Your turn
Each lab is one part of the video. Tap, slide and see what happens.
Each lab isolates one step. The models are simplified teaching sketches, not to scale.
Tap the buttons and slide the sliders.
Tap the sky over the square. What has to change for floating water to fall?
Clouds hold liquid water yet float. Toggle the rain and think: what must happen to the droplets first?
Tap the sky. Splash! Now tap again to stop the rain.
Drag the bubble up. How high does it go before the cloud appears?
Lift the parcel. It cools about 9.8 °C per km while unsaturated; watch it reach the dew point and condense on nuclei.
Move the bubble up high. What happens when it gets cold?
Slide the droplet size. When does it beat the rising air? Try strong rising air too.
Terminal velocity grows with drop size. Find the diameter where the drop overcomes a gentle, then a strong, updraft.
Make the drop bigger. When does it fall?
Drag left and right to steer the falling droplet into the others. How many does it swallow?
Steer the collector drop. Each merger raises its fall speed and sweep rate.
Move the big drop so it bumps the small ones.
Tap a droplet to freeze it, and watch it steal from its neighbours.
Freeze one supercooled droplet. Vapour deposits on the ice while nearby droplets evaporate.
Drag the air over the mountain. Where does it rain? Where is it dry?
Push the parcel over the range: windward lift and rain, lee-side descent, warming and a rain shadow.
Slide the air over the mountain. Where does the rain fall?
A cloud is made of water. But most clouds just float. So how does rain fall?
The air has water in it, but you can't see it. The sun warms the ground, and warm air goes up, like a bubble. High up, it is cold. The water in the air turns into tiny drops. Each drop grows on a speck of dust. Lots of tiny drops make a cloud.
Cloud drops are super tiny and light. The air holds them up, like a feather. A raindrop is made of lots and lots of cloud drops.
In the cloud, drops bump into each other and join. They get bigger and heavier. When they are heavy enough, down they fall. That's rain!
Mountains make rain too. Wet air goes up the mountain, cools, and rains on one side. The other side stays dry.
Want more? See the water cycle, rainbows and fog.
It rains when air rises and cools. Cooling makes the invisible water vapor in the air condense onto tiny specks of dust, salt or smoke, forming cloud droplets.21 But cloud droplets are far too small to fall.3 Rain only happens when those droplets grow, to about a million times their original amount of water,6 by crashing into each other and merging, or by growing first as ice crystals that melt on the way down.37 Once a drop is heavy enough to fall faster than the air is rising, it falls out of the cloud as rain.4
So the real question isn't "why does water fall?" but "how do floating droplets get heavy enough to fall?"
Rain comes from clouds, and cloud water comes from water that evaporated from oceans, lakes, soil and plants.1 Air always carries some of that water as vapor, a gas you can't see, even on a perfectly clear day.4 Winds carry moist air from the oceans inland.3 For the full journey of water from sea to sky and back, see how the water cycle works.
Oddly, only about a thousandth of one percent of all the water on Earth is in the air as vapor at any moment.12 It keeps moving, though, and that small amount feeds every rain shower on the planet.
Clouds form when air rises. As it goes up, the air pressure around it drops, so the air expands, and expanding air cools. Dry rising air cools by about 9.8 °C for every kilometer (5.5 °F per 1,000 feet).2 Once it cools to its dew point, it can't hold all its vapor, and the extra vapor condenses into liquid.28
Weather scientists list several ways air gets lifted:58
| What lifts the air | How it works | Typical rain |
|---|---|---|
| Convection (heating) | The Sun heats the ground, the ground warms the air above it, and the warm air rises like a bubble | Summer showers and thunderstorms |
| Mountains (orographic lift) | Wind is forced up and over hills and mountains | Steady rain on the windward side |
| Weather fronts | Warm air slides up over colder air, or cold air wedges under warm air | Wide areas of rain lasting hours |
| Convergence | Winds flowing together, as in a low-pressure area, have nowhere to go but up | Unsettled, rainy weather |
Water molecules on their own are too small to clump into droplets. They need a surface: a tiny particle called a cloud condensation nucleus. These are specks of dust, soil, smoke, sea salt from ocean spray, or volcanic particles, about a hundredth of the size of the droplet that forms on them.2 So every cloud droplet has a speck of dust, dirt or salt at its heart.210
Fog forms in exactly the same way, just at ground level. If you want the details of dew point and humidity, see what causes fog.
