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242 METEOROLOGT FOR MARINERS

salt is deposited, leaving purc icc containing pockets of solid salt, thc icc gains in strcngth so that at temperatures below — 23°c sca icc is a vcry tougli materiał.

This process is rcverscd in summcr whcn, as a rcsult of rising temperatures, thc deposited salts go back into solution as brinc. The pockcts containing thc brine gradually enlarge as the surrounding icc begins to melt so that the ice becomcs honcycombed once morę with pockcts of brine. Eventually a great number of these pockets interlink and some break through the lower surface of the icc, rcsulting in an accclcrated ratę of brine drainage. It is at this stage that most of the salt trapped in the process of frcczing is drained from the ice. Should this icc survive thc summer melt and bccome sccond-year ice its salt contcnt will be smali. Survival through another summer season, whcn morę salt is drained away, results in multi-year ice which is almost salt-free. Because of its very Iow salt contcnt, multi-year icc in winter is extremcly tough.

Ordinarily, first-year icc found floating in thc sca at thc end of six months is too brackish for making good tea but is drinkable in thc sense that the fresh water in it will rclieve morę thirst than thc salt creates. When about ten months old and floating in thc sca the salt-water ice has lost most of its milky colour and is nearly fresh. A chunk of last ycar’s icc that has bcen frozen into this ycar’s ice will givc water fresh enough for tea or coffee. Usually thc water from sca ice does not become ