There is something magical about going to sleep in the winter and waking to the landscape covered in hoarfrost. It is one of my favourite times to go on a winter wander as everything from a blade of grass to a supersized spruce tree seems to be glowing in the brilliant white of the frost. I feel like I’m walking through a scene on someone’s christmas card. It reminds me of the white paint on the fake trees of my dad’s old train set, and leaves me with the same feeling of how can this be real? But nature is full of surprises and science.

Hoarfrost itself is a delicate crystalized frost that clings to solid objects. It gets its name from the old English meaning of “hoar” which means “showing signs of age”, which if you have talked to any middle aged person, it makes sense as they are always teased when they get their first white or grey hair for getting old. I wonder if the trees feel old as they wear their wigs of frost?

In order for hoarfrost to form you need some weather conditions to line up. The first is a lot of moisture in the air. This could be from a moist air mass moving into an area, an open source of water like a lake or even from the snow that is on the ground. The next thing you need is a cool, clear night, indicating the moisture in the air is gaseous water vapour. The water vapour is supercooled, which means it is below zero degrees when water should be frozen, but because it is so tiny it cannot freeze. That is until it touches a solid surface. As soon as it contacts a frozen object the water changes from its gaseous state to a solid ice state instantly in a process called sublimation. These tiny ice crystals build up as new water vapour hits them leaving behind the intricate patterns we see in the morning.

The second set of crystals we can find, which can be mistaken for hoarfrost, is rime ice. It is a similar process except it happens on foggy nights, and because we can see the fog it means the moisture in the air is actually liquid water droplets. They are still tiny and supercooled and will freeze instantly when they meet a solid frozen object. If the wind is blowing when rime ice forms you can actually tell from which direction in the morning as all the spikey ice crystals will have formed on the wind facing side of the object. Very cool, pun intended.

‘Weather’ you have awakened to the fluffy crystals of hoarfrost or the spikey frost fixtures of rime ice, it is a delight on the eyes. I encourage you to take a hike and try to figure out what way the wind was blowing the night before, see how high some of the crystals grew, or just marvel at the beauty of nature.

Photo by Laura Griffin