2. When sea ice starts to melt, a vicious cycle
frequently starts. The water absorbs more
sunlight as more ice melts and reveals more
black water. The water is then heated by the sun,
melting more ice. This cycle of positive feedback
has the potential to affect the climate over many
years.
3.
4. Frazil are millimeter-wide crystals that form when
seawater starts to freeze. Whether the oceans are
calm or boisterous affects how the ice crystals
combine to form larger masses. The crystals in
calm oceans create nilas, which are thin sheets
of ice that are so smooth they resemble oily or
greasy. These imperceptibly thin ice sheets move
over one another to create rafts of thicker ice. Ice
crystals condense into mushy pancakes amid
choppy waters. These pancakes either clash with
one another, generating ridges on the top and
keels on the bottom, or they slide over one
another to form smooth rafts.
5.
6. Some sea ice holds fast to a coastline or the sea
floor—“fast ice”—while pack ice drifts with winds and
currents. Because pack ice is dynamic, pieces can
collide and form much thicker ice. Leads—narrow,
linear openings ranging in size from meters to
kilometers—continually form and disappear.
7. Larger and more persistent openings, polynyas, are
sustained by upwelling currents of warm water or
steady winds that blow the ice away from a spot as
quickly as it forms. Polynyas often occur along
coastlines when winds blow persistently offshore.
As water and air temperatures rise each summer near
the Poles, some sea ice melts. Differences in
geography and climate cause Antarctic sea ice to melt
more completely in the summer than Arctic sea ice.
Editor's Notes
Sea ice begins as thin sheets of smooth nilas in calm water (1st) or disks of pancake ice in choppy water (2nd image). Individual pieces pile up to form rafts and eventually solidify (3rd image). Over time, large sheets of ice collide, forming thick pressure ridges along the margins (4th image)
