Clouds Are Made of Water —
So Why Don't They Fall?
Clouds are made of water. Water is heavier than air, so how can it stay up in the sky? And why does that water, which seems to hang there forever, suddenly turn into rain and fall one day? The truth is, it isn't "floating" at all — it's falling incredibly slowly.
On a clear day, white clouds drift in the sky. For hours, they stay up there, roughly the same shape.
But that cloud isn't water vapour, a gas. If you can see it, it must be droplets of liquid water (or ice). Water is almost a thousand times heavier than air, yet it doesn't fall.
Try working out how much water is in a big thundercloud, and the answer is said to be the equivalent of several swimming pools. That much water is floating right above your head.
Then, a few hours later, that water falls as rain. What changed?
The droplets that make up a cloud are about a tenth the width of a human hair. Air resistance is so strong on something that small that it only falls at around 1 centimetre per second. The slightest updraft pushes it right back up.
The water in one raindrop is about a million cloud droplets' worth. Rain falls once this "million droplets become one" process is finished.
In other words, rain isn't about new water arriving in the sky — it's existing water droplets growing bigger. Let's look at this 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 air pressure is lower higher up. Expanding cools it (the opposite of a bicycle pump, which heats up when you compress it). The cooled air can no longer hold all the water vapour it was carrying, and the excess turns into liquid droplets. That's a cloud.
In perfectly clean air, water vapour struggles to turn into droplets. Water molecules forming that very first tiny droplet on their own is surprisingly hard. In lab experiments, air can hold several times the normal limit of water vapour without any droplets forming at all.
Real air is full of dust, sand particles, dried sea-spray salt crystals, pollen, and smoke particles. Water vapour uses these particles as a foothold to turn into liquid. This is called a condensation nucleus.
In other words, there's usually some kind of "core" at the centre of a cloud droplet. In a world with absolutely clean air, rain would be far harder to produce than it is now.
Why it doesn't fall — the smaller it is, the stickier the air feels
This is where intuition goes wrong. 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 more 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 only drops with the square. The smaller something gets, the more surface area it has relative to its weight.
For cloud droplets (around 0.02 mm across), this effect is extreme. They fall at only about 1 centimetre per second — just 36 metres in an hour. Inside a cloud, there are usually updrafts of tens of centimetres to several metres per second. That easily pushes the droplet back up.
So a cloud isn't "floating" — it's falling slowly while being pushed back up at roughly the same speed, over and over.
They're falling too slowly to ever actually land.
Becoming rain — turning a million droplets into one
A raindrop (around 2 mm across) is roughly 100 times the diameter of a cloud droplet. Its volume is 100 × 100 × 100, about a millionfold larger. So a million cloud droplets' worth of water must gather into a single drop.
Simply growing by picking up more water vapour isn't nearly fast enough. There are mainly two routes that get the job done.
The upper part of a cloud sits below 0°C. There, ice crystals mix with droplets that haven't frozen. Only the ice crystals grow, at the expense of the surrounding water droplets. The grown ice starts to fall, melting on the way down into rain.
Slightly larger droplets fall faster, so they catch up to and swallow smaller droplets below them. The bigger they get, the faster they fall, and the more they swallow. They grow like a rolling snowball.
Even the rain that falls on the lowlands of Japan is said to be mostly snow up in the clouds, aside from summer downpours. It just melts on the way down to the ground — it started out as ice.
Raindrops aren't teardrop-shaped
Illustrations draw raindrops with a pointed top, teardrop-shaped. That isn't the real shape.
Small raindrops are held nearly perfectly round by surface tension. As they grow and fall faster, air pushing up from below flattens and dents the underside. It ends up shaped like a bun or a burger roll.
Grow larger still, and the dent deepens into a parachute shape, which splits apart at around 5 mm across. So truly huge raindrops simply don't exist. That's why raindrop size has an upper limit.
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 single breath of incense smoke into the bottle and close the lid right away (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 lets it expand and cool. The instant it cools, it can no longer hold all its water vapour, and tiny water droplets form using the smoke particles as a foothold — that's exactly what the white haze is, and it's the same mechanism that makes clouds in the sky. Without the smoke, the haze barely forms, which also confirms the role of condensation nuclei.
Summary
Clouds don't fall not because water is light, but because the droplets are too small to overcome air resistance. And rain doesn't fall because new water has arrived in the sky — it falls because droplets that were already there have grown to a millionfold their original size.
Look up at the sky, and several swimming pools' worth of water
are falling, right now, at 1 centimetre per second.
The same mechanism behind cloud formation — "air cools, and water vapour turns to droplets" — is also behind why your breath looks white in winter. You can read more about that in this article.
When cloud droplets pile up thick enough, a cloud seen from below looks black rather than white. That's explained in Why Do Rain Clouds Look Black?
So once raindrops start falling, 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? — terminology, formulas, and textbook connectionsFrom middle-school science to open research questions, each level is clearly labelled
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics" / "Basic Earth Science"
- HS+Covered in high-school "Physics" / "Earth Science," or treated as advanced/column content in textbooks
- UniNot covered in high school — university-level specialist content (cloud physics, meteorology)
- ResearchNot even settled "fact" at university level — an active research question
MSTerminology: 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 cools, water vapour starts turning into liquid water.
- Condensation nucleus: a tiny airborne particle that acts as a foothold for water vapour turning liquid. Dust, sea salt, pollen, etc.
- Cloud droplet: the water or ice particles that make up a cloud. Roughly 0.01–0.05 mm across.
- Raindrop: roughly 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 and it stops accelerating.
MSHSWhy rising air cools
When air rises, the surrounding pressure drops, so it expands. As it expands, the air does work pushing its surroundings outward. That uses up energy, so the temperature drops even without losing heat to the outside. This is called adiabatic expansion.
