There are many shapes of snow crystals. [Illustrations on the overhead or computer projector] Most likely you are familiar with these that have six branches and branches on the branches. [lower right of illustration] We call them dendrites. There are also some crystals that have only the main six branches and nothing on their sides. We call them stellars, which means star-shaped. Those branches can be broad. They can be stubby. Maybe there are no branches at all, and we have a simple hexagonal plate. [moving progressively to the lower left; then move to the upper right]
Those plates could be thick. If they are thicker still, we could turn them on their side and see a rectangular shape. We call them columns. They can be long. If they are very long and narrow we call them needles. [moving progressively to the upper left] If they have pointed ends we call them bullets. Those are the basic snow crystal shapes. There are many combinations of them.
There is another way in which snow crystals can grow. If they are big enough they may hit water droplets before those droplets have a chance to evaporate. [refer to the upper left of the illustration] If so, then the water droplets will freeze instantly to the surface of the ice crystal, building up a white deposit that we call "rime". That rime may nearly obscure the crystal inside. [refer to the lower center] If it does so, then we have a snow pellet composed almost entirely of frozen water droplets. We call it "graupel". [refer to the right] Most likely you have seen this type of snow in the Autumn and Spring. It tends to bounce on the sidewalk during certain snow showers. If the process of riming continues and some clear ice layers are added, then we get hail.
So how do we get the various shapes of snow crystals? This is a diagram that I memorized in high school. It was not taught there; I learned it at home. Along the bottom axis is a temperature scale ranging from -40 to melting. We scientists like to use the Celcius temperature scale, and in this case it helps when memorizing the diagram. The vertical axis indicates moisture, from dry cloud to wet cloud.
We see that the pretty, highly branched dendrites grow only at -15C, give or take a couple of degrees. Beside them are the broadbranched crystals, then the hexagonal plates. The columns are near -9 and -20C. Needles are only at -5C. Bullets are colder than -20C. So temperature controls most of the snow crystal shape.
With that diagram in mind we can figure out the combinations of shapes. For example, if a crystal starts growing at -18C, then it will have a hexagon in its center. [bottom] If it continues to grow at -15C, then on the tips of the hexagon there will be branches. If the crystal starts at -15C, it will be a dendrite. If that crystal continues to grow to -12, then on the tips of the branches there will be hexagons. There are many other combinations of temperatures and shapes.
So in the wintertime you could have fun by looking closely at the snow crystals falling on your sleeve. Look at the design changes from the center to the edge. Then you can tell the temperatures in the clouds where the crystals are growing. You don't even have to go up into the cloud and measure the temperature there; the crystal shapes tell you what the temperatures are. I had fun doing this in high school, and I still do it.
[Back to the freezer for a repeat of the instant snowstorm with another group of students.]
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