The water cycle: the sun lifts water as invisible vapour, cold air turns it into cloud drops, the drops grow until they fall, and rivers carry the rain back to the sea.
☀️ The sun warms the sea. Some water floats up as tiny bits you cannot see.
☁️ Up high it is cold. The bits turn into tiny drops. Lots of drops make a cloud.
🌧️ The drops bump and join. They get big and heavy. Down they fall as rain!
Where does the water come from?
The sun warms oceans, lakes, and puddles. Some of the water turns into vapor, a gas you cannot see. It floats up into the sky.
How does a cloud form?
High up, the air is cold. The vapor turns back into tiny water drops. Millions of tiny drops together make a cloud.
Why does it rain?
Inside the cloud the drops bump into each other and join up. When a drop gets big and heavy, the air cannot hold it up any more. Down it falls as rain.
Where does the rain go?
Rain runs into rivers, lakes, and the sea. Then the sun warms it again, and the trip starts over. This loop is called the water cycle!
Up, Up, and Invisible
Every drop of rain started somewhere else. The sun warms the oceans, lakes, rivers, and even wet leaves, and some of that water turns into water vapor, a gas you cannot see. Warm air rises and carries the vapor with it, and plants add more through tiny holes in their leaves. Most of the vapor in the sky, more than eight tenths of it, came from the ocean.
Making a Cloud
The higher air goes, the colder it gets. Cold air cannot hold as much vapor, so the vapor turns back into liquid. It needs something to grab onto, and the sky is full of tiny specks: dust, sea salt, pollen, and smoke. A drop forms around each speck. A cloud is millions of these drops, each about as wide as a hair is thick.
From Drop to Raindrop
A cloud drop is far too small to reach the ground. To become a raindrop it has to grow about a million times bigger. Drops bump into each other and join, and big drops fall faster and sweep up more small ones on the way, like a snowball rolling downhill. In tall clouds the top is below freezing, and ice crystals grow even faster than drops. Most rain outside the tropics starts as snow and melts on the way down.
The Water Cycle
Rain runs into streams, rivers, and the sea, or soaks into the ground. Then the sun warms it and it rises again. Water has been going round this loop for billions of years, so the rain on your window today may once have been drunk by a dinosaur.
Try This!
Put a lid on a pot of hot water and wait a minute. Lift the lid and look underneath. The drops are a cloud that formed on the cold lid, and the ones that drip off are rain.
The Sky's Invisible Water
About 505,000 cubic kilometres of water evaporate from Earth's surface every year, and roughly 86 percent of it comes from the oceans, with the rest from lakes, rivers, soil, and the leaves of plants, which release vapour through pores called stomata in a process called transpiration. That vapour is a gas, and it is invisible, so the white you see in a cloud is not vapour at all but liquid drops and ice crystals. At any moment the atmosphere holds about 12,900 cubic kilometres of water, and a molecule stays up there for about nine days on average before it comes back down.
Cooling Makes Clouds
Air holds less vapour when it is cold, so the way to make a cloud is to cool moist air, and the atmosphere does that mostly by lifting. Air rises when the sun heats the ground beneath it, when wind pushes it up a mountainside, when a warm air mass slides over a cold one along a front, or when winds converge and have nowhere to go but up. Lifting is the key. Rising air expands and cools by about 10 °C for every kilometre, and once it is cold enough the vapour condenses into droplets on cloud condensation nuclei: specks of sea salt, dust, smoke, or pollen. Without those specks, air can hold far more vapour than it should before anything forms, so a cloud is usually a sign of slightly dirty air, in the most harmless sense.
