Though the winter snows upon the plains, and the slopes of mountains of
medium height, disappear during the warm season; yet, in all latitudes,
the tops of high mountains are covered with a layer of permanent snow,
which the summer heat of these great altitudes is not sufficient to
melt.
The lower limit of perpetual snow, called the snow line, is found,
within the tropics, about three miles above the level of the sea. In
temperate latitudes it occurs at the height of a little less than two
miles; and at the northern limit of the continents, it is about half a
mile above the level of the sea, or, perhaps, even less than this.
On the Arctic Islands, vast fields of snow remain permanently, at a few
hundred feet above the sea level.
The winter snows, falling into the icy waters of the polar oceans, are
but partially dissolved; and, remaining upon the freezing surface, they
help to form those vast ice floes which encumber the polar seas at all
times.
The following table gives the observed height of the snow line in the
different latitudes:--