Growing Snow in Pasadena

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Growing Snow in Pasadena
Credit: Ken Libbrecht


All snowflakes are broadly categorized as either columns, which are long and thin, or plates, which are flat. Needles are considered a type of column and form only when the temperature is near 23 degrees Fahrenheit and the humidity level is high.

Credit: Ken Libbrecht


Columns like these—whether hollow or solid—typically appear at around 23 degrees Fahrenheit. Unlike needles, they can appear at varying levels of humidity and also at temperatures below 8 degrees Fahrenheit.

Credit: Ken Libbrecht

Stellar Dendrites

Plate-like snowflakes, such as this stellar dendrite, typically appear under conditions of very high humidity—but only when the temperature is near 5 degrees Fahrenheit. For the record, Libbrecht employs no Photoshop trickery to create his colorful images. Instead, he shines colored lights in from different angles behind the crystal. The ice acts like a complex lens and bends the light to accentuate structural details.


Credit: Ken Libbrecht

Stellar Plates

Snowflakes such as this stellar plate are common at lower levels of humidity, usually at temperatures between 26 degrees Fahrenheit and freezing, or below −14 degrees Fahrenheit. Why plates fail to form between these temperature ranges—or, more generally, why snow crystals grow into such different shapes at different temperatures—remains a scientific mystery.


Credit: Ken Libbrecht

Sectored Plates

Libbrecht created this sectored plate snowflake in his lab when the weather outside was frightful—for snowflakes, anyway. "It was summer and about 95 degrees outside when this crystal was growing," he said. Inside the lab, this flake grew at temperatures between 5 and 8 degrees Fahrenheit, and varying levels of humidity.

Credit: Ken Libbrecht

Capped Columns

The capped column starts out as a columnar crystal growing near 21 degrees Fahrenheit. If the crystal then falls into colder air, plates may grow from both ends of the column. The final crystal looks like a spool of thread with a hexagonal center column.


Credit: Ken Libbrecht

Bullet Rosettes

The capped bullet rosette forms in much the same way as a capped column but has at least three sections sprouting from a common center. This one grew columns at 21 degrees Fahrenheit, then Libbrecht lowered the temperature to 8 degrees Fahrenheit to create conditions favorable to the formation of plates on the ends.


For a city so proud of its mild, sunny winters that it created the Rose Parade to tout its climate, Pasadena—or, at least, Caltech—has become, somewhat paradoxically, renowned for its snowflakes.

Every year, when flurries begin falling across the country, members of the media begin flocking to speak with Caltech physicist and snowflake guru Ken Libbrecht, who literally wrote the book—or, rather, books—on the science and beauty of the frozen crystals. In his lab, he creates countless snowflakes to investigate and explain how subtle changes in temperature, pressure, and humidity give rise to an impressively varied menagerie of intricate snowflake forms.

In recent years, we have showcased his research in articles that explain the physics of snowflake formation in clouds and even how to grow your own snowflakes at home. This year, we are pleased to share some of his most recent micrographs documenting the fascinating and beautiful results of his investigations.

Displayed below are some of Libbrecht's images that show representative samples of some common types of snowflakes: needles, stellar dendrites, stellar plates, columns, sectored plates, capped columns, and bullet rosettes.

To learn more, visit Libbrecht's comprehensive site. It features sections on the science, aesthetics, and history of snowflakes—and it also answers the age-old question of whether any two snowflakes are alike. (Spoiler alert: they aren't.)

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