Sculpt­ing a snowflake

How do tem­per­a­ture and hu­mid­ity af­fect flake for­ma­tion?

How It Works - - SCIENCE -

Nee­dles

Nee­dle-like struc­tures form when the tem­per­a­ture is in the col­umn for­ma­tion range, but the water sat­u­ra­tion in­creases to pro­duce these slen­der ici­cles.

Den­drites

As the sat­u­ra­tion of water in­creases, branches pro­trude from plates to create fern-like den­drite snowflakes, with sub branches form­ing from the larger ones.

Plates

At the early stages of de­vel­op­ment, these sim­ple hexag­o­nal crys­tals form at around -2°C.

Hol­low col­umns Stel­lar plates Com­pos­ite shapes

If the con­di­tions change while the crys­tal is still form­ing, hy­brid shapes can de­velop. For ex­am­ple, capped col­umns be­gin in the same way as hol­low col­umns, but if the tem­per­a­ture drops be­low -15°C, their hol­low ends fill and grow to form flat cov­ers, a bit like a bobbin. Plates will branch out to form elon­gated, star-like snowflakes at be­tween -10°C and around -20°C. At a sim­i­lar at­mo­spheric water level as plate crys­tals but with a de­crease in tem­per­a­ture (be­tween -5°C and -10°C), plates ex­tend to form hol­low col­umns.

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