Wednesday, December 28, 2011

Ice


Note: A different version of this post originally ran on July 14, 2009.

Ice has been a challenge to mariners from ancient times right up until today. Two of the most infamous maritime disasters of all time, the Titanic sinking and the Exxon Valdez oil spill, were caused by either a collision with an ice berg or an attempt to avoid one.


Calving glaciers. The photos at left show the South Sawyer glacier in Tracy Arm, a fjord in Southeast Alaska. The picture on top was taken in May of 1995, the bottom picture in June 2009. South Sawyer was always good for a show, but about five years ago it started calving at a much increased rate and went into what scientists call "catastrophic retreat." The pictures weren't taken at the exact same distance (lots of seal pups were hauled out on the ice in the more recent picture, so the Spirit of Yorktown kept her distance to avoid disturbing them) but it's obvious how much the glacier has retreated in the intervening years.

Ice words. It's a myth that Eskimos have more than a dozen words for "snow," but mariners have developed a huge vocabulary to describe ice of various sizes, age, and composition. Large chunks of floating ice are called either bergy bits or growlers. Bergy bits are the size of a small building, reaching up to five meters above the water and an area of up to 300 square meters. Growlers -- named for the sound they sometimes make as they bounce in the waves -- reach less than a meter above the water and take up about 20 square meters. Sea ice -- ice formed by saltwater -- can be described as frazil, grease, nilas, rind, pancake, young, old, etc.

Great Lakes Ice. During a particularly cold winter, Lake Superior can be completely frozen over for some periods, with ice as thick as 100 centimeters. During a mild winter, Lake Ontario can be basically ice-free, possibly with some forming around the entrance to the St. Lawrence River in early January. Despite this variability, recent years have seen a decline in the amount of overall Great Lakes ice formed and the time it stays frozen. While this may seem like a boon for shipping at first, the increased time the water of the Lakes stays in liquid form increases the evaporation rate in a given year, ultimately lowering the lake level.

Ice breakers. Coast Guard and research vessels (or cruise ships that are converted research vessels) rated as ice breakers have especially heavy and reinforced hulls. These vessels don't break ice by plowing through it, they break ice by running their bows up on the ice and using the weight of the vessel to break up the ice from above. Ice breakers are mainly used in areas where sea ice has formed, or in large frozen areas of fresh water like the Great Lakes. In places like Tracy Arm, ship captains go slow, contact ice chunks at an angle, and take into account the material and thickness of their hull. The above video shows the Canadian Coast Guard cutter Samuel Risley in action in Thunder Bay.

Michael Scott of The Cleveland Plain Dealer filed an excellent article on the consequences of the reduction of Great Lakes ice in March 2009. Find it here.

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