The Million-Fold Problem
A cloud droplet is about 0.02 millimetres across and falls at about a centimetre per second, slower than the air rising beneath it, so it never reaches the ground. A raindrop is about a hundred times wider, which makes it a million times heavier. Getting from one to the other is the real question, and there are two answers. In warm clouds, such as those over the tropics, slightly larger droplets fall faster, collide with smaller ones, and merge, and each merger makes them fall faster still, so the growth runs away with itself; this is called collision and coalescence. In colder clouds the top is below freezing, but the droplets stay liquid, supercooled, down to about minus 40 °C unless they find an ice nucleus. A few crystals form, and then a strange fact takes over: air that is saturated for water is supersaturated for ice, so vapour leaves the droplets and grows the crystals, which become snowflakes, fall, and melt into rain below the freezing level. Most rain outside the tropics is born as snow. Snow first, rain later.
Where the Rain Lands
Mawsynram in India receives about 11,900 millimetres a year, because moist monsoon winds are forced up the Khasi Hills, while parts of Chile's Atacama Desert have gone decades without measurable rain, sitting in the rain shadow of the Andes beside a cold ocean current that keeps the air from rising. The global average is about a metre of rain a year.
Think About It
If cloud droplets need a speck to form on, what would happen to rain over a perfectly clean ocean, and why does the smell of rain, which comes from soil bacteria, arrive before the first drops rather than after them?
A Phase Change in the Sky
Rain is the last step of a chain that begins with radiation. Sunlight heats the sea surface, water molecules gain enough energy to escape the liquid, and the resulting vapour is mixed upward by turbulence and convection. The air's capacity to hold vapour is set by the Clausius–Clapeyron relation: saturation vapour pressure rises roughly exponentially with temperature, about 7 percent per degree Celsius near the surface. Lift the air and it cools at the dry adiabatic lapse rate of 9.8 °C per kilometre until it reaches saturation, after which condensation releases latent heat (2.26 megajoules per kilogram) and the cooling slows to the moist rate of about 5 to 6 °C per kilometre. That released heat is the fuel of deep convection.
Nucleation
Pure vapour does not condense at 100 percent relative humidity. The Kelvin effect raises the equilibrium vapour pressure over a curved surface, so a droplet forming from nothing would need supersaturations of several hundred percent. Real clouds form at supersaturations of a fraction of a percent because the air carries cloud condensation nuclei, and soluble ones, sea salt above all, lower the vapour pressure by the Raoult effect. Köhler theory combines the two: each nucleus has a critical supersaturation above which it activates into a droplet that keeps growing. Over remote oceans there may be only 50 nuclei per cubic centimetre; over a city, thousands. Polluted clouds therefore have more, smaller droplets, which is why they are brighter and, all else equal, slower to rain.
Ice
Cloud droplets do not freeze at 0 °C. Homogeneous freezing needs about minus 38 °C; between there and 0 °C a droplet freezes only on an ice nucleus, and those are scarce, so mixed-phase clouds full of supercooled water are ordinary. Once a few crystals exist, the Wegener–Bergeron–Findeisen process takes over. Saturation vapour pressure over ice is lower than over liquid water (about 10 percent lower at minus 12 °C), so air in equilibrium with droplets is supersaturated for ice; the crystals grow by deposition, the droplets evaporate to feed them, and the crystals aggregate into snowflakes or rime into graupel. Wegener sketched the idea in 1911, Bergeron argued it in 1935, and Findeisen quantified it in 1938. Cloud seeding with silver iodide, first tried in 1946, works by adding ice nuclei to clouds that lack them, with effects that remain modest and hard to measure.
Falling
A drop's terminal velocity is set by drag: about 0.3 metres per second at 0.1 millimetres, 6.5 at 2 millimetres, and 9 at 5 millimetres, where aerodynamic pressure flattens the base into an oblate bun and internal oscillations shatter anything larger. Between cloud base and ground a drop may evaporate entirely; the trailing curtain of virga under a high cloud is rain that never arrives. On Venus, sulfuric acid virga evaporates some 25 kilometres above a surface too hot for any liquid; on Titan, methane rains onto a landscape of hydrocarbon lakes.
What Is Still Unknown
How aerosols change rainfall is the largest uncertainty in the cloud part of climate models; the first steps of collision growth in turbulent clouds are still being measured in laboratories; and ice nucleation, which particles trigger it and at what temperature, remains a field where a single new measurement can move the models.
