Saturday, November 7 · Tromsø
Every day, the sky grows one snowflake. This is today's.
Snow crystals are messengers from the sky, as Ukichiro Nakaya put it. Their shape records the temperature and the humidity of the cloud where they grew, a kilometre or more above your head.
Stellar dendrite
Near -15 °C and with plenty of spare vapour, the six corners of a tiny hexagonal plate grow faster than its faces and sprout branches, which sprout branches of their own. These are the fern-like stars that make the classic snowflake.
Snow is falling in Tromsø right now: flakes like this one are landing.
Where it grew
| Temperature at 850 hPa (about 1.5 km) | -14.0 °C |
|---|---|
| Relative humidity at 850 hPa | 98 % |
| Supersaturation over ice (estimate) | 12.7 % |
| Ground temperature | -4.4 °C |
| Snow in the next 24 hours | 5.9 cm |
| Model parameters (β, γ) | 0.42, 0.0011 |
Supersaturation is estimated from the forecast humidity aloft with the Magnus formulas for water and ice. It is a honest estimate, not a measurement: real clouds vary from one updraft to the next.
The solid line is the air you walk in, the dotted line the air where flakes are born. Bars show the chance of precipitation and the snow that falls each hour.
Each crystal is grown from that day's forecast at cloud level, at the snowiest hour or at noon. Snow totals are the model's daily sum.
Sunrise, sunset and the minutes the day gives up as winter closes in. Local time in the selected city.
Plates near 0 °C, columns and needles around -5 °C, plates and stars again from -10 to -22 °C, thick plates and columns below. The more spare vapour, the more branches. Today's crystal and this week's are marked.
A farmer with a bellows camera made the first photomicrograph of a single snowflake and went on to record more than 5,000 of them on glass plates.
Grew the first artificial snow crystal on a rabbit hair in a cold chamber and mapped crystal shape against temperature and humidity. Kenneth Libbrecht has since refined the map.
Saturation vapour pressure over water and over ice (Magnus):
ew(T) = 6.112 · exp(17.62 T / (243.12 + T)) hPa ei(T) = 6.112 · exp(22.46 T / (272.62 + T)) hPa σice = RH · ew(T) / ei(T) - 1A cloud saturated with liquid water at -15 °C holds about 16 % more vapour than ice can tolerate: that surplus is what builds branches.
Every crystal on this site is computed in your browser, cell by cell, by a published model of snow crystal growth. Nothing is pre-drawn.
Water vapour sits on a honeycomb of cells. A seed of seven frozen cells sits in the middle. Ice molecules stack in hexagons, so the grid does too.
Following Clifford Reiter's 2005 model, vapour diffuses between cells (α), the far field is refilled at level β, and cells touching ice gain γ each step. A cell freezes when it reaches 1.
Cloud temperature and supersaturation choose β and γ along the Nakaya diagram. A seed made from the date and the city makes each day's flake unique, and the same for everyone.
Each day's flake is added here when it finishes growing. Like Bentley's plates, no two are alike. Your collection stays in this browser only.
Your first flake is growing on the stage above. It will appear here once it has finished.
Perfect stellar dendrites fall only a few hours a winter. We watch the cloud-level forecast for your city and tell you when to step out with a dark sleeve and a hand lens.
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Today's snowflake, the 7-day forecast and your personal collection.
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A notification when a fine dendrite, or a freezing rain risk, is forecast over your city.
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For science teachers: the live model with every dial unlocked, and lesson sheets.