Clouds are made of water.
So why don't they fall out of the sky?
Clouds are made of water. Water is heavier than air, so how can it stay up there? And why does water that seemed to hang in the sky forever suddenly turn into rain and fall? The truth is, it's not "floating" at all — it's falling incredibly slowly.
On a clear day, a white cloud sits in the sky. For hours, it barely changes shape.
But that cloud is not water vapour, a gas. If you can see it, what's actually there is liquid water droplets (or ice crystals). Water is nearly a thousand times denser than air, yet it doesn't fall.
Try estimating how much water sits inside a large storm cloud, and you'll find figures like several swimming pools' worth. All of that is floating above your head.
Then, a few hours later, that water falls as rain. What changed?
The water droplets that make up a cloud are about a tenth the width of a human hair. Air resistance is so dominant at that scale that they fall at only about 1 centimetre per second. The faintest updraft pushes them back up.
One raindrop's worth of water is about a million cloud droplets. When that job — merging a million droplets into one — is finished, rain falls.
In other words, rain isn't water arriving in the sky — it's water that was already there, growing bigger. Let's look at how, step by step.
So why do water droplets form in the sky at all?
Air holds invisible water — water vapour. But it can only hold so much, and that limit shrinks as temperature drops.
When air near the ground warms and rises, it expands because pressure is lower higher up. Expanding cools it (the opposite of a bicycle pump getting hot when you compress it). The cooled air can no longer hold all the vapour it had, and the excess turns into liquid droplets. That's a cloud.
In perfectly clean air, water vapour struggles to form droplets. Getting water molecules to form that very first tiny droplet is surprisingly hard. In lab experiments, air can hold several times the normal limit of vapour without any droplets forming at all.
Real air is full of dust, sand particles, salt left behind by dried sea spray, pollen, and smoke particles. Water vapour uses these as a scaffold 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? Smaller things stick to air more
Here's where intuition misleads us. The smaller something is, the more air resistance matters.
Crumple a sheet of paper and it falls fast; spread it flat and it falls slowly. Same weight, but more surface touching the air means more drag. As a droplet shrinks, its weight drops with the cube of its diameter, while the surface area that drives drag drops only with the square. The smaller it gets, the more surface it has relative to its weight.
For cloud droplets (about 0.02 mm across), this effect is extreme. They fall at only about 1 cm per second — just 36 metres in an hour. Inside a cloud, updrafts typically run from tens of centimetres to several metres per second. They're pushed back with ease.
So a cloud isn't "floating" — it's falling slowly while being pushed back up at roughly the same rate, over and over.
They're falling too slowly to ever land.
Becoming rain: merging a million into one
A raindrop (about 2 mm across) is roughly 100 times wider than a cloud droplet. Its volume is 100 × 100 × 100 — about a millionfold greater. So a million cloud droplets' worth of water must gather into one drop.
Simply picking up more vapour isn't nearly enough. Two main routes handle the rest.
The upper reaches of a cloud sit below 0°C, holding a mix of ice crystals and unfrozen droplets. There, only the ice crystals grow, at the expense of the surrounding water droplets. Once large enough, the ice falls, melting into rain along the way.
Slightly larger droplets fall faster, so they catch up with and swallow smaller droplets below. Bigger means faster, which means swallowing even more — growth snowballs.
Most rain falling on lowland Japan — aside from summer downpours — is said to have been snow higher up. It simply melts on the way down; its origin is ice.
Raindrops aren't teardrop-shaped
Illustrations often draw raindrops as teardrops with a pointed top. That's not their real shape.
Small raindrops are held nearly spherical by surface tension. As they grow and fall faster, air pushing from below flattens and dents the underside. The result looks more like a bun or a burger bun.
Grow further, and the dent deepens into a parachute shape, which then splits apart at around 5 mm across. That's why truly enormous raindrops don't exist — this is what caps their size.
