Frost Flowers on Lake Ice: How the Crystals Form

On a cold, still morning, new ice on a lake can sprout a whole meadow of white "flowers". These frost flowers are clusters of ice crystals only a few centimetres across, and a Reddit user's photos of a frozen lake in Finland, covered in them at sunset, showed just how spectacular they can be.

What are frost flowers on ice?

Frost flowers are ice crystals that grow on young sea ice and thin lake ice in cold, calm weather. They form in patches about 3 to 4 cm across and look much like hoar frost. Spread across a whole lake, thousands of them create the pale, field-like pattern in these photographs.

How frost flowers form

The recipe is a big temperature gap between the ice and the air. Brand-new ice stays close to 0 °C because liquid water lies just beneath it. When still air at least 15 °C colder sits on top, for example −30 °C over 0 °C ice, water vapour rises from the surface and the thin layer of air just above it becomes supersaturated. The vapour then condenses as ice on tiny bumps and ridges, and crystals start to grow.

• Calm air is essential. Wind strips away the supersaturated layer, and blowing snow buries the ice.
• Only thin, new ice works. As ice thickens, its surface cools and the temperature gap shrinks.
• Humidity shapes the crystals. Lower supersaturation produces branched, tree-like crystals; higher supersaturation makes star-like crystals without side branches.

On freshwater lakes they are relatively rare, because they need a sudden, sharp drop in air temperature that freezes the lake fast and leaves fresh ice exposed to very cold air.

Frost flowers on sea ice and the atmosphere

On the Arctic and Antarctic oceans, frost flowers are common on newly formed sea ice, and they are remarkably salty. Brine wicks up into the crystals from the ice surface: near Barrow, Alaska, researchers measured salinities of 16 to 105 parts per thousand, averaging about 62, roughly three times saltier than seawater. Their huge surface area and concentrated chemistry have made them a subject of polar research, with studies linking them to sea-salt particles in the air and to springtime ozone depletion events.

The other frost flowers: ice ribbons on plants

The same name is also used for a completely different winter wonder. In autumn and early winter, thin, curling ribbons of ice can grow out of the stems of certain plants, such as white crownbeard (also called frostweed) and dittany. Sap in the stem expands as it freezes and splits it; water drawn up through the cracks freezes on contact with the air and is pushed outward as a petal. These ribbons need freezing air while the ground is still unfrozen, and they are best seen early in the morning or in shade before the sun melts them.

Enjoy lake frost flowers from the shore: they grow on new, thin ice, so wait for local confirmation that the ice is thick enough before stepping out. For more ice crystals, see snow-crystal patterns formed in clouds and crystals growing across a soap bubble.

Related reading

• Snow Roller: A Strange Meteorological Phenomenon
• Ten Snow, Ice and Water Landscapes: Glaciers, Mountains and Frozen Shores
• Roof Ice Falling at a Lodge in Grapevine, Texas
• Ice Driving in Western Norway: Tracks Across a Frozen Surface
• An Ontario Ice Fort Built One Batch at a Time

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