No two Christmas cards are alike

After a very unpolite break, I am happy to announce that “Melting the ice” is back. And I’m not goinna start posting without wishing you all a Merry Christmas and a Happy New Year!

I chose this image as a Christmas card because, despite is not stylish, since I was a child I found it quite amazing how snowflakes can really look like that. I mean, this is the coolest design ever, isn’t it? And a but of scientific chatting before going to complete the daily knowledgement dose:

Ice crystals formed in the appropriate conditions can often be thin and flat. These planar crystals may be simple hexagons, or if the supersaturation is high enough, develop branches and dendritic (fern-like) features and have six approximately identical arms, as per the iconic ‘snowflake’ popularised by Wilson Bentley. The 6-fold symmetry arises from the hexagonal crystal structure of ordinary ice, the branch formation is produced by unstable growth, with deposition occurring preferentially near the tips of branches.

The shape of the snowflake is determined broadly by the temperature, and humidity at which it forms. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.

Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.

Interestingly, if a crystal has started forming in a column growth regime, say at around −5 °C (23 °F), and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called ‘capped columns’.

There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it’s more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance.

See you soon, and thanks for waiting.