of the ice canopy; the counterpart of a hummock.
13.2 Friendly ice: From the point of view of the sub-
13.5 Ice keel: From the point of view of the submari-
mariner, an ice canopy containing many large
ner, a downward-projecting ridge on the under-
skylights or other features which permit a sub-
side of the ice canopy; the counterpart of a ridge.
marine to surface. There must be more than ten
Ice keels may extend as much as 50 m below
such features per 30 nautical miles (56 km) along
the submarine's track.
13.6 Skylight: From the point of view of the subma-
13.3 Hostile ice: From the point of view of the sub-
riner, thin places in the ice canopy, usually less
mariner, an ice canopy containing no large sky-
than 1 m thick and appearing from below as rela-
lights or other features which permit a subma-
tively light, translucent patches in dark surround-
rine to surface.
ings. The under-surface of a skylight is normally
flat. Skylights are called large if big enough for
13.4 Bummock: From the point of view of the subma-
a submarine to attempt to surface through them
riner, a downward projection from the under-side
(120 m), or small if not.