Clouds are made of water,
so why don't they fall?
Clouds are made of water. Water is heavier than air, so how does it stay up in the sky? And why does water that seemed to float forever suddenly fall as rain one day? The truth is, it's not "floating" at all — it's falling incredibly slowly.
On a clear day, white clouds hang in the sky. They can stay there, roughly the same shape, for hours.
But that cloud is not water vapour gas. Since you can see it, what's actually there is liquid water droplets (or ice particles). Water is nearly a thousand times heavier than air, yet it doesn't fall.
Try estimating the amount of water in a large thundercloud, and you'll find it can add up to several swimming pools' worth. That much water is floating right over your head.
Then, hours later, that water falls as rain. What changed?
The water droplets that make up clouds are about one-tenth the width of a human hair. Air resistance is so effective on them that they fall at only about 1 centimetre per second. Even a weak updraft pushes them back up.
The water in a single raindrop is about 1 million cloud droplets' worth. Rain falls once this job of "combining a million into one" is finished.
In other words, rain isn't water newly arriving in the sky — it's water already there "growing bigger". Let's look at it step by step.
So why do water droplets form in the sky at all?
Air contains invisible water (water vapour). But there's a limit to how much it can hold, and that limit shrinks as temperature drops.
When air near the ground warms and rises, it expands because air pressure is lower higher up. Expanding cools it (the opposite of a bicycle pump heating up when you compress it). The cooled air can no longer hold all the water vapour it carried, and the excess becomes liquid droplets. That's a cloud.
In very clean air, water vapour struggles to form droplets. It's surprisingly hard for water molecules alone to form the very first tiny droplet. In lab experiments, air can hold several times the "limit" of water vapour without any droplets forming at all.
Real air, though, is full of floating dust, sand particles, dried sea-salt grains from spray, pollen, and smoke particles. Water vapour uses these particles as a foothold to turn liquid. This is called a condensation nucleus.
In other words, most cloud droplets have some kind of "core" at their centre. In a world with perfectly clean air, rain would be far rarer than it is now.
Why don't they fall? — air gets "stickier" for smaller things
Here's where intuition fails us. The smaller something is, the more air resistance affects it.
Drop a crumpled ball of paper and it falls fast; spread the paper flat and it falls slowly. Same weight, but the larger surface touching the air means more resistance. As a droplet shrinks, its weight drops with the cube of its diameter, while the surface area that resistance acts on drops only with the square. The smaller it gets, the more "surface relative to weight" it has.
For cloud droplets (about 0.02 mm across), this effect is extreme. Their fall speed is only about 1 cm per second. In an hour, they fall just 36 metres. Inside clouds, updrafts of tens of centimetres to several metres per second are common. They get pushed back up with ease.
So a cloud isn't "floating" — it's falling slowly while being pushed up at about the same speed, over and over.
They're falling too slowly to ever actually land.
Becoming rain — combining a million into one
A raindrop (about 2 mm across) has roughly 100 times the diameter of a cloud droplet. Its volume is 100 × 100 × 100, or about a million times larger. So the water from 1 million cloud droplets must gather into a single drop.
Simply growing by water vapour sticking on isn't nearly fast enough. There are mainly two routes that make this happen.
The upper parts of a cloud sit below 0°C. There, ice particles mix with unfrozen water droplets. Here, only the ice particles grow large, at the expense of the surrounding water droplets. Grown ice begins to fall, melting on the way down into rain.
Slightly larger droplets fall faster, so they catch up with smaller droplets below them, collide, and swallow them. The bigger they get, the faster they fall, and the more they swallow. They grow like a snowball.
Even the rain that falls on Japan's lowlands is said to have mostly started as snow aloft, except for summer evening downpours. It simply melts before reaching the ground — it was originally ice.
Raindrops aren't teardrop-shaped
Illustrations usually draw raindrops with a pointed top, teardrop-shaped. That's not their actual shape.
Small raindrops are nearly perfectly round thanks to surface tension. As they grow larger and fall faster, air pushes up from below, flattening and then denting the underside. It ends up shaped like a bun or a hamburger bun.
Grow larger still, and the dent deepens into a parachute shape, and the drop splits apart at around 5 mm across. That's why there's no such thing as a truly huge raindrop — there's an upper limit to raindrop size for exactly this reason.
Something you can check in your kitchen
- Pour a little water into a clear plastic bottle (just enough to wet the bottom) and shake it
- Puff a small amount of incense smoke in, then quickly close the cap (this acts as the condensation nuclei)
- Squeeze the bottle hard with both hands to flatten it, hold for a few seconds, then release suddenly
- The instant you release it, the inside turns white and cloudy. Squeeze again and the haze disappears
Squeezing compresses and warms the air inside; releasing it lets it expand and cool. The moment it cools, the air can no longer hold all its water vapour, and tiny water droplets form using the smoke particles as a foothold — this white haze is exactly the same mechanism that forms clouds in the sky. Since it barely clouds without the smoke, this also confirms the role of condensation nuclei.
