You might call these “snowflakes”. But let's be specific here with the names.

Anyway, a close associate who has been sweating over his snow-crystal growth machine in Japan for many years has just published his latest results. His machine is a vertical supercooled-cloud wind tunnel, which is like a thin column of cloud in which one can observe a single floating crystal. The crystal is in a climate-controlled box containing a droplet-infused stream of uprising air.

For this study, he focused on the crystal habit (i.e., shape) in the temperature range that we tend to get the large six-pointed stars. In addition to varying the temperature, he varied the density of droplets in the air. Nobody had done this before, certainly not as carefully and complete as he. Though he found lots of interesting results, I'll focus here on the following diagram:


Given its similar layout to the well-known Nakaya habit diagram, let's call this the Takahashi habit diagram. Whereas Ukichiro Nakaya showed us the crystal shape as he varied the temperature and humidity, Prof. Tsuneya Takahashi varied the temperature and droplet density (“LWC” on the vertical axis stands for liquid water content).

The basic findings: As the temperature decreases from left to right in the diagram, the crystal form goes from the very wide-branched sector crystal (see the “sectors”?) to the longer branches of the broad-branch crystal, to the even longer and narrower branches of the stellar crystal, to the crystal with sidebranches on each branch, called a “dendrite”, and then the pattern reverses back to sectors.

In the middle of this temperature range, something else happens. If he increased the density of cloud droplets in the dendrite regime, he got the more finely sidebranched fern forms. Ferns differ from dendrites only in the number of sidebranches they have. Fern crystals look more like fern plants because they have more offshoots. But at other temperatures, adding more droplets did not change the crystal form. In fact, the crystal size did not increase with more droplets, although the mass did increase.

If you observe some crystal that looks similar to one on the Takahashi diagram, you may infer that it grew under the indicated conditions. But this is not yet the final story: for instance, the amount of sidebranching is also thought to depend upon the temperature and humidity fluctuations. More fun ahead!

- JN