Here's the part most explanations skip. A newly formed cloud droplet is tiny, on the order of 0.01 millimeters across,8 and falls only about 1 centimeter per second.7 Air inside a cloud is rising, sometimes at a few centimeters per second and sometimes at several meters per second,7 so a cloud droplet simply floats. There are usually a few hundred droplets in every cubic centimeter of cloud,8 and that's why clouds can hang in the sky for hours without raining.
An average raindrop holds as much water as about one million cloud droplets.6 Because width grows as the cube root of volume, a raindrop is only about 100 times wider than a cloud droplet, but that's enough to change everything. Raindrops fall at roughly 2 to 10 meters per second,9 fast enough to beat the rising air and survive the trip to the ground.6
| Typical width | Falls at about | |
|---|---|---|
| Condensation nucleus (speck) | ~0.0001 mm | floats |
| Cloud droplet | ~0.01–0.02 mm | 1 cm per second |
| Drizzle drop | 0.2–0.5 mm | 0.7–2 m per second |
| Raindrop | over 0.5 mm, usually 1–3 mm | 2–10 m per second |
1 raindrop ≈ 1,000,000 cloud droplets (100× wider)
Sources for the table: NOAA (nucleus size),2 Britannica (droplet, drizzle and raindrop sizes and speeds)789 and USGS (typical raindrop sizes).4 Just condensing more vapor can't close this gap on its own. Condensation alone makes drops of drizzle size at most.8 Clouds use two tricks instead.
In clouds warmer than freezing, droplets grow by collision and coalescence, sometimes called the warm rain process.3 Cloud droplets are never all the same size, because they form on specks of different sizes and grow in slightly different conditions. A droplet that's a little bigger than average falls a little faster, so it catches up with smaller droplets below it, collides with them, and they fuse into one.7
Each merger makes the drop bigger, so it falls faster and sweeps up even more droplets. A droplet can even be carried upward by the rising air, growing on the way up, and keep growing on the way back down. If the cloud is deep enough, it comes out of the bottom as a raindrop.7 This happens most easily in the tropics, where clouds can be deep and warm all the way up.7
Not a teardrop! Small raindrops, up to about 1 millimeter wide, are round, held together by surface tension. At around 2 to 3 millimeters, the air pushing up from below flattens the bottom until the drop looks like a hamburger bun.410 At about 4 to 5 millimeters, the dent in the bottom balloons into a sort of parachute, and the drop breaks up into smaller drops.4
That's why raindrops rarely grow past about 6 millimeters.914 In a deep, vigorous cloud, big drops keep splitting, and each piece grows again. The number of raindrops can shoot up within minutes until the rising air can't hold them, and the cloud dumps a heavy shower.7
High up, many clouds are colder than 0 °C. Surprisingly, their droplets usually stay liquid anyway. They're supercooled. Pure droplets only freeze on their own below about −40 °C. Above that, a droplet needs a special speck called an ice nucleus (probably clay particles blown up from the ground) before it can freeze.7
Once a few ice crystals appear among the supercooled droplets, the mix is unstable. Water vapor deposits onto ice more easily than onto liquid water, so the ice crystals grow while the droplets around them shrink.37 This is called the Bergeron process (or ice-crystal process), after the meteorologist Tor Bergeron.6 In a fast-growing cloud, a crystal can reach about 0.13 millimeters in three to five minutes,6 and after several minutes the crystals fall at tens of centimeters per second, clumping into snowflakes.7
If the air below the cloud is cold all the way down, you get snow (see how snow forms). If it's warmer than freezing, the snowflakes melt into raindrops.3 Weather radar regularly shows this melting layer inside rain clouds. Widespread rain from big cloud sheets rarely falls unless the cloud tops are colder than about −12 °C, so outside the tropics a lot of ordinary rain starts its life as snow.7 In violent storms, frozen particles can grow into hail instead.