Summary
Clouds don't fall not because water is light. It's because the droplets are too small, and air resistance keeps them from ever landing. And rain falls not because new water arrives in the sky, but because droplets that were already there have merged into something a million times bigger.
Look up at the sky, and several swimming pools' worth of water
are falling at 1 centimetre per second, right now.
The same mechanism behind cloud formation — "air cools, and water vapour turns into droplets" — is also why your breath turns white in winter. For more on that, see this article.
When those cloud droplets pile up thick, a cloud you look up at stops looking white and turns black instead. We explain why in Why do rain clouds look black?
So once raindrops start falling together, how fast do they get by the time they reach the ground? Find out in "Raindrops fall from 1,000 metres up — so why don't they hurt?"
Want to know more? — terms, equations, and how this connects to textbooksFrom junior-high science to open research questions, each level is clearly marked
- JHSCovered in junior-high science
- High SchoolCovered in high-school "Basic Physics" / "Basic Earth Science"
- High School+Covered in high-school "Physics" / "Earth Science", or treated as advanced/column content in textbooks
- UniversityNot covered in high school — university-level specialist content (cloud physics, meteorology)
- ResearchNot even settled "fact" at university level — something researchers are actively investigating
JHSTerms: words around clouds and rain
- Saturation vapour amount: the maximum amount of water vapour air at a given temperature can hold. It shrinks as temperature drops.
- Dew point: the temperature at which, as air is cooled, water vapour starts turning into water.
- Condensation nucleus: a tiny airborne particle that acts as a foothold for water vapour to turn liquid. Dust, sea salt, pollen, and so on.
- Cloud droplet: the water or ice particles that make up a cloud. About 0.01–0.05 mm across.
- Raindrop: about 0.5–5 mm across. Anything smaller is called drizzle.
- Terminal velocity: the speed a falling object settles at once air resistance and weight balance out, so it stops accelerating.
JHSHigh SchoolWhy rising air cools
As air rises, the surrounding pressure drops, so it expands. While expanding, the air does work pushing its surroundings outward. That takes energy out of it, so its temperature drops even without losing heat to anywhere else. This is called adiabatic expansion.
As a rough guide, dry air before a cloud forms cools by about 1°C for every 100 m it rises. Once a cloud starts forming, water vapour turning liquid releases heat (latent heat of condensation), so the cooling slows down.
High SchoolWorking it out: how many cloud droplets make one raindrop
Some days have clouds but no rain. Are cloud droplets and raindrops really that different? We can check this using nothing more than the volume formula from junior high.
V = (4 ÷ 3) × π × r³
| V Volume of a sphere | in units like mm³ |
| r Radius | in mm |
| π Pi | about 3.14 |
Notice the r³. Volume grows with the cube of the radius. Double the radius and volume is 8 times bigger; 10 times the radius means 1000 times the volume. The difference inside is far larger than the difference you'd guess from looking.
When comparing two spheres, the (4 ÷ 3) × π part is the same for both, so all you need is the ratio of radii, cubed.
| Cloud droplet diameter | about 0.02 mm |
| Raindrop diameter | about 2 mm |
| Diameter ratio | 2 ÷ 0.02 = 100 times |
| Volume is that cubed | 100 × 100 × 100 = 1,000,000 |
| Answer | 1 raindrop = about 1 million cloud droplets |
It takes a million cloud droplets combining to finally make one raindrop. The size difference is only 100-fold, yet the number required explodes far more. That one line explains why a cloud doesn't rain the instant it forms.
There's another condition: can it beat the updraft? If its fall speed is slower than the updraft, the droplet can never reach the ground.
| Updraft inside a cloud | say, about 1 m/s |
| Cloud droplet fall speed | about 0.01 m/s (1 cm/s) |
| How far short | 1 ÷ 0.01 = 100 times too slow |
| Raindrop fall speed | about 6 m/s |
| How much it wins by | 6 ÷ 1 = 6 times |
A cloud droplet falls at only 1/100th the needed speed, so in effect it stays up indefinitely. That's how a cloud stays in the sky. A cloud isn't "floating" — it's "falling too slowly, and getting pushed back up by updrafts."
Only once it has grown a millionfold does it reach 6 m/s, finally beating the updraft and reaching the ground. Rain happens when both of these conditions are cleared at once.
| Cloud droplet (0.02 mm) | about 1 cm/s |
| Drizzle (0.2 mm) | about 0.7 m/s |
| Raindrop (2 mm) | about 6 m/s |
| Large raindrop (5 mm) | about 9 m/s (splits beyond this) |
Look closely at this table: a 100-fold increase in diameter only gives a 600-fold increase in speed — even though volume increases a millionfold. Fall speed doesn't scale as simply as volume does.
