|Thin and Flat|
often-overlooked quality of stellar snow crystals is that they are
remarkably thin and flat. This photo shows one seen from the side. This
is why we call them snow flakes.
again, snow crystals are not always thin, flat, plates. Some look
like slender needle crystals. The different forms appear at
different temperatures, as shown in the snow crystal morphology diagram.
all begs the question: How is it that snow crystals can grow into such
extreme shapes -- extremely thin, flat plates and extremely long,
|The Sharpening Instability|
Part of the answer to this question lies in an edge-sharpening growth instability.
It sounds complicated, and it is a bit. Turns out the growth of a
faceted surface depends on the width of the facet. When last molecular
terrace is less than 100 molecules wide (roughly), it becomes
especially easy for molecules to attach to the top of the terrace.
Strange, but that seems to be part of how ice works.
quality tends to sharpen the crystal edges. As shown in this diagram,
when a corner grows, it produces narrow faceted terraces. A narrow
terrace grows faster than a wide terrace, and the growth then adds more
terraces that are even narrower than before. The result is an
edge-sharpening growth instability.
|Plates on Needles|
study how all this works by growing ice crystals in the snowflake lab.
The images on the left shows a thin plate growing on the tip of an
electric ice needle.
The composite image shows a side view at different times. The other
photo shows the plate face on. The edges of the plate grow out rapidly
once the edges become thin and sharp.
From the imaging data we
measure the grow velocities, and then try to model everything on the
computer. The sharpening instability is necessary to make sense of all
these measurements, so we are fairly confident this is a real thing.
Still, we are collecting more data, under all different conditions,
just to be sure.
|Cups on Needles|
This shows a photo of a cup-shaped crystal growing on the end on an electric ice needle. This time it is the edges of the column that become thin and sharp from the edge sharpening instability.
It's unfortunate that ice crystal growth is so complicated, with faceting, branching,
and edge sharpening all going on simultaneously. But, all this
complicated physics is why snow crystals look so intriguing. Next
time you see it snow, try to imagine all the things going on in the