Crystals have been on my mind a lot the last week or two.
I was asked a fun question this week about the formation of snowflakes. This morning I see that the Cascades are socked in, hidden behind heavy clouds, and I'm beginning to think about snow again, as I look forward to ski season. Last Friday and the week before the Organic class was doing recrystallization in lab. I've got their reports on my desk, ready to grade. My introductory class has been learning about ions and ionic compounds, and the resulting crystals. Crystals are everywhere!
The question I was asked is about snowflake types, and whether there is a connection between snowflake type and the temperature where the snowflake forms.
I'm no expert on this stuff, and I doubt the situation is simple. I have browsed Barnes and Noble enough to know that there are field guides to weather, and that they include some neat stuff about snow, but I don't own one of these books and I haven't ever taken the time to dig very far into it. Experience tells me that cold and dry air leads to tiny and sparkly-perfect but tiny crystals (champagne powder), while wetter and warmer conditions can lead to big flakes.
I would guess that both temperature and humidity would have a dramatic effect on snow type. Other things like shearing from winds might also matter.
As I think about this now I'm trying to link the formation of snow crystals to what we see in the lab, in solutions --which may be importantly different!-- but to make big crystals in the lab we cool the solution slowly and don't disturb the flask while they form. This results in a situation where there is more opportunity for crystal growth and the nucleation events (which is when the crystals start forming) are more rare, so there are fewer, bigger crystals. If the solution is cooled rapidly, you get lots of nucleation and little chance for crystals to grow.
Wikipedia has a good and pretty technical page on snow, here.