds! Here's how the calculation works:

Finding the Weight of a Cloud

Clouds form when the temperature becomes too cold for the air to hold water vapor. The vapor condenses into tiny droplets. Scientists have measured the density of a cumulus cloud at about 0.5 grams per cubic meter. Cumulus clouds are fluffy white clouds, but the density of clouds depends on their type. Lacy cirrus clouds may have a lower density, while rain-bearing cumulonimbus clouds may be denser. A cumulus cloud is a good starting point for a calculation, though, because these clouds have a fairly easy-to-measure shape and size.

How do you measure a cloud? One way is to drive straight across its shadow when the sun is overhead at a fixed rate of speed. You time how long it takes to cross the shadow.

Distance = Speed x Time

Using this formula, you can see a typical cumulus cloud is about a kilometer across or 1000 meters. Cumulus clouds are about as wide and tall as they are long, so the volume of a cloud is:

Volume = Length x Width x Height

Volume = 1000 meters x 1000 meters x 1000 meters

Volume = 1,000,000,000 cubic meters

Clouds are huge! Next, you can use the density of a cloud to find its mass:

Density = Mass / Volume

0.5 grams per cubic meter = x / 1,000,000,000 cubic meters

500,000,000 grams = mass

Converting grams into pounds gives you 1.1 million pounds. Cumulonimbus clouds are considerably more dense and much larger. These clouds may weigh 1 million tonnes. It's like having a herd of elephants floating over your head. If this worries you, think of the sky as the ocean and clouds as ships. Under ordinary conditions, ships don't sink in the sea and clouds don't fall from the sky!

Why Clouds Don't Fall

If clouds are so massive, how do they stay in the sky? Clouds float in air that is dense enough to support them. Mostly this is because of variations in the temperature of the atmosphere. Temperature affects the density of gases, including air and water vapor, so a cloud experiences evaporation and condensation. The interior of a cloud can be a turbulent place, as you know if you have flown through one in an aircraft.

Changing the state of matter of water between a liquid and a gas also absorbs or releases energy, affecting temperature. So, a cloud doesn't just sit in the sky doing nothing. Sometimes it does become too heavy to stay aloft, which leads to precipitation, such as rain or snow. Other times, the surrounding air becomes warm enough to convert the cloud into water vapor, making the cloud smaller or causing it to vanish into the air.

If you're interested in learning more about how clouds and precipitation work, try making a homemade cloud or making snow using boiling hot water