When scientists talk about the cryosphere, they mean the places on Earth where water is in its solid form, frozen into ice or snow. Read more ...
Cryosphere Glossary
ice skylight
from the point of view of the submariner, thin places in the ice canopy, usually less than 1 meter (3.3 feet) thick and appearing from below as relatively light, translucent patches in dark surroundings; the under-surface of an ice skylight is normally flat; ice skylights are called large if big enough for a submarine to attempt to surface through them (120 meters, 131 yards), or small if not.