Why Does Snow Stop at a Certain Height? A Line That Moves With the Season

The term covers two things that are frequently confused. The seasonal snow line is the lower edge of continuous snow cover at a given moment, which rises through spring and summer as melting proceeds and falls in autumn, and which anybody can see. The permanent snow line, more usefully called the equilibrium line, is the elevation above which snow persists through an entire year, meaning that as much accumulates in winter as melts in summer, and it is the boundary that matters glaciologically, since a glacier can only exist where that line intersects ground capable of holding ice. Its elevation is what changes when climate changes, and tracking it is a standard measure.

Several factors move the line and they interact:

• •Temperature, which falls with height and determines whether precipitation falls as snow and whether it melts
• •Precipitation, since more snowfall requires more melting to remove it and lowers the line
• •Latitude, with the line descending towards the poles and reaching sea level in polar regions
• •Aspect, with shaded slopes holding snow hundreds of metres lower than sunlit ones
• •Wind, which strips snow from ridges and deposits it in hollows
• •Continentality, since dry interior ranges have higher lines than wet maritime ones at the same latitude

The elevation does not simply fall from the equator towards the poles, and the exception is instructive. The highest snow lines on earth occur not at the equator but in the subtropical dry belts around twenty to thirty degrees, reaching around six thousand metres in the Andes and in Tibet, because those regions sit under descending air that produces extremely low precipitation, so very little snow falls and what does falls in intense sunlight. Equatorial mountains receive far more precipitation and have lower lines despite higher temperatures. That pattern demonstrates that the line depends on the balance between accumulation and melting rather than on temperature alone, which is the single most useful thing to understand about it.

The zone above the permanent line is worth describing because it is not simply more of the same. Snow accumulating there compacts under its own weight into a denser granular material and then into ice over years, which is how a glacier is fed, so the region above the line is where every glacier originates. Bare rock projecting through the ice, which mountaineers call a nunatak, supports its own isolated communities of plants and invertebrates cut off from anything else. Surfaces there experience enormous temperature swings between sun and shade. Snow algae colour old snow pink in summer in many ranges, which is a living organism rather than dust and which darkens the surface and accelerates melting measurably.

The line is a practical measure across several fields. Glaciologists use its position on a glacier at the end of summer as a direct indicator of that year's mass balance, since a line higher than usual means more of the glacier melted, and satellite imagery makes that observable for glaciers nobody visits. Reconstructing past positions from moraines and from the extent of former glaciers estimates past temperature and precipitation, which is a standard palaeoclimate method. Water resource planning in mountain regions depends on how much of the catchment lies above the seasonal line, since snow stored in winter releases in spring. And mountaineering route planning depends on where snow conditions begin, which shifts through the season and between years.

The seasonal line is the visible lower edge of snow at a moment, while the equilibrium line is the elevation where a year's accumulation equals a year's melting, and the second determines where glaciers can exist. Aspect moves it hundreds of metres. The highest lines occur in the subtropical dry belts rather than at the equator, because so little snow falls there.