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This example shows one of my first successful attempts to grow a pair of identical-twin snowflakes. 

The first image on the right shows a small number of seed crystals that fell onto the substrate at the start of this growth run. These tiny crystals floated through the air and fell randomly, so I had no control over where they landed. Instead I moved the substrate around while I looked for two nicely formed hexagons sitting next to one another. When I found this pair of seed crystals, I started the growth process. The second image on the right shows a picture taken at the end of the run. Click on either image to see a larger view.

Note that both these images are completely unretouched, except for one thing: I resized the images by a factor of four in both dimensions, just because the originals were quite large for downloading on the web (5760x3840 pixels, taken using a Canon EOS 5D Mark III). In other words, this is pretty much what came directly out of my camera.

The initial seed crystals in the first image are about 70 microns in diameter -- about the same size as the cross-section of a human hair. The final crystals are about 2.0 millimeters from tip to tip. The angular placement of the crystals is random, and these happened to land with one crystal rotated about four degrees relative to the other. This small angle may seem improbable, but remember that 30 degrees is the maximum possible misalignment. Thus landing with an alignment angle of less than four degrees will happen about 13 percent of the time.

Next I rotated, cropped, and adjusted the brightness of each image in the series using the batch processing feature in PaintShopPro. At this stage I had 94 still images, covering a time span of 14 minutes. I applied the same rotation, crop, and brightness adjustment to each image. Since this was an early attempt at identical-twin movie-making, I focused on growing dendritic branches, since these grow quickly. Plates are pretty, but they take longer to grow. I also tripped the shutter by hand as the crystals grew, with about 10 seconds between shots. In later movies I used an automatic time-lapse shutter release, usually with a faster cadence. 

Next I used some custom software I wrote in Matlab to scale the brightness of the images, so the movie would not noticeably flicker. This is a fairly small adjustment, but it makes for a nicer movie. After that I used more custom software to shift each image slightly, so the crystal centers were fixed in the frame. My camera is bolted onto a heavy aluminum frame, but still it moves slightly during the shoot, and the motion of the crystals is distracting. I could skip both these image adjustments, but they are both fairly easy to do, and the final movie looks nicer with them.

Note, however, that none of these adjustments altered the shapes of the crystals; the crystal shapes remained unretouched throughout.

Finally, I did a tighter crop, rescaled the images to 1920x1080 pixels (standard TV size), tweaked the brightness a bit more, and added a bit of image sharpening. This image shows the final image after these adjustments, this time at full size. 

Finally, I converted the still images into a movie using Windows Movie Maker. Press the play button on the left to download and play the video (about 10 MByte). The movie jerks a bit because I started with only 94 stills. Here they are displayed at 0.2 seconds each, so about 50 times faster than real life.

I like to call these pairs "identical-twin" snowflakes. Like identical-twin people, the two crystals are clearly related, but they are not absolutely identical. If you pause the video at various times, you will see many small differences between these crystals, and you can easily see that they are not perfectly identical.

What you cannot see in this video is the temperature and humidity, which I changed as the crystals grew. However you can see the humidity changes indirectly -- when the humidity was high, water droplets condensed on the substrate around the crystal (like fog on a mirror). When the humidity was low, the droplets evaporated.

Each time I changed the temperature and/or humidity, the crystal growth changed. Since the same changes were experienced by all twelve arms of the two snow crystals at the same time, all twelve grew in synchrony. This is essentially what happens for the growth of natural snowflakes. However, natural snowflakes are carried by the turbulent wind as they grow, so each crystal takes a slightly different path through the clouds. My identical twins experience nearly identical conditions over time, because they are stationary on the substrate.