Why Your Kid Asked
Because rain is the most visible thing the sky does, and the usual answer ("clouds get full and it falls out") skips the interesting part: a cloud is already full of water, and the whole question is how drops a hundredth of a millimetre wide become drops big enough to fall.
The Short Version
The sun evaporates water, mostly from the ocean; rising air cools, and the invisible vapour condenses on specks of salt and dust into cloud droplets so small that rising air holds them up. Rain happens when droplets grow about a million times in mass, either by colliding and merging in warm clouds or, in clouds whose tops are below freezing, by ice crystals growing at the droplets' expense and falling as snow that melts on the way down. Rain then runs to the sea or into the ground, and the loop repeats. A water molecule spends about nine days in the air between trips.
The Numbers
Roughly 505,000 cubic kilometres of water evaporate each year, 86 percent from the oceans, and the global average rainfall is about a metre. A cloud droplet is about 0.02 millimetres across and a raindrop 0.5 to 5; a 2 millimetre drop falls at about 6.5 metres per second and a 5 millimetre one at 9, the practical maximum before drops break up. Warm air holds about 7 percent more vapour per degree, which is why a warming climate brings heavier downpours. Mawsynram in India averages about 11,900 millimetres a year and parts of the Atacama have recorded none for decades.
Two Common Misconceptions Worth Correcting
First, "clouds are made of water vapour": vapour is invisible, and what you see is liquid droplets and ice, which is worth a demonstration with a boiling kettle, where the clear gap just above the spout is vapour and the white plume beyond it is droplets. Second, the teardrop. Small drops are spheres and large ones are flattened buns, and no falling drop has a pointed tail, whatever the weather icon shows.
When to Pay Attention
Rain itself is harmless; what carries risk is water on the move. Flash floods kill more people than lightning in most years, and about half of flood deaths involve a vehicle driven into moving water, so the rule for children and adults is the same: do not walk or drive through water you cannot see the bottom of, and treat a storm drain or culvert in a downpour as you would a river.
Something to Do Together
Make rain in the kitchen: boil water in a pan, hold a plate of ice cubes above the steam, and watch drops form on the cold underside and fall. That is the whole cycle in one minute, with the stove as the sun, the plate as the cold upper air, and the drops as the cloud growing until it rains. Then, on the next rainy day, catch drops on a tray of flour: each one rolls into a ball you can dry and measure, which is how meteorologists first measured raindrop sizes in the 1890s.
Sources
- Wallace, J.M. and Hobbs, P.V. Atmospheric Science: An Introductory Survey, 2nd ed., chapter 6 (Cloud Microphysics). Academic Press, 2006.
- Rogers, R.R. and Yau, M.K. A Short Course in Cloud Physics, 3rd ed. Pergamon, 1989.
- Pruppacher, H.R. and Klett, J.D. Microphysics of Clouds and Precipitation, 2nd ed. Kluwer, 1997.
- Wegener, A. Thermodynamik der Atmosphäre. Barth, Leipzig, 1911.
- Bergeron, T. "On the physics of cloud and precipitation." Procès-Verbaux de l'Association de Météorologie, IUGG, Lisbon (1935): 156–178.
- Findeisen, W. "Die kolloidmeteorologischen Vorgänge bei der Niederschlagsbildung." Meteorologische Zeitschrift 55 (1938): 121–133.
- Trenberth, K.E., Smith, L., Qian, T., Dai, A. and Fasullo, J. "Estimates of the global water budget and its annual cycle using observational and model data." Journal of Hydrometeorology 8 (2007): 758–769.
- Bear, I.J. and Thomas, R.G. "Nature of argillaceous odour." Nature 201 (1964): 993–995.
- Becher, P.G. et al. "Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal." Nature Microbiology 5 (2020): 821–829.
- U.S. Geological Survey Water Science School. "The Water Cycle." usgs.gov.