Thursday, December 27, 2012

Weather Whys: Snowflakes

Keeping with a wintry theme, let's discuss snowflakes.

What is the first thing you may know about snowflakes? The old adage is that "no two snowflakes are exactly alike," and that is true. Snow is pretty awesome in that every single flake is unique. Can we describe the different types of flakes in one blog post? Considering an entire field guide has been written on the topic: Not a chance. But let's at least give you an overview. Sidebar: The book linked to above is highly recommended as a must have on a winter weather enthusiast's bookshelf.

Microscopic View of Snowflake. Credit: Wikipedia
So a snowflake is simply just a congolmerate of ice crystals. Because they form so high up, they will drift down through different humidity and temperatures, which will help form them into the unique shapes they take on. Water that's supercooled will freeze onto some sort of particle in the atmosphere (be it dust, sand, pollen, etc.). This will form an ice crystal. On its way down, additional water vapor freezes onto that ice crystal, which builds more crystals. The structure of an ice crystal is six-fold. Each of the six arms will grow independently. And more often than not, you'll see some semblence of symmetry in a snowflake. Each arm will form its own shape, but since the entire crystal is exposed to the same weather, that shape is usually replicated on all six arms.

For an in depth discussion and some references on how the snowflake process unfolds (the Bergeron Process), check out the Wiki article here:

The various shapes of the snowflakes depend on multitudes of factors, but most importantly the temperatures. The author of the book I linked to above has a website devoted to sdifferent snowflake types, and does an excellent job explaining the right conditions for them to form, as well as how they do form. Libbrecht has broken them into 35 different classifications. There are other ways of doing it. The International Commission on Snow and Ice (how does one get on that commission?) broke things into seven basic classifications: Plates, stellar crystals, columns, needles, spatial dendrites (from the word for tree), capped columns, and irregular forms (graupel, ice pellets, hail). Simplistic, but only scratches the surface. Other meteorologists have come up with other classification schemes. But in general, Libbrecht's is, to me, the most thoughtful and useful. Check it out.