Snow is snow until it melts or otherwise stops being snow. We use density to differentiate snow from ice for the most part; somewhere between .70 and .80
g/cm^3 snow becomes ice. Snow that has managed to to survive a melt season becomes Firn, which is generally between .50 and .8 g/cm^3 as well. Firn is
is the start of glacier ice. As it is compacted by subsequent layers of snow from above, it's density increases to the point were air can no longer escape
from pores in the crystals and that is the point in which it becomes glacier ice. Glacier ice is unique in that it is formed from multi-year events. Other
ice is seasonal, and is mostly composed of freshwater ice and saltwater ice.
This is ice made from simulated sea water. If you were a fish and were looking up, this is the view you would have. The bubbles present in the ice were
not trapped when it froze, but are actually pockets of very salty water. As the ice froze, it shed some of the salt into the surrounding water, which is
too salty to freeze at the temperature of the surrounding ice. Ice forming in water grows downwards. This block had thawed partially before refreezing,
so the ice no longer grew straight down, but rather at awkward angles, as seen in the somewhat chaotic pattern on this sample.
Contrast that with freshwater ice, which doesn't have salinity messing with crystal formation:
This is a block of freshwater ice. It was formed under the same conditions as the sea ice, but is thicker and clearer. Though the ice appears to have
been severely cracked in several places, that is not the case. Those are the grain boundaries between individual ice crystals. That block is a 25 cm
across, meaning the crystals are 10-15 cm wide and almost double that in height. Without impurities to arrest the growth, the crystals grew from top to
bottom of their container.