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How do clouds float?

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📓 The short answer

Clouds stay up in the sky because their tiny water droplets fall so slowly that rising air can blow them back up. Additionally, clouds' moist air is lighter and more buoyant than dry air, and heat released during condensation helps keep them aloft.

📚 The long answer

Cloud watching is a calming activity – until you think about how clouds are made up of water, and water is heavy. In fact, the average cumulus cloud weighs about 1.1 million pounds (roughly 500,000 kg). Now cloud watching has become a puzzling activity. How do clouds not immediately fall back down to the ground under the force of gravity?

First, a quick refresher on how clouds form. Clouds form when warm, humid air rises up through the lower atmosphere. As the temperature drops at higher elevations, the water vapor cools and condenses into tiny liquid droplets.

Clouds are made up of billions to trillions of these liquid water droplets (or, when they get freeze, ice crystals), which together become visible to our eyes as white puffs. But water and ice are pretty heavy, so what's keeping those things afloat?

Well, don't listen to the people telling you to get your head out of the clouds — it's time to get your head in the clouds. Let's dig into the key reasons why clouds are able to float if they're so heavy.

Reason #1: Rising air holds up falling water droplets.

The water droplets that make up clouds aren't immune from the effects of gravity. But they're so tiny (typically around 5-50 millionths of a meter in radius) that they fall incredibly slowly. The terminal velocity, the speed at which they fall, is as slow as 18-36 meters/hour (or 60-120 feet/hour).

The slow descent of a cloud's water droplets allows the rising air currents, known as updrafts, to counteract the gravity pulling them down. A good frame of reference here is dust particles. Similarly to water droplets, dust particles are technically falling, but even the lightest updraft can send them flying back up, making them appear like they're hovering.

As warm air continues to rise up, this updraft velocity counteracts or even exceeds the droplets' terminal velocity, creating the illusion that the clouds are simply hovering in place, even though they are technically falling down.

Reason #2: Moist air weighs less than dry air.

The moist air that makes up a cloud is filled with water molecules. Since water is heavier than air, you might assume that moist air is heavier than dry air, but the opposite is true.

Dry air is primarily made up of nitrogen and oxygen molecules, which are heavier than water molecules. As air becomes more moist, these heavier nitrogen and oxygen molecules are replaced by lighter water molecules. Thus, moist air is less dense than dry air, which makes it more buoyant. The increased buoyancy of clouds helps it stay aloft above drier air.

Reason #3: Condensation releases heat, which helps clouds stay afloat.

Even after wind carries clouds away from updrafts of warm air, the effects of condensation help keep it afloat. When sweat evaporates off of you, you feel cold because water evaporating absorbs heat from your skin. Conversely, when a gas condenses into a liquid, it releases heat.

So as water vapor cools, it turns into liquid water droplets. This condensation warms the cloud from the inside and keeps it up in the air, similar to how a hot air balloon stays afloat.

When do clouds "actually" fall?

Clouds, as we just covered, are always falling. They're just falling super, super, super slowly. But when the size of the water droplets has grown large enough for the terminal velocity to exceed the upward force of rising air, the droplets can no longer stay suspended. It's at this point that they fall to ground as precipitation (and you need to grab an umbrella).

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

📖 Book of the week

​Between Two Kingdoms: A Memoir of a Life Interrupted by Suleika Jaouad​

"Everyone who is born holds dual citizenship, in the kingdom of the well and in the kingdom of the sick." – Susan Sontag

This is a book about how you can only truly appreciate health when you are not well, how healing goes well beyond when your body improves, how the idea of "normal life" can be taken from you in an instant. Jaouad was nearly 23 when her diagnosis of leukemia came. And with it, the life she had imagined for herself gone in a flash. It's truly one of the most beautifully written memoirs I've ever picked up.

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​Check out the full list of books I've recommended here.

👀 Catch up on other curious questions

• ​How does your phone track steps?​​
• ​Why does the wind blow?​​
• ​Is peanut butter a liquid?​​

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