A set of points forming the lower limit of a snow-covered area; on a glacier, the line separating snow surfaces from ice or firn surfaces, and also separating the percolation zone from either the superimposed ice zone or the ablation zone (see also zone). The set of points need not form a continuous curve. The snow-covered area of the glacier may include outliers (isolated patches of snow surrounded by firn or ice) and may exclude inliers (isolated patches of exposed firn or ice). The snowline is usually easy to see, because the snow above it is brighter than the firn or ice below it. It may therefore be mapped by analysis of suitable imagery. When, and only when, there is no superimposed ice, the snowline coincides with the equilibrium line. Unless qualified by a different adjective, references to the snowline are understood to refer to the Annual snowline. It is common for 'snowline' to be used as an abbreviation of 'average altitude of the snowline'.