Why clouds don't fall: The surprising weight of sky giants
20 August 2024 17:02
Clouds, though often associated with lightness, are not light at all. Physicists know how to determine their mass accurately. They are indeed heavy, yet they don't fall to the ground.
Though clouds in the sky look like light, fluffy puffs, in reality, they are heavy objects composed of water. Depending on the type of cloud, their mass can differ. This all depends on their density and volume.
A cumulus cloud has a density of 0.5 grams per cubic metre. To give an idea, this would mean having one drop of water per cubic metre of the cloud. A single cumulus cloud has a volume close to one cubic kilometre.
Not so light after all
This means that 550 tonnes of water are in one cumulus cloud. An African elephant weighs between 3 and 7 tonnes, so to simplify, we could say that a single cloud weighs as many as a hundred elephants or as many as 12 fully loaded semi-trailer trucks.
Cumulonimbus clouds, or rain clouds, are significantly denser and have a larger volume. Such a cloud can be several kilometres tall and around 10-20 kilometres wide. Assuming that such a cloud is nearly circular with a diameter of 10 kilometres, its volume is approximately 786 cubic kilometres. Let's assume the water density in this type of cloud is about 2 grams per cubic metre.
The cloud weighs as much as 150 Eiffel Towers
Under these conditions, the cloud's mass is about 1.57 million tonnes. This is an unimaginable mass. To better visualize it, we can compare it to the mass of the Eiffel Tower, which is about 11,000 tonnes. One rain cloud weighs as much as 143 Eiffel Towers. A single cloud can weigh as much as ten fully loaded container ships.
Cirrus clouds are the lightest clouds. Their density is only a few grams per cubic kilometre, and they do not cause rain.
Why don't clouds fall to the ground?
Although clouds are heavy, they do not fall. This is related to the size of individual droplets. They are maintained by air resistance. For water droplets, the terminal velocity of air, which allows clouds to stay afloat, is between 30 and 45 kilometres per hour. Updrafts prevent droplets from falling to the ground.
