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The Snow Crystal Morphology Diagram, shown on the right, indicates what kinds of snow crystals grow at specific temperatures and humidity levels. This is important information for understanding snow crystal formation, yet it remains something of a scientific mystery.

You can see two main trends in this diagram:

1) Snow crystal formation changes rather dramatically with temperature -- thin plate-like crystals appear at temperatures near -2 C (28 F), slender columns grow near -6 C (21 F), large thin plates grow near -15 C (5 F), and one can find either plates or columns at lower temperatures.

2) Snow crystals grow into simpler forms when the humidity is low and the crystals grow slowly. More complex, branched forms appear when the humidity is high and the crystals grow rapidly.

Click on the image for a larger version of the Morphology Diagram, or here for a still larger version.

The Snow Crystal Morphology Diagram was discovered in the 1930s by Japanese physicist Ukichiro Nakaya and his collaborators, and it is sometimes called the Nakaya Diagram for this reason. They grew snow crystals in their cold lab at different temperatures and humidity levels and observed the different forms. The photo on the right shows Nakaya peering through his microscope at some snow crystals growing in his refrigerated laboratory.

The diagram has been extended and refined over the years by other researchers, but its fundamental characteristics have remained unchanged.

Nakaya used to say that snowflakes are like hieroglyphs from the clouds, because you can infer the conditions in the clouds by examining the shapes of the falling snow crystals.

Snowflake Hunting
If you ever go looking for beautiful stellar snowflakes outside, remember the snow crystal morphology diagram. For example, you are most likely to find large, plate-like crystals when the temperature is *cold*, near -15 C (5 F). Plate-like crystals also appear near -2 C (28 F), but these are smaller and are prone to melting (because the temperature is so close to the melting point).

When I am photographing snowflakes, I am more likely to find spectacular specimens (what I like to call the Greatest Snow on Earth) in places where the average temperature is around -15 C (5 F) and it snows frequently.

Explaining Snowflakes
The morphology diagram provides a handy explanation for the formation of odd-looking snow crystal forms like capped columns. The crystal on the right, for example, must have started growing at temperatures near -6 C (21 F), forming a stout columnar crystal. Then the wind carried it to colder temperatures, so thin plates grew on the ends of the columns.

The Road to Understanding
Observing the growth of snow crystals in the lab is one thing, understanding it is something else entirely. The branching instability does a pretty good job of explaining the origin of complex structures in snow crystals, and it also explains how snow crystal morphologies change with humidity. Higher humidity means faster growth, and the branching instability is enhanced when growth is rapid. For the fastest growing crystals (fernlike stellar dendrites), the branching becomes essentially chaotic in nature, with sidebranches appearing somewhat randomly along the main branches.

What science still cannot explain is why snow crystals grow as thin plates or slender columns depending on temperature. The sharpening instability plays a role in this, but this instability is itself not well understood. In the final analysis, the growth morphologies depend on the detailed molecular structure and dynamics of the different ice surfaces, and surface physics is complicated. Someday it will all be understood, but for now there remains a bit of mystery in the formation of a snowflake.