Each winter, trillions upon trillions of snowflakes fall out of the sky. But no two flakes are exactly alike, and the weather is the reason why.

To the naked eye, falling snow may look like clumps of white, fluffy stuff. But if you take a much closer look, each individual flake has its own intricate design.

And Mother Nature makes sure that you won't find any duplicates.

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(MORE: Your first freeze is coming: Average dates across US by region)

The two key factors

It all starts in the clouds, where the snow crystals form.

Each crystal begins as a hexagonal prism. Remember a hexagon from geometry class? Imagine that but with some thickness behind it.

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Now, the ultimate appearance of a snowflake depends on two factors: the temperature and the moisture, otherwise known as humidity.

(MORE: Will this super El Niño push snow to the South? Here's the catch)

As the crystal grows in the cloud and ultimately falls through layers with differing temperature and humidity, the shape gradually changes and becomes more defined.

Dendrites are the most common shape — likely familiar to you because of their use in holiday decor.

Other types include columns, plates, needles and prisms, as well as some other even more exotic types.

Ukichiro Nakaya was the scientist who developed a system for snow crystal formation in the 1930s, which is known as the Nakaya diagram, shown below.

Once the flake reaches the ground, it becomes like a diary log or love letter from the atmosphere — its shape detailing the weather encountered on the snowflake's descent.

Other weather impacts

If the temperature in a cloud is close to the freezing point, the snow crystals will exist among supercooled water droplets.

These droplets will freeze as they make contact with the crystal (rime).

When a snowflake is heavily rimed, it can appear puffy and lose its defined shape, as you can see in the images below.

It is similar to a phenomenon known as rime ice, which occurs when supercooled water droplets make contact with a surface below freezing.

(MORE: The smell of snow isn't actually a smell. It's all in your mind)

Fun facts about flakes

• All snowflakes have six sides.
• Snow is not white; it's actually translucent.
• Some snowflakes are as small as the ball on a ball point pen.
• Freshly fallen snow can absorb sound waves, making for an eerie quiet.
• Identical snowflakes can be produced in a lab setting.

Caitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and a graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.