How do snowflakes grow symmetrically? What causes patterns in frost?

Illustration by Slug Signorino

Cecil replies:

Amanda, meet Pete. My guess is, soul mates you’re not.

Scientists have been studying snowflakes since the early 1600s, and while we don’t know everything about how they form, we know a lot, a fair amount of which I’ve conveyed in this column over the years. However, since many of the Teeming Millions weren’t yet alive in 1973, you’ll excuse a brief review.

As water vapor rises into the colder upper atmosphere it cools, and when chilled enough condenses into infinitesimal water droplets, which commonly form around particles of dust, sea salt, etc, known as nucleation sites. If the temperature keeps dropping, eventually the droplets freeze into ice crystals. These crystals start out as tiny hexagonal prisms — the hex shape resulting from the angle formed between oxygen atoms as the frozen water molecules line up. Under the right conditions, additional moisture condenses directly onto the crystal and freezes too. Since the corners where the crystal faces meet project further into the surrounding vapor than the flat sides, they accumulate ice faster, typically producing the familiar six-branched snowflake.

Why are the branches identical? They’re not always, or even usually, as you’ll discover if you examine a few actual flakes. The impressive symmetrical specimens you see in books were selected for their photogenic qualities, possibly by Vermont farmer Wilson Bentley, who published a volume of more than 2,400 snowflake images in 1931. Symmetry admittedly is pretty common, but there’s nothing magical about it. Crystal formation isn’t random but rather is rigidly dependent on temperature, density of water vapor, and other local conditions, all of which are likely to be uniform in the immediate neighborhood of the flake — what happens to one branch happens to the rest. I’m tempted to say spinning of the budding crystal as it falls contributes to symmetry, just as wet clay spinning beneath the potter’s hand on a wheel tends to assume a uniform shape, but absent experimental corroboration we’ll consign this conjecture to the drawer.

Frost on windows forms under different conditions than in a cloud — typically temperature and pressure are higher and, more important, the ice collects on a surface that may be dirty or rough instead of what for practical purposes is a dimensionless point in the air. As a consequence symmetry is rare. Still, the basic angle inherent in ice molecules imposes some structure on frost formation, which manifests itself in those feathery patterns you see on winter windowpanes or (I bet Pete knows this) chilled beer glasses, which is a lucky thing from the standpoint of scientific study, since they’re much more interesting to look at when catalyzed with C2H5OH.

Progress Report

Readers will recall my discussion a couple years ago of the fable, repeatedly reported as fact by parties who should know better, that 90,000 Chicagoans died of cholera and typhoid in 1885 after sanitary waste got washed into Lake Michigan by heavy rain and fouled the city’s offshore water intake. Supposedly the epidemic, which would be one of the worst public-health disasters of modern times if the story were true, was the impetus behind the massive canal-building project that reversed the Chicago River and sent the city’s offal to Saint Louis, which was used to taking crap from Chicago. Historian Libby Hill established in her 2000 book The Chicago River: A Natural and Unnatural History that the story was fiction — while there was a big rainstorm in 1885, favorable winds kept sewage out of the tap water — but didn’t know where it originated. Now she does. In the autumn 2006 Journal of Illinois History she reports that it was initially publicized in a 1957 pamphlet published by the canals’ proprietor, the Metropolitan Sanitary District of Greater Chicago, which was trying to cast its beginnings in a heroic light. Specious details were added later — in 1976 the district’s boss, arguing for an even more ambitious flood control project called Deep Tunnel, was the first to attribute 90,000 deaths to the mythical outbreak. (There’s no evidence the story was knowingly fabricated, although if not you have to wonder what these guys were on.) What put the thing over the top, Hill contends, was a 1977 front-page story entitled “Deep Tunnel: Monster or Miracle?” that appeared in a Chicago alternative weekly called … well, nobody’s interested in such trivia. But in case you’re wondering, the author wasn’t me.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.