High School+For very tiny droplets, air resistance is proportional to speed (F ∝ v). In this range, fall speed is proportional to the square of the radius, so a tenth the radius means a hundredth the speed. As droplets grow larger, resistance approaches being proportional to the square of speed (F ∝ v²), and fall speed becomes roughly proportional to the square root of the radius. Because the rule switches partway through, there's no single simple formula.
UniversityHow ice grows by "stealing" water
The main text said "only the ice particles grow, at the expense of the surrounding water droplets." This is called the Bergeron process (or ice-crystal process), and it's a cornerstone of cloud physics.
The key fact is that at the same temperature, the saturation vapour pressure over ice is lower than over liquid water. Around −12°C, for instance, the difference is roughly 10%. So from ice's point of view, the air is "supersaturated" with vapour, while from a water droplet's point of view, it's "undersaturated."
As a result, water keeps evaporating from the droplets, and that vapour keeps condensing onto the ice. The ice grows one-sidedly while the droplets vanish. This mechanism only works because clouds contain large amounts of water that stays liquid even below 0°C (supercooled water droplets).
Grown ice crystals can then collide to form snowflakes, which catch supercooled droplets as they fall (riming), growing into graupel or hail. If they pass through a layer warmer than 0°C on the way down, they melt into rain.
ResearchQuestions still unanswered
- Rain starts falling faster than theory predicts. This has long been a famous puzzle in cloud physics. Growth by vapour condensation works well up to about 0.03 mm across, after which its efficiency drops off sharply. Growth by collision and merging only kicks in from about 0.08 mm onward. In between lies a "growth gap" with no effective mechanism. Yet real cumulus clouds start producing rain in as little as 20 minutes — much faster than theory predicts.
- Turbulence is the leading explanation, but it isn't settled. One idea is that turbulent motion inside clouds locally crowds droplets together, boosting collisions; this is being studied through simulations and lab experiments. But how much it actually contributes, and how it combines with other factors (such as large sea-salt particles acting as triggers), is still unresolved.
- The relationship between clouds and air pollution is the biggest uncertainty in climate forecasting. More condensation nuclei split the same amount of water into more, smaller droplets. This makes clouds whiter and better at reflecting sunlight, while also potentially suppressing rainfall. This effect remains one of the single largest sources of error in climate models.
- Proving that cloud seeding works remains difficult. Seeding clouds with nuclei is practiced in various countries, but it's fundamentally hard to rule out "it might have rained anyway," so quantitatively evaluating its effectiveness is still debated.
Even rain, something we see every day, can't be fully explained in terms of exactly how it starts. The most familiar phenomena are often, surprisingly, the least understood.
How this connects to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science – states of matter / saturation vapour amount and dew point / weather observation | How clouds form, condensation nuclei, the bottle experiment |
| High School | Basic Earth Science – atmospheric structure / adiabatic change | Why rising air cools, about 1°C per 100 m |
| High School | Basic Physics – force balance / air resistance | Terminal velocity, why small droplets don't fall |
| High School+ | Physics – resistive forces and motion / surface tension | The v-proportional and v²-proportional regimes, raindrop shape and splitting |
| University | Cloud physics / meteorology | The Bergeron process, supercooled droplets, riming |
| Research | Cloud physics / climate science (unresolved) | The growth gap, the role of turbulence, aerosols and clouds, cloud seeding |
- Explanatory material on clouds and precipitation, and weather-observation guidance, from the Japan Meteorological Agency (気象庁).
- Pruppacher, H. R. & Klett, J. D., Microphysics of Clouds and Precipitation (the standard textbook of cloud physics; covers cloud/raindrop size, terminal velocity, and the Bergeron process).
- Grabowski, W. W. & Wang, L.-P., Growth of cloud droplets in a turbulent environment, Annual Review of Fluid Mechanics 45, 293–324, 2013 (a review of turbulence and collisional growth).
- IPCC Sixth Assessment Report, Working Group I (assesses aerosol–cloud interactions as a major source of uncertainty in climate projections).
- Beard, K. V. & Chuang, C., A new model for the equilibrium shape of raindrops, Journal of the Atmospheric Sciences 44, 1509–1524, 1987 (on raindrop equilibrium shape).
※ Droplet sizes and fall speeds vary with conditions. This article presents commonly cited representative values as a general guide.
※This article is a general-audience science explainer. The figures given are approximate guides for understanding the underlying mechanism and can vary with conditions. For weather forecasts, please check announcements from the Japan Meteorological Agency and other weather services.