This strange, knobbly object is a single snow crystal photographed with a low-temperature scanning electron microscope at the US Department of Agriculture’s Agricultural Research Service. It went viral as a “snowflake zoomed in 50,000 times” that looks like an alien virus. In fact it is a capped column covered in rime, and its rough coating records the cloud it fell through.
What a capped-column snow crystal is
Snow crystals take different shapes depending on the temperature and humidity in which they grow. Japanese physicist Ukichiro Nakaya mapped these patterns, and his diagram is still the basic guide:
- Around −3 °C and warmer, crystals tend to grow as flat plates.
- Between roughly −3.5 °C and −10 °C, they grow as columns, prisms and needles.
- From about −10 °C to −22 °C, plates and branching dendrites form again, the classic “snowflake” shapes.
A capped column records a journey through more than one of these zones. The crystal starts as a hexagonal column in air around −5 °C, then falls or drifts into conditions that favour plates. Flat plates then sprout from each end, so the finished crystal looks like a tiny spool or cotton reel. The six-sided outline at every stage comes from the hexagonal structure of ice itself.
How rime forms on a snowflake
Clouds often contain supercooled water: droplets about 10 micrometres across that stay liquid well below 0 °C, in some cases down to −40 °C. When a falling snow crystal collides with them, they freeze instantly on contact. That process is called riming, and the frozen droplets are the bumpy beads covering the crystal in the microscope image.
Light riming leaves the crystal’s shape easy to recognise. Heavy riming buries it completely. The result is graupel, soft white pellets 2–5 mm across that were once called “soft hail”. Rime has been observed on every main crystal type: plates, dendrites, columns and needles.
The same process builds the feathery ice on mountain trees and summit weather instruments. On a hill, rime ice grows on the windward side of branches when freezing fog blows through, usually between about −2 and −10 °C. Compare it with a rime-coated forest, or see supercooled water turn to ice in the instant-ice demonstration.
How the electron-microscope image was made
The USDA’s Electron Microscopy Unit at Beltsville, Maryland, photographs snow with a low-temperature scanning electron microscope (LT-SEM). The crystals are kept frozen throughout, so their fine structure survives, and the electron beam shows surface detail far beyond what a light microscope can reach. LT-SEM work has shown cloud droplets up to 50 micrometres sitting on snow-crystal surfaces.
The file name of this image gives its scale as 300 micrometres, about a third of a millimetre. A magnification number on a viral post changes with every screen the picture is shown on; the scale bar is the reliable measure.
Why rimed snow matters on the mountain
Riming is more than a curiosity for skiers. Graupel is dense and granular. A fresh layer 20–30 cm or deeper on a slope can release as a slab avalanche, and thinner layers can act like ball bearings under later snowfall until they bond, usually within one or two days. When the forecast mentions graupel, take it seriously.
For photographers, rimed crystals are harder to capture than clean dendrites but tell a richer story. Wilson Bentley began photographing snowflakes through a microscope in 1885, and today’s hobbyists still look for crystals with crisp edges and, now and then, a dusting of frozen droplets.
Related reading
- A Guide to Snowflakes: Plates, Dendrites, Columns and Rime
- Alexey Kljatov’s Snowflake Photography: Tiny Crystals, Resourceful Equipment
- 18 Snowflake Photographs: Patterns Worth a Closer Look
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