Penitentes are spikes of snow or ice, and some can be 4 meters high. They are common on high-altitude glaciers or ice where the air is dry, and the sun's rays can turn ice directly into water vapor without melting it first. This process of going from solid to vapor is known as sublimation. An initially smooth snow surface first develops depressions as some regions randomly sublimate faster than others. The curved surfaces then concentrate sunlight and speed up sublimation in the depressions, leaving the higher points behind as forests of towering spikes.