Something you can check in your kitchen
- Pour a small amount of water (just enough to wet the bottom) into a clear plastic bottle and shake it
- Blow one puff of smoke from an incense stick into the bottle and seal it right away (this acts as the condensation nuclei)
- Squeeze the bottle hard with both hands to compress it, hold for a few seconds, then release suddenly
- The instant you release it, the inside turns hazy white. Squeeze again and the haze clears
Squeezing compresses and warms the air inside; releasing lets it expand and cool. The instant it cools, it can no longer hold all its vapour, and tiny water droplets form using the smoke particles as a scaffold — that's the white haze, and it's exactly how clouds form in the sky. Without smoke, the haze barely forms, which also confirms the role of condensation nuclei.
In summary
Clouds don't fall because water is light — they stay up because the droplets are too small to overcome air resistance. And rain falls not because new water arrives in the sky, but because the droplets that were already there have merged to a millionfold their original size.
Look up at the sky, and swimming pools' worth of water
are falling above you, right now, at 1 centimetre per second.
The same mechanism behind clouds — "air cools, and vapour turns to droplets" — also explains why your breath turns white in winter. You can read more about that in this article.
When those cloud droplets pile up thick enough, a cloud viewed from below turns from white to black. Find out why in Why do rain clouds look black?
So once droplets merge and start falling as rain, 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 go deeper? Terms, formulas, and textbook connectionsFrom middle-school science to open research questions — each section is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" / "Basic Earth Science"
- High school+High-school "Physics" / "Earth Science," or advanced/sidebar textbook content
- UniversityNot covered in high school — university-level cloud physics / meteorology
- ResearchNot settled even at university level — an open question researchers are actively studying
Middle schoolTerms: the language of clouds and rain
- Saturation vapour capacity: 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 cools, water vapour starts turning into liquid.
- Condensation nucleus: A tiny airborne particle that water vapour uses as a scaffold 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 reaches once air resistance and weight balance out, so it stops accelerating.
Middle schoolHigh 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 energy is spent, so the temperature drops even without losing heat to anything outside. This is called adiabatic expansion.
As a rule of thumb, dry air before a cloud forms cools by about 1°C for every 100 m it rises. Once a cloud starts forming, the cooling slows, because condensing vapour into liquid releases heat (latent heat).
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? You can check this with nothing more than the middle-school formula for volume.
V = (4 ÷ 3) × π × r³
| V Volume of a sphere | units: mm³, etc. |
| r Radius | units: mm |
| π Pi | about 3.14 |
Notice r³. Volume grows with the cube of the radius. Double the radius and volume grows 8-fold; a tenfold radius means a thousandfold volume. The difference inside is far bigger than the difference you'd see.
When comparing two spheres, the (4 ÷ 3) × π part is shared by both, so all that's left is cubing the ratio of their radii.
| Cloud droplet diameter | ~0.02 mm |
| Raindrop diameter | ~2 mm |
| Diameter ratio | 2 ÷ 0.02 = 100× |
| Volume ratio (cubed) | 100 × 100 × 100 = 1,000,000 |
| Answer | 1 raindrop = about 1,000,000 cloud droplets |
It takes a million cloud droplets merging to make one raindrop. The size difference is only 100-fold, yet the number required balloons far more. That's why a cloud forming doesn't mean rain starts right away.
There's another condition: can it beat the updraft? If fall speed is slower than the updraft, that droplet never comes down.
| Updraft inside a cloud | assume ~1 m/s |
| Cloud droplet fall speed | ~0.01 m/s (1 cm/s) |
| Shortfall | 1 ÷ 0.01 = 100× too slow |
| Raindrop fall speed | ~6 m/s |
| Margin of victory | 6 ÷ 1 = 6× faster |
A cloud droplet's fall speed is only 1/100th of what's needed, so in practice it just stays up. That's how a cloud stays in the sky. A cloud isn't "floating" — it's "falling too slowly, and getting pushed back by the updraft."