| Collision and coalescence | Bergeron (ice-crystal) process | |
|---|---|---|
| Where | Warm clouds (above 0 °C) | Cold clouds with supercooled droplets |
| How drops grow | Bigger droplets fall faster and swallow smaller ones | Ice crystals steal vapor from droplets |
| Reaches the ground as | Rain or drizzle | Snow, or rain if it melts on the way |
| Most common in | The tropics | Cooler, mid-latitude regions |
When a moist wind meets a mountain range, it's pushed up the slope. It cools, clouds form, and most of the rain or snow falls on the windward side. Once the air crosses the top and sinks down the far side, it warms up and its clouds evaporate.1311 That dry side is called a rain shadow. Death Valley is so dry because it lies in the rain shadow of the Sierra Nevada, and the Tibetan Plateau sits in the rain shadow of the Himalayas.11
Rain shadows help explain why rainfall is so uneven. The wettest place on record, Mawsynram in India, averages about 1,187 centimeters (467 inches) of rain a year, while Arica in Chile once went 14 years without any rain at all.4
Because growing a raindrop takes time and the right conditions. A cloud has to be deep enough and last long enough, with droplets of mixed sizes or with ice crystals, for a few drops to grow a million times bigger. Pollution matters too. More particles in the air mean the same water gets shared among more, smaller droplets, which merge less easily. So a polluted cloud can produce less rain than a clean cloud of the same size.15
In US forecasts, the probability of precipitation is the chance that at least 0.01 inch of rain (or melted snow) will fall at any given point in the forecast area during the forecast period.16 A 40% chance doesn't mean it will rain for 40% of the day, or over 40% of the town.
Air carries invisible water vapor. When air rises, it cools, and the vapor condenses onto tiny specks of dust, salt or smoke, making cloud droplets. Those droplets are far too small to fall. Rain happens when they grow, by bumping into each other and merging, or by first growing as ice crystals that melt on the way down, until they're heavy enough to fall through the rising air.
Rain comes from clouds, and the water in clouds comes mostly from water that evaporated from oceans, lakes, soil and plants. Winds carry that moist air inland, where it's lifted and cooled into clouds. So the rain on your window may have been part of an ocean a few days earlier.
A cloud droplet is only about a hundredth of a millimeter across and falls about one centimeter per second, so even gentle rising air keeps it up. A raindrop holds as much water as about a million cloud droplets. If a cloud is too thin, too short-lived, or its droplets are too even in size (or too crowded by pollution particles), the droplets never grow big enough, and the cloud drifts by without raining.
No. Small raindrops, up to about 1 millimeter across, are round. Bigger drops get flattened by the air pushing up from below, so drops of 2 to 3 millimeters look like a hamburger bun. At 4 to 5 millimeters, the bottom caves in like a parachute and the drop breaks apart.
Rain means drops wider than 0.5 millimeters; smaller drops are drizzle. Most raindrops are 1 to 3 millimeters across, and they rarely grow past about 6 millimeters because larger drops break up as they fall.
In the US, the National Weather Service's probability of precipitation is the chance that measurable rain (at least 0.01 inch) will fall at any given point in the forecast area during the forecast period. It doesn't mean it will rain for 40% of the time or over 40% of the area.
Moist wind is forced up the windward side of a mountain, where it cools, forms clouds and drops much of its water as rain or snow. On the far side, the air sinks and warms, so its clouds evaporate. That dry side is called a rain shadow. Death Valley lies in the rain shadow of the Sierra Nevada.