Surface hoar

Apparently this is surface hoar:

Uploaded image — Spiky bits on snow, on top of a log seen on a walk in Wild Park. A small terrier gives it a sniff.

It’s been well studied because when buried it’s an avalanche risk (but not where I live).

According to Avalanche Canada it’s:

frost crystals that grow upward from the snow surface when the air just above is cooled to the dew point.

There’s a description of the process from the Woodland Trust:

It forms when the water vapour in the air comes into contact with solid surfaces that are already below freezing point. Ice crystals form immediately, and the ice continues to grow as more water vapour is frozen.

YouTube have various time-lapses, for example:

https://youtu.be/WCw8SEDdhpo

The Met Office has an explainer video on different types of frost.

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