How does snow form? The short answer
Snow forms inside clouds that are colder than freezing, 0 °C (32 °F).1 First, water freezes onto a tiny floating speck of dust, soot or pollen.411 Then water vapor, the invisible gas form of water, freezes straight onto that ice without becoming liquid first.2 The ice grows into a six-sided snow crystal. When crystals get heavy, they fall, often tangling together into snowflakes. If the air stays below freezing all the way to the ground, they land as snow.6
So a snowflake isn't a frozen raindrop. It's a crystal that grew, a little at a time, out of thin air.
Where does snow come from?
Snow starts as water vapor. Evaporation from oceans, lakes and rivers puts vapor into the air, and so do plants (and your breath).4 When air rises it cools, and its vapor condenses onto tiny particles to make cloud droplets. Every cloud droplet has a speck of dust, salt or smoke at its core.12 That's the start of the story, and it's one step of the water cycle.
How is snow made inside a cloud? Step by step
- A supercooled cloud. Winter clouds are mostly made of liquid droplets, even when they're below freezing. Water that stays liquid below 0 °C is called supercooled. The droplets start to freeze at around −10 °C (14 °F), a few at a time.4
- A seed. Freezing gets going on special particles called ice nuclei. Scientists have found mineral dust, soot, pollen, bacteria and fungi doing this job in clouds.11
- Vapor turns straight into ice. Once there's a speck of ice, water vapor freezes directly onto its surface. That's what makes a snow crystal different from sleet.2
- Droplets feed the crystal. As the ice grows, the droplets around it slowly evaporate, and their vapor ends up on the crystal. Bit by bit, the cloud's water moves from droplets to snow.4
- It falls. As crystals grow heavier, they fall toward the ground.7
Why do snowflakes have six sides?
When water freezes, its molecules lock into a pattern of hexagons, a bit like a honeycomb. (The same open honeycomb is why ice floats.) That six-fold pattern is where a snowflake's six-fold symmetry comes from.45 A brand-new snow crystal is a tiny hexagonal prism: two flat six-sided faces and six rectangular sides. It can be flat like a plate or long like a pencil.2
Then the corners take over. Vapor sticks to the first ice it touches, so any bump that pokes out catches more vapor and grows even faster. The bump becomes a branch.5 A hexagon has six corners, so as the plate gets larger, branches sprout from all six.2
What decides a snowflake's shape? Temperature and humidity
In the 1930s, the Japanese physicist Ukichiro Nakaya grew snow crystals in a cold lab and found that their shape depends on the temperature and humidity where they grow. His chart is now called the snow crystal morphology diagram, or the Nakaya diagram.3
Humidity matters too. In damp air crystals grow fast and form branches; in dry air they stay simple prisms.34 A crystal that grows as a column and then gets blown somewhere colder can end up as a "capped column": a column with a plate on each end.3 Oddly, scientists still can't fully explain why ice flips between plates and columns as the temperature changes. It depends on how water molecules attach to ice surfaces, and that physics is complicated.3
Nakaya said snowflakes are like hieroglyphs from the clouds: by looking at a crystal's shape, you can work out the conditions it grew in.3
Are no two snowflakes alike?
A growing crystal is blown around the cloud, through colder and warmer, damper and drier air. Every change alters how its arms grow. All six arms go through the same changes at the same moment, so they grow alike, even though nothing links them. Kenneth Libbrecht, a Caltech physicist who studies snow crystals, compares it to neighbors all carrying umbrellas on a rainy day: nobody planned it, they all just felt the same weather.2
Because no two crystals follow exactly the same path, no two end up exactly alike.29 Nobody has checked every snowflake, so it's a probability argument, not a proven fact. But even a small crystal contains about 1018 (a billion billion) water molecules, so a perfect match is astronomically unlikely.105
By the way, most real snow crystals aren't the neat stars you see in photos. Irregular, lopsided crystals are by far the most common kind.2
How cold does it have to be to snow?
The cloud must be below freezing. The ground doesn't have to be. Snow reaches the ground unmelted when the air stays at or below 0 °C from the base of the cloud to the surface.6 It can still arrive as snow when the air near the ground is slightly warmer, because melting takes heat from the air right around each flake, which slows the melting down.1 The UK Met Office says snow can fall at air temperatures up to about 2 °C (36 °F), and its heaviest snowfalls often happen between 0 and 2 °C.8
And it's never too cold to snow. Very cold air just holds very little water vapor, so extremely cold places can be surprisingly dry. Most heavy snowfalls happen when the air near the ground is about −9 °C (15 °F) or warmer.1
Snow, sleet or freezing rain?
