Snow and Ice Crystals

In winter, when it snows, you should take some time to study the white splendour more closely at some point. Once you have looked at several snowflakes, you will notice that the flakes consist of smaller building blocks: the ice crystals. The diversity and beauty of these crystals is really astonishing.

What is snow actually, and why are there so many different forms?

As we all know, snow is nothing but frozen water. However, to explain the diversity of the crystals, you have to take a closer look at the history of their origin. Ice crystals are formed when small water droplets in the clouds crystallise on dust particles or similar things - such contaminations are called crystallisation nuclei in this context - at temperatures down to -40°C. Then the crystals start to sink slowly towards the ground, growing more and more, due to the humidity of the air during the process. Then the well-known hexagonal shapes form. Above all, it is the temperature of the cloud that is responsible for these special coarse structures.

At lower temperatures, small plates are formed (left picture and second picture above), whereas at higher temperatures the typical stars, also called dendrites, are formed (left picture and first picture above).

It is remarkable that every snowflake is indeed unique. There are not two absolutely identical flakes. The reason is that the conditions of the origin vary constantly, and thus the fine structure of the crystals. There is only one thing that they all have in common, the already mentioned hexagonal shape.

But what is the reason for this?

The reason for this is that water molecules always assemble at a certain angle, namely 120°, when they crystallise out. And that is exactly the interior angle between two sides of a regular hexagon.

And how do these small crystals become a snowflake?

That is simple. If the air temperature is close to the freezing point, the single ice crystals are glued together by small water droplets and snowflakes form. At low temperatures, only very small flakes are formed, at higher temperatures really big snowflakes occur. So the humidity no only influences the growth of the crystals, but also works like glue that glues together the single crystals.

With this knowledge, there is nothing that stands in the way of a snowball fight!