Illustration: Gee whiz science
By contributor

It’s widely believed that no two snowflakes are alike — but it turns out that there are various forms of snowflakes, making the tale a bit more complicated. For better or verse, here’s the whole story, including the lowdown on rain, hail and sleet.

Because of the way that they grow,
Each frozen white crystal of snow
Is thought in our mind
To be one of a kind.
But guess what: It just isn’t so.

When H2O molecules freeze,
Hexagons form with great ease.
Molecular bonds
Like magical wands
Form the six-sided shapes snowfall sees.

Image: Plates and dendrites
The dendrites at right can cause a sensation. The plates at left exhibit a less unique relation. Photos are courtesy of the U.S. Bureau of Reclamation.

Dendrite flakes, with their six crystal wings,
Are among nature’s loveliest things.
These flakes are unique,
No two can you seek
Just the same in the white winter brings.

But at temperatures lower or higher
In air that’s more humid, or drier,
Different shapes grow:
Less unique flakes of snow
That develop much simpler attire.

Needles, and Columns, and Plates
Are flakes with identical mates.
Like wheels on a bike
These flakes look alike.
Each type has quite similar traits.

Image: Needles and columns
These closeups of needles and columns, of course, are courtesy of the same federal source.

But the turbulent winds of a storm
With temperatures both cold and warm
And mixed moisture sources
Create varying forces
That cause complex mixed crystals to form.

So while some flakes can all look the same,
Most are not nearly so tame.
They’re all based on a “hex,”
But they’re just as complex
As the Nature from whence each one came.

Now raindrops, and sleet, even hail
Are part of a simpler tale:
Though they vary in size
Like a bunch of french fries
Each forms the same way without fail.

Big drops fall from moist warmer air,
Or when smaller drops join, way up there.
Hailstones will grow
When strong updrafts blow.
A thunderstorm has those to spare.

Sleet’s just a raindrop that froze
Before it crashed into your nose
Wet rain that falls
And then freezes on walls
Is freezing rain. Traffic it slows.

Each type of cold precipitation
Has its own origination.
We suffer together
The wet winter weather
No matter our town, state, or nation.

David Ropeik is a longtime science journalist and currently serves at Director of Risk Communication at the Harvard Center for Risk Analysis. This article is drawn from the archives of “How and Why,” Ropeik’s column about scientific puzzlers. Send David Ropeik your suggestions for future columns via

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