Intricate fern-like “frost flowers”, said to be painted on windows and windscreens by Jack Frost, are a familiar feature of British winter. In Arctic regions there is an even prettier three-dimensional version.
These frost flowers are typically 3-4cm across and whole gardens of them grow on frozen lakes and seas. Like the window version, they are the result of ice crystals growing in a slow, orderly fashion.
Arctic frost flowers form when relatively warm ice is exposed to still air that is at least 15C colder. The ice sublimes into water vapour, producing a low-lying cloud of humid air, like steam from a coffee mug. As the temperature drops, this moisture crystallises into hoar frost, sprouting from protrusions on the ice and growing into complex shapes.
At higher humidity frost flowers form side branches and may look like miniature trees. If there is less moisture they become star-like crystals with no branches. Frost flowers start as pure water but quickly draw up salt from the ice below until they are saltier than seawater.
Frost flowers can only form in still air, and they are fragile. A strong wind will destroy them, which is why they typically only survive for a few days at most, and are rarely seen or photographed.
