How Does Snow Form? Snowflakes Explained
In short: snow forms when water vapour in a cloud turns into tiny ice crystals around microscopic particles, in air at or below 0°C. The crystals grow, stick together into snowflakes and fall once they are too heavy. Whether you get powder, wet snow or freezing rain then depends on the temperature of the air they fall through.
The essentials
| Type | Temperature | What it is like |
|---|---|---|
| Dry snow (powder) | Well below 0°C | Light, does not stick: skiers' favourite |
| Wet snow | Around 0°C, slightly above | Heavy, sticks to everything, disrupts traffic |
| Slushy snow | About +1 to +3°C | Very wet and heavy, can turn into black ice |
| Freezing rain | Warm layer aloft, freezing air at ground level | Rain that freezes on contact: be extremely careful |
Watching snow fall is always a quiet little bit of magic. But have you ever wondered how snow is formed, and why it sometimes snows when it does not feel that cold? Here is how a snowflake is made, step by step, and why not all snow is the same.
How does snow form? The conditions
Snow needs three ingredients: water vapour, cold air and tiny particles to freeze onto.
Cold alone is not enough. To get snowflakes, you need water vapour, low temperatures and microscopic particles in the air. Time for a quick reminder of the water cycle from school?
Water from the oceans and lakes evaporates in the heat and the vapour rises into the atmosphere. The higher it goes, the colder the air gets. When the vapour meets cold air in a cloud, it condenses into tiny droplets, which gather around specks of dust, sand or pollen.
Then the temperature decides what kind of precipitation you get. Where the air is above 0°C, the droplets fall as rain. Where the cloud is at or below freezing, the water freezes onto those particles and ice crystals begin to form. That is why the temperature inside the cloud matters so much. To see the other side of the story, read our article why does it rain?
From ice crystals to snowflakes
Ice crystals grow inside the cloud, then join together into snowflakes as they fall.
When an ice crystal first forms around a speck of dust, it is microscopic. Thanks to the water vapour and the cold inside the cloud, it can grow to several millimetres. Its shape depends on the temperature and the humidity at the moment it forms: plates, needles, hollow columns, stars or branching dendrites.
Once the crystals are too heavy to stay in the cloud, they fall. On the way down, they bump into one another and stick together, and that is how the snowflakes we know are made. Each flake follows its own path through the air, which is one reason why no two look exactly alike.
Why snow takes different forms
The air temperature, in the sky and on the ground, decides what kind of snow you get.
Right down to the last few metres, the temperature of the air changes the snow. Will it be perfect for skiing, or will it bring the day to a standstill? Time to get your barometer and thermometer ready. The main types are:
- Dry snow (powder): the skiers' dream. It falls in very cold weather, well below 0°C, as in the mountains. With no liquid water in it, it is light and does not stick.
- Wet snow: common on lower ground, when the air is close to freezing or slightly above. It is the most dreaded type: with a little liquid water in it, it sticks to everything and disrupts road, rail and air traffic. It is also heavy, so it can cause damage when it builds up. It can reach the ground even when part of the air is slightly above 0°C, because the flakes only partly melt on the way down.
- Slushy snow: soaked with water, it appears at around +1 to +3°C. It is easy to clear from roads, but it can quickly turn into black ice.
- Freezing rain: when snowflakes fall through a deep layer of warm air, they melt completely into raindrops. If the air at ground level is below freezing, those drops freeze on contact with the surface and coat roads and branches in ice. Be extremely careful. If the flakes only partly melt and refreeze before landing, you get sleet instead.
Now that snow has given up its secrets, you will never watch the flakes fall in quite the same way!
Keep exploring: predicting the weather with a barometer and our Arctic Christmas wishlist.
Read the sky before the flakes
Frequently asked questions
How does snow form?
Water vapour in a cloud freezes onto tiny particles such as dust or pollen, in air at or below 0°C. This makes ice crystals, which grow, join together into snowflakes and fall when they are too heavy.
Why does it snow even when it is not very cold?
Because what matters is the temperature in the cloud and on the way down, not only at ground level. Snow can reach the ground even when the air near the surface is slightly above 0°C, but it is then wet snow.
What is the difference between snow and rain?
It all comes down to temperature. Where the air is above 0°C, the water falls as rain. Where it is at or below freezing, it turns into ice crystals that become snow.
Why do snowflakes have different shapes?
The shape of a crystal (plate, needle, star, dendrite) depends on the temperature and humidity where it forms. Each flake also follows its own path through the air, so no two are exactly alike.
What is powder snow?
It is dry snow, which falls when the air is well below 0°C. With no liquid water in it, it is light and does not stick, which is why skiers love it.
Sources
