Glaciers form, advance, recede, and disappear in response to changing climate patterns. Alpine glaciers show the most direct response to climatic change of any surface phenomena in the mountains. This is apparent in the Sierra Nevada of California, where several small glaciers of less than 1 km² (0.386 mi²) exist under marginal conditions. Unlike larger ice masses, such as those in Alaska, which may typically display lag times of years or even decades in response to changes in climate, these glaciers respond rapidly, often showing measurable differences on a yearly basis.