As a rough guide, dry air (before any cloud forms) cools by about 1°C for every 100 m it rises. Once a cloud starts forming, water vapour turning into liquid releases heat (latent heat), so the cooling rate slows down.
HSWorking it out: how many cloud droplets make one raindrop?
Some days there are clouds but no rain. Are cloud droplets and raindrops really that different? You can check this using nothing more than the volume formula from middle school.
V = (4 ÷ 3) × π × r³
| V Volume of a sphere | Units like mm³ |
| r Radius | Units of 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, 1000 times the volume. The difference inside is far bigger than it looks from outside.
When comparing two spheres, the (4 ÷ 3) × π part applies equally to both, so you just need to cube the ratio of their radii.
| Cloud droplet diameter | ~0.02 mm |
| Raindrop diameter | ~2 mm |
| How many times wider? | 2 ÷ 0.02 = 100 times |
| Volume is that cubed | 100 × 100 × 100 = 1,000,000 |
| Answer | 1 raindrop = about 1 million cloud droplets |
A million cloud droplets have to gather to finally make one raindrop. The size difference is only 100-fold, yet the number required explodes far beyond that. That single line explains why clouds don't turn into rain right away.
There's another condition: can it beat the updraft? If a droplet falls slower than the updraft, it simply can't come down.
| Updraft inside a cloud | say, ~1 m/s |
| Cloud droplet fall speed | ~0.01 m/s (1 cm/s) |
| How far short? | 1 ÷ 0.01 = 100 times too slow |
| Raindrop fall speed | ~6 m/s |
| How much it wins by | 6 ÷ 1 = 6 times faster |
A cloud droplet falls 100 times too slowly, so in effect it just stays up there. That's how clouds manage to stay in the sky. A cloud isn't "floating" — it's "falling too slowly, so updrafts keep pushing it back up."
Only once it has grown to a millionfold its original size does it reach 6 m/s, fast enough to beat the updraft and reach the ground. Rain happens when both of these conditions are cleared.
| Cloud droplet (0.02 mm across) | ~1 cm/s |
| Drizzle (0.2 mm across) | ~0.7 m/s |
| Raindrop (2 mm across) | ~6 m/s |
| Large raindrop (5 mm across) | ~9 m/s (splits beyond this) |
Look closely at this table: a 100-fold increase in diameter only produces a 600-fold increase in speed, even though volume increases a millionfold. Fall speed doesn't scale as simply as volume does.
HS+For very small droplets, air resistance is proportional to speed (F ∝ v). In this regime, 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, it can't be written as a single simple formula.
UniHow ice grows by "stealing" from water
The main text said "only the ice crystals grow, at the expense of the surrounding water droplets." This is called the Bergeron process (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, that difference is roughly 10%. So from the ice's point of view, the air is "supersaturated" with vapour, while from the water droplet's point of view, it's "undersaturated."
As a result, water keeps evaporating off the droplets, and that vapour keeps condensing onto the ice. The ice grows one-sidedly while the droplets disappear. This mechanism only works because clouds contain large amounts of water that stays liquid even below 0°C (supercooled droplets).
Grown ice crystals then collide to form snowflakes, which can pick up 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 been a thorny problem in cloud physics. Growth by picking up water vapour works well up to about 0.03 mm across, but efficiency drops sharply beyond that. Meanwhile, growth by collision and merging doesn't kick in until about 0.08 mm. There's a "growth gap" in between where neither mechanism works well. 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 motion inside clouds concentrates droplets locally, boosting collisions, and this is being studied through both simulations and lab experiments. But exactly how much it matters, and how it combines with other factors (such as larger sea-salt particles acting as triggers), is still unresolved.
- The relationship between clouds and air pollution is among the most uncertain parts of climate prediction. More condensation nuclei spreads the same amount of water across more, smaller droplets. That makes clouds whiter and better at reflecting sunlight, while also potentially making rain less likely. This effect remains one of the largest sources of uncertainty in climate models.
- Proving cloud seeding works remains difficult. Various countries scatter nuclei into clouds to try to trigger rain, but it's fundamentally hard to rule out "it might have rained anyway," so quantifying the effect is still debated.
Even rain, something we see every day, can't be fully explained in detail — how exactly it starts falling. The things closest to us are often the least understood.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: states of matter / saturation vapour amount and dew point / weather observation | How clouds form, condensation nuclei, the bottle experiment |
| HS | Basic Earth Science: atmospheric structure / adiabatic change | Why rising air cools, ~1°C per 100 m |
| HS | Basic Physics: force balance / air resistance | Terminal velocity, why small droplets don't fall |
| HS+ | Physics: resistive forces and motion / surface tension | Regimes where resistance ∝ v versus ∝ v², raindrop shape and splitting |
| Uni | Cloud physics / meteorology | Bergeron process, supercooled droplets, riming |
| Research | Cloud physics / climate science (unsolved) | The growth gap, role of turbulence, aerosols and clouds, cloud seeding |
- Japan Meteorological Agency (気象庁) explanatory materials on clouds and precipitation, and its weather observation handbook.
- Pruppacher, H. R. & Klett, J. D., Microphysics of Clouds and Precipitation (the standard textbook on cloud physics; cloud and raindrop sizes, terminal velocity, 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 (assessing aerosol-cloud interactions as one of the largest sources 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 (the equilibrium shape of raindrops).
※ Droplet sizes and fall speeds vary with conditions. This article presents commonly cited representative figures as a general guide.
※This article is a general-audience science explainer. The figures given are approximate guides for understanding the mechanism, and vary with conditions. For weather forecasts, please check the Japan Meteorological Agency and other weather services.