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METEOROLOG! FOR MARINERS

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Antarctic Bcrgs

The breaking away of ice from the Antarctic continent takes place on a scalę quite unknown in the Arctic, so that vast numbers of bergs are found in the adjaccnt waters. Bcrgs are formed by the calving of masses of ice from ice shelves or tongues, or from a glacier face.

Antarctic bergs are of several distinctivc forms. The following descriptions should be regarded as covcring only thosc terms which are likcly to be of interest to mariners.

Tabu lar Bergs. This is the most common form and is the typical berg of the Antarctic, to which there is no cxact parallel in the Arctic. Thcsc bcrgs are largcly, but not all, dcrivcd from ice shelvcs and show a characteristic horizontal banding. Tabular bcrgs are flat-toppcd and rcctangular in vcrtical cross-scction, with a pcculiar whitc colour and lustre, as if formed of plaster of Paris, duc to their relativcly large air contcnt. They may be of great sizc, larger than any other typc of berg found in either of the polar regions. Such bcrgs, excccding 1 nautical mile in length, occur in hundreds. Many havc been measured up to 20 or 30 nautical miles in length, while bcrgs of morę than twicc this length have been reported. The largest berg authcntically reported is one about 90 nautical miles long, observed from the whaler Odd I on 7 January 1927, about 50 nautical miles north-east of Clarence Island in the South Shctlands. This great tabular berg was about 35 metres high. According to recent data the averagc height of tabular bcrgs abovc the water is about 50 metres.

The number of tabular bcrgs set free varies in difTerent years or periods of years. There appears to have been an unusual breakup of icc-shclf in the Weddcll Sca region during the years 1927-33, when the number and size of the tabular bergs in that region was exccptional. The giant berg described aljove was one of these. It is probable that the calving of the larger tabular bergs is achieved by sca surface disturbanccs causcd by underwater movements of the carth’s crust (i.c. tsunainis) or, indeed, directly by earth tremors originating within Antarctica. Should these be pariicularly vigorous then the configuration of the ice shclveson long strctches of the coastline may be drastically altered in a matter of hours.

The glacier bergs of the Antarctic are similar to thosc of the Arctic region.

Black and White Bergs. A unique form of iccberg, called black and white bcrgs, has been obscrvcd north and east of the Weddell Sea. They are of two kinds, which are difłicult to tell apart at a distance: (a) morainic, in which the dark portion is black and opaque, containing mud and stones, (b) bottle-green, in which the dark part is of a dcep green colour and translucent, mud and stones appearing to be absent. In both kinds the dcmarcation of the white and dark parts is a clear-cut piane, and the dark portion is invariably smoothly roundcd by water action. Such bergs have frcqucntly been mistaken for rocks.

Weathered Bergs. This namc is given to any berg in an advanced State of disintegration in either hcmisphcrc. Large variations occur. The length of lifc of a floating berg is determincd partly by the time spent in the pack beforc it cmcrges into the open sea. Thereafter its period of survival is determined largcly by the rapidity of its transport to lowcr latitudes.

Melting of the underwater surface is a continuous process and this, aided by the mcchanical action of the sea, produces caves with intervening spurs near

FORMATION and ciiaracter of ice

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the water-linc. This finally leads to the scaling off of a portion of the berg or to a change in its equilibrium, whereby tilting or even complete capsizing may occur, thus presenting ncw surfaces to the sca and the wcather. The prcscncc of crevasscs, earth particles or rock debris greatly enhancc the processes of mclting or evaporation and produce plancs of wcakness, along which further calving occurs. In grounding, a much crevassed berg may be wrecked. Other bcrgs, in passing over a shoal, may dcvclop strain cracks, which latcr accclcratc their weathering.

Movcment of Icebergs

Since about 9/10 of the volume of an iccbcrg is submerged, it follows that its movement is chicfly controlled by the water movement or current. However, strong winds may exert a considerablc influence on the movement of icebergs, partly by direct action on the berg, and partly through their effect upon the current. Off Ncwfoundland the chief factor governing the severity of any iccberg season (March to July) is the frcąuency of north-north-wcstcrly winds along the Baffin Island and Labrador coasts during the months immcdiatcly preccding and early in the season. For example, in 1972 north-westerly winds prevailed from latc Deceinber to May, resulting in 1587 bcrgs drifting south of the 48th parallcl on the Grand Banks of Ncwfoundland. In 1966, however, strong north-easterly (on-shorc) winds occurred in January, February and March, resulting in the grounding and cventual decay on the Labrador Coast of the 1966 iccberg ‘crop’.