Most precipitation that forms in winter clouds starts out as snow. What lands depends on the air it falls through:6
Hail is a different kind of falling ice altogether: it grows inside thunderstorms. See how hail forms.
Types of snow: wet snow vs dry snow
Snowflakes are usually clumps of crystals. The University Corporation for Atmospheric Research says a snowflake can be made of as many as 200 crystals,7 and Kenneth Libbrecht notes that hundreds or even thousands can collide and stick together.2 In cold, dry air, branchy crystals hook onto each other when they bump. That makes light, powdery dry snow: great for skiing, easily blown about, and useless for snowballs.98 When flakes fall through air a little above freezing, their edges melt slightly, and that thin film of water glues them together into big, heavy flakes: wet snow, the kind that's perfect for a snowman.9
Most snowflakes are less than 1.3 cm (half an inch) across, though clumps near 5 cm (2 inches) can form when it's close to freezing and the wind is light.1 On average about 10 inches of snow melts down to 1 inch of water, but that varies a lot from storm to storm.7
Why is snow white?
A layer of snow is made of many tiny ice grains with air between them. Clean snow reflects almost all the visible light that hits it, without favoring any color, so it looks white.1
Common misconceptions
"Snow is frozen rain." Frozen raindrops are sleet. Snow crystals grow straight from water vapor, which is how they get their detailed shapes.
2
"It has to be below freezing on the ground to snow." Snow can fall with air temperatures up to about 2 °C (36 °F). What matters is the air all the way from the cloud down.
81
"It can be too cold to snow." It can't. Very cold air holds little moisture, so there's less snow, but snow can still form.
1
"The six arms 'know' what the others are doing." Nothing connects them. They grow alike because they share the same conditions at the same time.
2
"Every snowflake is a perfect star." Most snow crystals are irregular. The perfect ones in photos are carefully picked out.
2
FAQ
How does snow form, in simple words?
High in a cloud that is below freezing, water freezes onto a tiny speck of dust or pollen. Water vapor in the air then freezes straight onto that bit of ice, and it grows into a six-sided snow crystal. When crystals get heavy enough they fall, often sticking together into snowflakes, and if the air stays below freezing all the way down they land as snow.
What causes it to snow?
Snow needs two things: moisture in the air and cold. Air rises and cools, clouds form, and in clouds colder than 0 °C (32 °F) ice crystals grow from water vapor. Snow reaches the ground when the air stays at or below freezing from the cloud down to the surface, or only slightly above it.
How cold does it have to be to snow?
The cloud has to be below freezing, but the ground doesn't. Snow can reach the ground when the air near the surface is a little above freezing, up to about 2 °C (36 °F), because melting cools the air around the falling flakes. It can never be too cold to snow, but very cold air holds little water vapor, so most heavy snowfalls happen when the air near the ground is about −9 °C (15 °F) or warmer.
Is snow frozen rain?
No. Frozen raindrops are sleet: hard, clear pellets. Snow crystals grow when water vapor turns straight into ice without becoming liquid first, which is how they get their detailed six-sided shapes.
Why do snowflakes have six sides?
When water freezes, its molecules lock into a hexagonal (six-sided) pattern. A new snow crystal copies that pattern as a small hexagonal prism, and when it grows into a branching plate, arms sprout from its six corners.
Are no two snowflakes alike?
It's almost certainly true, but nobody has checked every snowflake. Each crystal takes its own path through the cloud, and even a small crystal holds about a billion billion water molecules, so an exact match is astronomically unlikely. Simple, tiny crystals can look almost identical, though.
Where does snow come from?
From water vapor in clouds. That vapor evaporated from oceans, lakes, rivers and plants, rose into the sky and cooled. Snow is one step of the water cycle, and when it melts the water flows back into rivers and the ground.
Sources
- National Snow and Ice Data Center, Science of Snow
- Kenneth Libbrecht (Caltech), Snowflake Science
- Kenneth Libbrecht (Caltech), The Snow Crystal Morphology Diagram
- Kenneth Libbrecht (Caltech), A Snowflake Primer
- NOAA NESDIS / NASA Space Place, How Do Snowflakes Form?
- NOAA National Severe Storms Laboratory, Types of Winter Weather
- UCAR Center for Science Education, Snowflakes
- Met Office, How does snow form?
- Alexandria Blanchard (Purdue University), The Conversation: How is snow made?
- Kenneth Libbrecht (Caltech), No Two Alike?
- Coluzza et al. (2017), Perspectives on the Future of Ice Nucleation Research, Atmosphere 8:138
- NOAA JetStream, How Clouds Form