Only once it has grown to a millionfold its size does it reach 6 m/s, break free of the updraft, and make it to the ground. Rain falling means clearing both of these hurdles.
| Cloud droplet (0.02 mm) | ~1 cm/s |
| Drizzle (0.2 mm) | ~0.7 m/s |
| Raindrop (2 mm) | ~6 m/s |
| Large raindrop (5 mm) | ~9 m/s (splits beyond this) |
Look closely at this table: a 100-fold increase in diameter only yields a 600-fold increase in speed — even though volume grows a millionfold. Fall speed doesn't scale as simply as volume does.
High school+For very small droplets, air resistance is proportional to speed (F ∝ v). In this range, fall speed scales with the square of the radius, so a tenth the radius means a hundredth the speed. As droplets grow larger, resistance approaches proportionality to the square of speed (F ∝ v²), and fall speed scales roughly with the square root of the radius. Because the governing rule switches partway through, no single simple formula covers the whole range.
UniversityHow ice grows by "stealing" from water
Earlier, we said "only the ice crystals grow, at the expense of the surrounding water droplets." This is known as the Bergeron process (or ice-crystal process), a cornerstone of cloud physics.
The key fact is that at the same temperature, the saturation vapour pressure over an ice surface is lower than over a water surface. Around −12°C, for instance, the gap is roughly 10%. That means air can be "supersaturated" with respect to ice while simultaneously "undersaturated" with respect to liquid water droplets.
The result: water droplets keep evaporating, and that vapour keeps condensing onto the ice. The ice grows one-sidedly while the droplets vanish. This mechanism relies on clouds containing large amounts of water that stays liquid below 0°C (supercooled droplets).
Later, grown ice crystals can collide to form snowflakes, which capture 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 unsolved
- Rain starts falling faster than theory predicts. This has long puzzled cloud physicists. The process of droplets growing by picking up more vapour works well up to about 0.03 mm across, then suddenly loses efficiency. Meanwhile, collision-and-merging growth doesn't kick in until about 0.08 mm. There's a "growth gap" in between with no efficient mechanism. Yet real cumulus clouds start producing rain in about 20 minutes — far faster than theory predicts.
- Turbulence is the leading explanation, but it's not settled. One idea is that turbulent eddies inside a cloud locally concentrate droplets, increasing collisions — an active area of numerical and laboratory research. But how much it actually contributes, and how it combines with other factors (such as larger sea-salt particles acting as triggers), remains unresolved.
- The relationship between clouds and air pollution is the biggest source of uncertainty in climate forecasting. More condensation nuclei spread the same amount of water across more, smaller droplets. That makes clouds whiter and more reflective of sunlight, while also potentially suppressing rainfall. This effect remains one of the largest sources of error in climate model projections.
- The effectiveness of cloud seeding remains hard to prove. Various countries scatter nuclei into clouds to trigger rain, but it's fundamentally hard to rule out "it might have rained anyway," and quantifying the effect is still debated.
Even rain, something we see every day, still isn't fully explained in terms of exactly how it starts falling. The most familiar phenomena are often the least understood.
Textbook connections, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: states of matter / saturation vapour capacity 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, ~1°C per 100 m |
| High school | Basic Physics: force balance / air resistance | Terminal velocity, why small droplets don't fall |
| High school+ | Physics: drag force and motion / surface tension | Regimes where drag ∝ v and drag ∝ v², raindrop shape and splitting |
| University | Cloud physics / meteorology | Bergeron process, supercooled droplets, riming |
| Research | Cloud physics / climate science (unsolved) | The growth gap, the role of turbulence, aerosols and clouds, cloud seeding |
- Japan Meteorological Agency (気象庁) explanatory materials on clouds and precipitation, and its guide to weather observation.
- Pruppacher, H. R. & Klett, J. D., Microphysics of Clouds and Precipitation (the standard textbook on cloud physics, covering 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 (raindrop equilibrium shape).
※ Droplet sizes and fall speeds vary with conditions. This article presents commonly cited representative figures.
※This article is a general-audience science explainer. The figures given are approximate, meant to convey the underlying mechanism, and can vary with conditions. For weather forecasts, please check the Japan Meteorological Agency or other weather services.