Penitentes: Incredible blades of ice that point towards the sun

My walk to the railway station this morning was quite an ordeal. Not only was I freezing cold and miserable about the fact it was no longer the Christmas break, the soles on my shoes have no grip whatsoever. So I basically slid all the way to the station (the closest I’ll ever come to a winter sport).

This is what happens when it’s cold in Essex. But when it’s cold in the Dry Andes, something a little more impressive happens…

Credit: ESO

Penitentes or nieves penitente (which is Spanish for “penitent-shaped snows”) are pointy blades of hardened snow or ice. They only form at high altitudes in the Dry Andes in South America, above 13,120 feet (4,000 metres), and they always point in the general direction of the sun.

Credit: Mark Sanderson

They were first described by none other than Charles Darwin himself, who had to squeeze his way through them in 1835 on his way from Santiago de Chile to Mendoza in Argentina. (Did Charles Darwin have travel insurance I wonder?) The name comes from their similarity to the white pointy hats worn during the Processions of Penance in Spanish Holy Week.

Penitents during Spanish Holy Week. Credit: Frank Kovalchek

Being a curious sort, I looked this phenomenon up on Wikipedia, hoping to find a nice and easy-to-understand explanation for how they form and why. Well, this is what I got instead:

Louis Lliboutry noted that the key climatic condition for the differential ablation that leads to the formation of penitentes is that the dew point is always below freezing. Thus, snow will sublimate. Once the process of differential ablation starts, the surface geometry of the evolving penitente produces a positive feedback mechanism, and radiation is trapped by multiple reflections between the walls. The hollows become almost a black body for radiation, while decreased wind leads to air saturation, increasing dew point temperature and the onset of melting. In this way peaks, where mass loss is due only to sublimation, will remain, as well as the steep walls, which intercept only a minimum of solar radiation. In the troughs, the ablation is enhanced, leading to a downward growth of penitentes.

Got that? Something to do with the temperature, the wind and radiation from the sun… Yep.

Credit: Vrac

However they form, I think penitentes are quite pretty. Who knows? Maybe I’ll see some on my walk to the station tomorrow morning…