Snowflakes 101: How Do Snowflakes Form?

Snowflakes 101: How Do Snowflakes Form?
Posted in: Weather 101

Snowflakes 101: How Do Snowflakes Form?

Snowflakes might be considered unique objects that span the range from awe-inspiring works of natural art to a real pain when too many of them collect on your favorite highway. There’s a time-honored saying that no two snowflakes are alike — but let’s check out how snowflakes form and see if this saying is true.

How Do Snowflakes Form?

Snowflakes take shape only when the air temperature is 32 °F (0 °C) or less, but it can be snowing when it’s above freezing. How? Well, the temperature at cloud level governs whether or not snow forms, not the temperature at the ground since cloud level temperatures are usually colder than the surface. Here’s a deeper look at snowflake formation:

  • An ice crystal forms around a particle of dust, smoke, or pollen from tiny water droplets suspended in the cloud.
  • Snowflakes start out as hexagons (six-sided) because that’s the way water molecules are arranged.
  • The crystal shapes are regulated by the temperature and humidity around them — they might be six-sided plates, columns, or the classic dendrite flake we see on greeting cards.
  • Additional ice crystals collect around the central crystal; they tend to accumulate along the sharp edges, becoming the arms of the snowflake.
  • The size of a snowflake depends on available moisture, its growth rate while it is falling, and collision with other ice crystals, so a single snowflake may change its appearance a number of times on the way to the ground. Once fallen, snow may appear powdery, soggy, or icy depending on temperature and moisture conditions.
Snowflake Ice Crystal Shapes

Ice crystals come in different versions within a snowflake. In fact, there are 35 basic snowflake shapes! Temperature and humidity conditions when the snowflake forms determine its initial appearance, but as the flake runs into differing moisture or temperature conditions, its appearance changes on the way to the ground. Here is some basic information on how temperature impacts ice crystal shapes:

  • Ice crystals within the flake will appear as either dendrites or flat plates when the air temperature is closer to freezing.
  • Ice crystals within the snowflake will appear as hollow columns or needles when temperatures are between 23 °F and 14 °F (-5 °C and -10 °C).
  • Dendrites and thin plates appear as it gets colder, between 14 °F and - 4 °F (-10 °C and -20 °C).
  • Finally, for an extremely cold snow event, oddly, columns and plates return as the dominant crystal shape below -4 °F (-20 °C).
blowing snowflakes

Can Two Snowflakes Be Alike?

You’ve probably heard the aforementioned expression, “no two snowflakes are alike.” Is it really true?

Let’s imagine 10 snowflakes falling from the same cloud, at the same time, with the same temperature, humidity, and so forth. But this situation is impossible — no two snowflakes will experience the same conditions, and these tiny variations in moisture, temperature, collision with other snowflakes, and wind currents shape how the flake develops. And then there’s this: a typical snowflake contains 10 quintillion water molecules! The snowflake’s shape is determined by how the water molecules line up and what sticks to the flake as it grows, so the odds are essentially zero that two of them will be identical in nature.

But hold on a second. Scientists have found that very small ice crystals can resemble each other — but a complete snowflake finds no copies in nature.

It Can Be Too Cold To Snow

As the air gets colder, moisture condenses out as snow or ice crystals. This is why supercold places like the South Pole get less than 2 inches of snow per year. On the other hand, the snow that falls in these polar deserts never melts, so it piles up thousands of feet deep over many millennia. In fact, the ice cap at the South Pole is almost 2 miles thick!

Explore Snowflakes Yourself

Snowflakes will vary in size and shape depending on the temperature, wind, and humidity. Use your home weather station to track how they change in appearance as the outdoor readings vary. Capture a snowflake on a clear sheet of plastic or a piece of glass and take a close-up with a cell phone or camera, or just check it out with a magnifying glass. After a few snowfall events, you’ll start to see a pattern in the ice crystal shapes. Pretty cool (pun intended). Enjoy the snow!

Steve LaNore is a certified broadcast meteorologist with more than three decades in the field. His work ranges from forecasting tornadoes to insurance and disaster consulting to freelance science publishing. He has won nine awards during his career, including three author awards for his children's book, “Weather Wits and Science Snickers: Corny Jokes and Cool Facts,” available on Amazon. He and his wife Gera live near beautiful Lake Texoma.
February 25, 2022
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