Let’s not understate the complexity of a simple hexagonal plate snowflake. Roughly a millimeter in size, these nearly microscopic snowflakes wouldn’t get a passing glance to the naked eye – you wouldn’t be able to even see it. The connection of two features here reveal a hidden mystery! View large!
Let’s start with the outer edge of the snowflake and work our way in. You might notice, especially along the top edge of the snowflake, that the outer edge is thicker than the inner areas. This plays an important role in generating the circles that echo in towards the center. A thick outer edge can do two things: grow outward into traditional snowflake-like patterns (60-degree growth, branches, etc.) but it also has an inside edge that can collect water vapour as well. This growth starts to round out the corners, becoming more circular as it reaches closer to the center of the snowflake. Circles in the snow are always caused by inward growth from a thick edge! It took me a long time to wrap my head around this when I was first studying them.
The lighter areas of the snowflake are bubbles, or cavities, forming inside the ice. This happens when the center of the prism facet (the thin sides) grows slower than the outer edges of the facet. The patterns change by subtle fluctuations in humidity, and can close entirely when the humidity is higher. You can see how thick these bubbles are by looking at the thin edge of the snowflake. Notice the white line? It’s less than 3 microns (0.003mm) thick. It creates additional ice/air boundaries which allow for more reflection from the flash – which is why these areas appear brighter. They also function to create thinner “layers” of ice on the top and bottom… which is where the turquoise colour comes in.
The colour is caused by the phenomenon known as “thin film interference”, which is the same physics that puts rainbows in soap bubbles and oil spots. If Ice is just the right thickness, light bouncing off the surface of the snowflake will interfere with ice that entered into the snowflake for a brief period of time (slowing down as it passes through a denser material) and the combination of constructive (amplifying) and destructive (dampening) interference will cause white light to be revealed as a specific colour. Further reading on some of pages of my book Sky Crystals as a freebie: http://skycrystals.ca/pages/optical-interference-pages.jpg
This is where those rings come in. Thin film interference is relatively rare in snowflakes with only a handful of snowfalls a year creating this type of snowflake. You’ll notice that the turquoise colouring happens only in one of the “rings” created by inward crystal growth. At exactly this thickness, we get proper interference patterns. Adjust the thickness slightly on either side and we see no interference at all. The snowflake growing back in on itself allowed it to meet the very strict requires for colours to show up in a snowflake.
All of this is a tiny hexagon falling from the sky. If you like this sort of deep dive into the building blocks of nature, you’d love a copy of my book Sky Crystals:
2018 Ice Crystals coin, produced by the Royal Canadian Mint and designed by me: http://www.mint.ca/store/coins/coin-prod3040427
Bonus: How the heck to I measure a snowflakes features? Simple algebra and some practical observations. You’ll a few things:
- The magnification factor. This is easily obtained by photographing a ruler and counting the number of millimeters you cover. With a full-frame camera at 1:1 magnification you’ll see 36mm, since the sensor is 36mm wide.
- The resolution of your camera sensor (horizontal number of pixels)
- The image of the subject you wish to measure.
In this case, I see 3 millimeter markings, so 36 / 3 = 12:1 magnification. Actually, I don’t even really NEED this number, all I need is to know that the horizontal number of pixels of the sensor and divide that by 3. I get 1824px per mm.
Then you count the number of pixels across your subject. This snowflake measures 1880px or just over 1mm, and the thin white line measures 5px. 5 / 1824 = 0.00274mm, or 2.74 microns. Math!