Saturday, 17 December 2022
I observed a curious phenomenon on my run today. I have been unable to find an authoritative account or name for it, so I will call them snow crenellations.
I noticed them on a long underpass with a wooden rail that passes beneath Lyndale Ave along Minihaha creek. At first glance it looks as though a very methodical child brought a stick down upon the snow every foot or so — but gently so that a depression was created that did not go all the way to the rail. At second glance one becomes impressed with the energy of the child, because the underpass is perhaps a quarter of a mile long. At third glance, one notices that the (increasingly hypothetical) child only acted on the rail on the south side, and entirely skipped a segment of the rail that veered off at an angle as the underpass approached its end. I think nature is the more likely culprit.
About a quarter of a mile west on the path, I noticed another section of fence (albeit with a different top) that showed similar crenellation, although it was not so clean or distinct. On returning home I took a short walk and saw other instances of crenelation, though sometimes it devolved into humps or subtle ripple-patterns.
For now I assume that this is due to snow ablation due to melting or sublimation that that creates a surface texture that leads to a positive feedback loop, similar to what one sees with suncups or penitente.
Ablation hollows can be formed either by turbulent heat transfer or by radiative heating. With dirty snow, formation by turbulent transfer is favoured. Under a heat budget dominated by sensible heating, accumulations of detritus and their concentration by the normal-trajectory path can result in the initiation of a positive feed-back mechanism for the formation of ablation hollows; this occurrence is dependent on the thickness of the detritus, its albedo, and its heat conductivity.
— https://www.cambridge.org/core/journals/journal-of-glaciology/article/mode-of-formation-of-ablation-hollows-controlled-by-dirt-content-of-snow/F12E89574ABAD5FC84600BDE7C6133C3