The lower boundary of the area with perennial snow cover is called the snow line. Above the snow line, the annual supply of snow and ice exceeds the amount that melts, forming a region of perennial snow cover; below the snow line, the annual supply of snow and ice is less than the amount that melts, preventing the accumulation of multi-year snow, resulting in only seasonal snow cover.

The distribution height of the snow line varies greatly across different regions of the Earth, primarily due to the combined effects of climatic and geomorphological factors. Climate-related elements such as temperature and precipitation both play a role. Surface temperatures decrease from the equator toward the poles, so the general trend of snow line elevation also decreases from the equator to the poles. For example, the snow line in tropical Africa is between 4,500 and 5,200 meters, dropping to 2,400–3,200 meters in the Alps, and only around 200 meters within the Arctic Circle.

Precipitation is closely related to the height of the snow line. The greater the snowfall, the lower the snow line, and vice versa. Therefore, the regions with the highest snow lines in the world are not near the equator but in the subtropical high-pressure zones. For instance, the Andes Mountains in South America, within this zone, have very high snow lines, including the highest in the world, reaching up to 6,400 meters. Another example is the Tianshan–Qilian Mountain range in China, where moisture mainly comes under the influence of the westerlies. As a result, moving from the western section of the Tianshan Mountains eastward, precipitation decreases and the snow line rises, exceeding 5,000 meters in the western section. However, as you move further east to the eastern Qilian Mountains, increased moisture from the Pacific leads to a lower snow line.

Geomorphological factors mainly influence the snow line through mountain shape and slope orientation. Steep mountains are not conducive to snow accumulation, resulting in a higher snow line; in the Northern Hemisphere, southern and western slopes receive more sunlight, leading to greater snowmelt and thus a higher snow line, while northern and eastern slopes have lower snow lines. For example, the southern slope of China’s Tianshan Mountains has a snow line height of 3,900–4,200 meters, compared to 3,500–3,900 meters on the northern slope. However, climatic factors such as temperature and precipitation generally have a greater impact on the snow line than geomorphological ones, which is why there are also cases where the southern slope of a mountain range has a lower snow line. For instance, the southern slope of the Himalayas, being on the windward side of the southwest monsoon, receives abundant precipitation, resulting in a snow line at about 4,600 meters, while the northern slope’s snow line is roughly one kilometer higher.