Photo of snowy Sussex woodland trail Photo of snowy Sussex woodland trail

Frost Shadows and Compasses

It is that time of year when nature likes to get out her frost compasses for us to admire.

I took this photo last Friday in the middle of the day.

We are looking due west.

The southern sun has thawed the areas it can get to, but unlike other suns, it cannot refresh the parts it cannot reach. (My apologies, I think a retro marketing slogan tried to creep in there.)

The frost shadow on the sand itself is quite straightforward, but the shadows and frost on the logs are more interesting, particularly when we focus on the middle log.

Note how there is a strong relationship between the gradient of the wood and the areas of frost. Looking from left to right on the middle log, where the wood slopes at all up there is no frost left as this face will absorb the warmth of the sun very quickly. Conversely, where there is a downward slope, even a very gentle one, the frost remains, even in sunlight, because it only receives a ‘glancing blow’ of sunlight which is much weaker.

Of course, when the sun is out like this in the middle of the day, we don’t need these frost shadows to find direction. But this is a very good way of getting a feel for how the sun effects frost and dampness more generally. The frost helps paint a picture of the areas that receive the least warmth and light, and this points out the spots that are more prone to moisture-loving organisms like the mosses, algae and some lichens.