Why heavy clouds don't fall: The mysteries of atmospheric physics
20 August 2024 13:37
Clouds, though often associated with lightness, are actually quite heavy. Physicists have the tools to accurately determine their mass. Despite their weight, they don't fall to the ground.
Although clouds in the sky appear as light, fluffy puffs, they are, in reality, heavy objects composed of water. Depending on the type of cloud, their masses can vary significantly, largely depending 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 500,000 kilograms of water are in one cumulus cloud. An African elephant weighs between 3,000 and 7,000 kilograms, so to simplify, we could say that a single cloud weighs as much 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 10-19 kilometres wide. If we assume that such a cloud is nearly circular with a diameter of 10 kilometres, its volume is approximately 780 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 mass of the cloud is about 1.56 billion kilograms. This is an unimaginable mass. To better visualize it, we can compare it to the mass of the Eiffel Tower, which is about 10 million kilograms, meaning one rain cloud weighs as much as 156 Eiffel Towers. A single cloud can also weigh as much as ten fully loaded container ships.
The lightest clouds are cirrus clouds. Their density is only a few grams per cubic kilometre. These clouds 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.
