THE ice flowersscientifically called frost flowersare ice crystals that typically form on top of a thin layer of recently formed sea ice, especially when the air is very cold (below –15°C) and dry. They are mainly observed in the Arctic areas on thin ice on seas or more rarely lakes, and have a high concentration of salt because they derive from the condensation of aerosols.
What are ice flowers and how are they formed?
Ice crystals are crystalline formations of ice with a pointed shape and dimensions of a few centimeters, whose name recalls the fact that they definitely resemble flowers that “bloom” on frozen waters, especially Arctic ones, during the winter. They contain large quantities of microorganisms, so much so that they constitute a sort of temporary ecosystem.
In summary, frost flowers can form and grow in a dry atmosphere as long as the temperature difference between the ice surface and the atmosphere is large enough. In general we can say that the favorable conditions for the formation of ice flowers are:
- presence of dry and very cold air with an air temperature of at least –15 °C;
- almost total calm wind;
- ice that is not too thick and/or has recently formed;
- absence of snow on the frozen surface.
The main factors involved in the training process are the quantity of water vapor available, the temperature of the air and the surface involved and the wind. To these factors we must also add the vapor pressure or saturated vapor pressurei.e. the pressure exerted by the water vapor in conditions of saturation of the air mass, in this case while it passes into the solid state, when the two phases (gas and solid) are in contact with each other thermodynamic equilibrium.
From a physical point of view, vapor pressure can be interpreted as the result of the tendency of a particular substance – in this case water – to pass from the condensed (or frozen) phase to the gaseous phase and vice versa (i.e. evaporate or sublimate): it increases as the temperature increasesas the molecules acquire a higher kinetic energy and tend to evaporate more easily.
Therefore, what matters in this specific case are the variations with height in temperature and vapor pressure above the ice surface. This surface must be in conditions such as to sublimate, i.e. in a phase during which the ice transforms directly into vapor without passing through the liquid phase.
This process occurs when the air above the ice is quite dry and, obviously, cold. At this point the conditions are created for an ice cream to develop above the same ice surface supersaturation zoneor with an excess of water vapor which can subsequently “frost” passing directly from the gaseous state to the solid state.
Ice flowers on freshwater lakes
Much less common, however, is their training on frozen surfaces of freshwater lakesdue to the fact that here the quantity of salts available is, obviously, much lower. Let us remember that salt, a bit like atmospheric dust for microscopic cloud droplets, accelerates the process of aggregation and growth of the microscopic ice crystals in formation.
Frost flowers, in fact, require a sort of “condensation nucleus” on which to start growing, exactly like the water vapor that forms the microscopic cloud droplets. In this case, however, we are talking about “freezing nuclei”. In most cases, irregularities in the frozen surface or small debris on top of the ice itself act as freezing nuclei.
More rarely, the freezing nucleus can be represented by a small ice crystal in the air, the so-called “diamond dust”. In this case, surface frost begins to form on a small grain of diamond dust, which increases its mass and causes it to settle on the frozen surface, and the process continues from there.
How big are ice flowers and what their size depends on
In the situation just described, frost flowers can therefore form precisely because, although the air is dry and, theoretically, the water vapor is not sufficient for the formation of frost, the humidity content in the form of water vapor sublimated by the frozen surface creates the optimal conditions and environment for the growth of very large frost crystals.
Once the growth of the frost flower begins, its size will depend on the thickness of the thin supersaturated layer described above. This thickness is determined both by environmental conditions of the air massas we have seen, both from air turbulence. The presence of wind, in fact, tends to create turbulence on the frozen surface which practically destroys the thin portion of supersaturated air necessary for the formation of crystals.
The dimensional development of crystals in the form of points or flat surfaces with fern-like appearance it occurs precisely thanks to the large quantity of water vapor available in the thin layer of supersaturated air in contact with the ice. This happens because there is no contact with the surface of the ice and the crystals are therefore not affected in the slightest by the “heating” by the ice itself, despite its high thermal conductivity.
It goes without saying that what we have just described is the most evanescent there can be in nature: in fact, all it takes is for your fingers or a ray of sunlight to just touch these wonderful efflorescences of frozen filaments to dissolve them into many, tiny droplets of water. And the magic dissolves instantly.
