If you are someone who has a child, plays some other role in a child's life, or are simply near one at a bus stop, at some point you are going to be asked how clouds stay in the sky by a toddler.
Science lessons from your youth fade pretty quickly, we get it. There's also the chance you have been told the incorrect explanation at some point, that clouds float because they are less dense than the air beneath them.
As air near the Earth's surface heats up it does indeed begin to rise, carrying water vapor with it, being less dense than the surrounding (mostly) nitrogen and oxygen. This is the same principle that lifts hot air balloons off the ground. Think of how a balloon held underwater will rise to the surface, where it stops, as the air within it is roughly as dense as the air around it. Or at least, it is not dense enough to sink in water, and not sparse enough to keep on flying upwards. If you filled it with the less dense helium, it would continue to float upwards.
But air – and the water vapor contained within it – does not forever fly upwards and out of our atmosphere like helium, reaching a height where it is roughly the same density as the air around it. As air cools, water and ice droplets are formed within it in big enough volumes to make the clouds we see, as sunlight reflects off the large numbers of water droplets.
Droplets of water within clouds, though, make up only a tiny proportion of the cloud's volume. A common cumulus cloud, for example, has approximately 0.5 grams (0.018 ounces) of water per cubic meter. This adds up quite quickly given the enormous volume of clouds. A 1-cubic-kilometer (0.24-cubic-mile) cloud would weigh 500,000 kilograms (1.1 million pounds).
So, how does something that heavy stay up there, when throwing a truck out the window of an airplane would result in a truck smashing to the ground and probably a few lawsuits?
Water droplets inside the clouds are not concerned with how heavy the air around them is in total. The numerous droplets within a cloud may make a huge total, but that doesn't matter for the individual droplet. These stay aloft (until they don't) due to their large surface area.
The upwards force of air resistance (aka buoyant force) on small droplets of water is enough to keep their terminal velocity – where the drag force of the air an object is moving through is equal to the downward force of gravity – low, for small droplets. The result is that the smaller droplets are constantly falling, but easily pushed upwards by the air below them.
This doesn't last forever – just ask anyone who lives in the UK. The droplets within the cloud collide and become larger droplets. As this happens, the weight of the droplets compared to the surface area increases, and the droplets fall, the air resistance no longer enough to slow them.
"A droplet of 10-micron radius falls at a speed of 1 cm/sec [0.02 mph]," Louis J. Battan explains in the book Cloud Physics, "while droplets of 50-micron radius fall at a speed of 26 cm/sec [0.6 mph]."
As soon as the radius of the raindrop goes above 0.1 millimeters, the air resistance of upward-flowing air isn't enough to counter gravity, and it's time to fetch an umbrella.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.