From Glossary of Meteorology
A spike or pillar of compacted snow, firn, or glacier ice caused by differential melting and evaporation.
Necessary for this formation are 1) air temperature near freezing; 2) dewpoint much below freezing; and 3) strong insolation. Consequently, penitent ice is most developed on low-latitude mountains, especially the Chilean Andes, but has been found in polar regions. Penitents are oriented individually toward the noonday sun, and usually occur in east–west lines. The term is derived from the Spanish nieve penitente (penitent snow), which is still widely used throughout the literature.