h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Snow is sometimes also translucent(1). Like eskimo's have may words for snow, so do scientists (2). Snowflakes are known as aggregates which means that many ice crystals have stuck together. There are many different types of ice crystal.

The ones you will see on all the christmas cards are known as Planar Dendritic (also known as Stellar)(3). Each of these will almost certainly be different from one another(4). Planar crystals are usually hexagonal (but I have seen triangular) and tend to grow only width ways; for active site and water molecule mobility reasons(5). The interesting thing about the dendritic (tree like) type is that they form branches on the corners of the central hexagon, and on each of these branches another branch forms.... (6). This in itself is probably sufficient to prevent the growth of two identical Stellars due to the range of different conditions under which it will grow; but just to be on the safe side, each of the main branches is likely to be slightly different to each of the other branches (7); to be even safer it is likely that the structure of the crystal will contain holes in random places but you could only see these with an electron microscope.

Another intersting crystal type with a whole range of subtypes is the column which is a prismatic and grows mainly lengthways by the use of "skrew dislocations"(8). You can also get a mixture of crystal types within the same crystal.

These crystals are all grown from the available water vapour in the air, how they actually come into existance is the subject of many a PhD Thesis. Growth can also can occur by the capture of water droplets which is known as riming, which will produce heavily rimed(9)crystals, graupel or hail.

Of course the best thing about snow is you can ski on it.

(1) Which is a hip word meaning you can see through it.
(2) Good for party conversations we are never invited to.
(3) I found this photo on the web, it is also a very good site http://www.its.caltech.edu/~atomic/snowcrystals/photos/pp04.htm
(4) Even if you went to sub atomic scales you wouldn't get a scientist to say definately.
(5) In other words we don't really know, but we want to make you think we do.
(6) This is caused by temperature anomalies and vapour competition
(7) They are not symmetrical, but crystalographic.
(8) Last time I heard.
(9) Was going to put a pun here but I doubt the moderator would like it.