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Snow and Ice - The Frozen Continent

Aurora Australis With 98% of its surface covered with various forms of snow and ice, it's no wonder that the continent of Antarctica attracts "cold weather" scientists from all over the world. Basically, Antarctica is a snow and ice "factory" with ice depths on the Polar Plateau reaching 15,000 feet (the continent's average ice thickness is 7,000 feet). Thus, one of Antarctica's most important resources is its ice. It is said that Antarctica's ice accounts for 70% of the world's fresh water. Some people have considered towing icebergs from Antarctica to parts of the world in need of fresh water.

As strange as it sounds, however, Antarctica is essentially a desert. The average yearly total precipitation is about two inches. So, where did all this snow and ice come from? The answer lies in Antarctica's unique location at the bottom of the world and the unique weather conditions that exist there.

The Many Forms of Snow and Ice
Moving bodies of ice formed mainly of snow that has thawed, refrozen, and become more crystalline and dense.
Vast shields of thick continental ice that have formed through the accumulation of snow over millions of years. Ice sheets are frequently domed shaped and gradually sloped.
Ice Shelves
Large continentally based ice sheets which have flowed to the coast where they then float in the ocean.
Chunks of ice of all sizes which break off continental ice sheets and ice shelves in a process known as "calving". Individual icebergs range from a few meters to tens of miles in diameter.
Ice Crystals
Also known as "Diamond Dust," ice crystals are essentially crystallized water vapor, found in areas of low humidity and extreme cold. At the South Pole, they are often observed suspended on currents of air, and sparkling with reflected sunlight.
Wind packed drift snow in the form of small hard ridges which resemble frozen waves. They can make surface travel very rough.
Sea Ice Ice which forms on the surface of the ocean, starting at coastlines and extending outward. It can take many forms from a "greasy" sheen on the water's surface to vast discontinuous masses of pack ice several meters thick.

Where does the snow come from?

The precipitation is carried in by the storm systems. These cyclonic systems carry warm moist air from the lower latitudes. So, most of the snow falls within 120 to 190 miles of the coast. Average precipitation on the coast is 20 to 50 inches of snow (7 to 16 inches of water equivalent). The Antarctic Peninsula has highest precipitation of the continent, (36 inches water equivalent).

Precipitation declines inland because of the increased altitude and distance from the sea. Storms cannot penetrate far into the continental interior except in the low lying regions. Most snow fall occurs in winter when the westerlies are strongest and the storm systems can reach inland farther and more often. When warm moist air does make it all the way to the Polar Plateau, the air cools considerably. Eventually it becomes supersaturated with ice crystals. Ice crystals account for 90% of the accumulation on the plateau. The annual snow fall at the South Pole is less than 1 inch (water equivalent) or 3 centimeters.

The Antarctic Ice Sheet
The Antarctic Ice Sheet is a thick, ancient sheet of ice with a maximum depth of nearly 3 miles (15,000 feet). It is the iceberg 'factory' of the Southern Ocean. This icesheet contains over 5 million cubic miles (30 million cubic km) of ice. The weight of the Antarctic ice is so great that in many areas it actually pushes the land below sea-level. Without its ice cover Antarctica would eventually rise up another 1500 feet (450 m) above sea-level. The Ice Sheet is very gradually moving, in this case towards the sea in a radial pattern.

Glaciers form when the yearly snowfall exceeds the yearly melt. Because Antarctica's temperatures rarely exceed the freezing point, thick ice sheets can form if given enough time. In Antarctica there are both alpine and continental glaciers. Alpine glaciers are found in the high basins of mountain ranges and flow down into valleys. By contrast, a continental glacier forms on a continental land mass and flows outward from its source region. Over time the snow on glaciers becomes more dense and more granular in texture as freeze/thaw cycles and pressure cause it to recrystallize and compact into ice.
Gravity pulls all glaciers and ice sheets downhill and the Antarctica ice sheets are no exception. The ice in Antarctica is flowing to the sea at a rate of about one to ten meters per year. When glaciers flow over changing slopes or uneven land, the brittle top layers crack and form deep fractures called crevasses.

Ice Shelves
Ice shelves are areas of floating, fresh water ice (formed from snow accumulation), attached to the major ice sheets which make up the ice cover of Antarctica. Ice shelves terminate where ice calves off to form icebergs.The Ross Ice Shelf is the largest covering an area of 150,000 square miles, and is about 500 miles across. Ice shelves typically range in thickness from 1000 feet near the edge to 3000 feet at the boundary between floating ice and grounded (land-based) ice.

Icebergs come in all shapes and sizes. They can be categorized into tabular, irregular or rounded icebergs and their shape is usually an indication of their age. Antarctica as a rule has much larger icebergs than the Arctic. A large Antarctic iceberg may weigh 400 million tons, tower ten stories above the surface of the water and contain enough fresh water to supply a city of three million people for a year. After erosion from wind and waves, and melting from the warmer sea temperatures away from the Antarctic coast, the tabular icebergs become unstable and roll over to form jagged irregular icebergs, sometimes with spikes towering up to 180 feet into the air and with even greater protrusions deep under the ocean surface. Eventually, icebergs melt completely as they drift to more northerly, warmer water.

Pack Ice or Sea Ice
In winter the sea around the Antarctic freezes (sea water usually begins to freeze at 28°F or -1.8°C) eventually covering an area larger than the continent itself. Ocean swells and wind break the ice into large pieces termed pack-ice that move under the influence of wind and currents. (Fast-ice is sea-ice that is held fast to the continent.) Pack ice can change in a matter of hours from being open and navigable to densely packed and impassible. There are distinct stages in the transition from sea water to sea-ice. First, crystals form on the surface of the brine creating an oily sheen known as grease-ice. This further evolves into a slush known as frazil-ice.The sea-ice gradually thickens as more and more water from below freezes and as snow falls from above, but it is by no means a continuous mass.
Did you know?
  • Aurora AustralisThe frozen water contained in a snowflake falling at the South Pole would take up to 50,000 years to reach the ocean.
  • Ice cover doubles the area of Antarctica each year -- extending the continent to approximately 30 million square miles.
  • The most famous Antarctic glacier is the Beardmore, which served as a pathway for early explorers such as Scott and Shackleton on their way to the South Pole.
  • Over time salt is gradually leached from sea ice into the surrounding ocean to the point where the water from melted sea-ice is quite drinkable. Melt water from old sea ice and icebergs was the main way early Antarctic ships were able to replenish their water supplies.
  • One of the longest icebergs ever recorded, designated B9, broke away from the Ross Ice Shelf in October 1987. The size of the state of Delaware when it first calved, it measured 86 x 22 nautical miles (160 x 40 km).



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