Penitentes, or nieves penitentes (Spanish for "penitent-shaped snows"), are a snow formation found at high altitudes. They take the form of tall thin blades of hardened snow or ice, closely spaced with the blades oriented towards the general direction of the sun.
The name comes from the resemblance of a field of penitentes to a crowd of people kneeling, as when doing penance. The name refers to the tall, pointed habits and hoods worn by brothers of religious orders in the Procession of Penance of the brotherhood to which they belong in Spain, in the processions of Spanish Holy Week. In particular the hats are very tall, narrow, and white with a sharp tip.
These pinnacles of snow or ice grow over all glaciated and snow-covered areas in the Dry Andes above 4,000 metres or 13,120 feet. They range in size from a few centimetres to over 5 metres or 16 feet.
Penitentes were first described in scientific literature by Charles Darwin in 1839. On March 22, 1835, he had to squeeze his way through snowfields covered in penitentes near the Piuquenes Pass, on the way from Santiago de Chile to the Argentine city of Mendoza, and reported the local belief (continuing to the present day) that they were formed by the strong winds of the Andes.
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, because sublimation requires a lower energy input than melting. 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. A mathematical model of the process has been developed by Betterton, although the physical processes at the initial stage of penitente growth, from granular snow to micropenitentes, still remain unclear. The effect of penitentes on the energy balance of the snow surface, and therefore their effect on snow melt and water resources has also been studied.
Penitentes may be present on Europa, a satellite of Jupiter. 
Field of penitentes (1.5–2 metres or 5–7 feet high); upper Rio Blanco, Central Andes of Argentina.
Small penitentes in the summit crater of Mount Rainier.
Penitentes ice formations at the southern end of the Chajnantor plain in Chile.
- "Planetary Analogue". Retrieved 8 June 2015.
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- Lliboutry, L. (1954b). "The origin of penitentes". Journal of Glaciology 2: 331–338.
- Lliboutry, L. (1965). Traité de Glaciologie, Vol. I & II (in French). Paris, France: Masson.
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- Darwin, C. (1839). Journal of researches into the geology and natural history of the various countries visited by H. M. S. Beagle, under the command of Captain Fitz Roy, R.N., 1832 to 1836. London, UK: H. Colburn.
- Betterton, M.D. (2001). "Theory of structure formation in snowfields motivated by penitentes, suncups, and dirt cones". Physical Review E 63: 12. doi:10.1103/physreve.63.056129.
- Corripio, J.G. (2003). Modelling the energy balance of high altitude glacierised basins in the Central Andes (PDF) (PhD. thesis). Edinburgh, UK: University of Edinburgh. p. 151. Retrieved 7 September 2013.
- Corripio, J.G.; Purves, R.S. (2005). "Surface Energy Balance of High Altitude Glaciers in the Central Andes: the Effect of Snow Penitentes". In de Jong, C.; Collins, D.; Ranzi, R. Climate and Hydrology in Mountain Areas (PDF). London, UK: Wiley & Sons. p. 18. Retrieved 7 September 2013.
- Vance Bergeron, Charles Berger, and M. D. Betterton, 2006. "Controlled Irradiative Formation of Penitentes", PRL 96, 098502 (2006), webpage: Arxiv-PDF-601184.
- Kotlyakov, V. M. and Lebedeva, I. M.: 1974, "Nieve and ice penitentes, their way of formation and indicative significance", Zeitschrift für Gletscherkunde und Glazialgeologie, Vol. X, 111-127.
- Lliboutry, L.: 1998, "Glaciers of the Dry Andes", in R. S. J. Williams and J. G. Ferrigno (eds), Satellite Image Atlas of Glaciers of the World - South America, United States Geological Survey Professional Paper 1386-I, webpage: USGS-p1386i.
- "Spiky glaciers are slower to melt", New Scientist (March 7, 2007).