Hexenkessel
Ice crystals on a window by softsimu on Flickr.Via Flickr:
Too bad couldn’t get a bit closer with my camera but nevertheless: the photo shows ice crystals on a window. The dendritic and needle-shaped patterns are very clearly present. 
How does this happen? Moisture condenses on the window and once the outside temperature drops to subzero (Celsius), ice crystals start to nucleate and grow. How fast they happen depends on, for example, how quickly and how low the temperature drops. Here, we went quickly from about +9C to -10C. The shape and growth patterns (dendritic, needles, hexagonal, etc, i.e., pattern selection) depends on temperature and moisture. 
Once crystals start to form, they grow when the moisture on the window’s surface diffuses to the already crystallized area. Why the branches, then? The diffusing moisture may have to diffuse from fairly far away and arrives at the crystal at random locations and random times. Once the diffusing microdroplets touch the crystal, that particular part of the crystal grows - it is then farther away from the initial seed than some of the neighboring parts. Since such areas may acquire extra moisture, and since they then prevent moisture from reaching other areas, we get branched patterns. This is called  branching instability. 
Scratches or impurities on the window: they help and direct crystallization.

Ice crystals on a window by softsimu on Flickr.

Via Flickr:
Too bad couldn’t get a bit closer with my camera but nevertheless: the photo shows ice crystals on a window. The dendritic and needle-shaped patterns are very clearly present.

How does this happen? Moisture condenses on the window and once the outside temperature drops to subzero (Celsius), ice crystals start to nucleate and grow. How fast they happen depends on, for example, how quickly and how low the temperature drops. Here, we went quickly from about +9C to -10C. The shape and growth patterns (dendritic, needles, hexagonal, etc, i.e., pattern selection) depends on temperature and moisture.

Once crystals start to form, they grow when the moisture on the window’s surface diffuses to the already crystallized area. Why the branches, then? The diffusing moisture may have to diffuse from fairly far away and arrives at the crystal at random locations and random times. Once the diffusing microdroplets touch the crystal, that particular part of the crystal grows - it is then farther away from the initial seed than some of the neighboring parts. Since such areas may acquire extra moisture, and since they then prevent moisture from reaching other areas, we get branched patterns. This is called branching instability.

Scratches or impurities on the window: they help and direct crystallization.

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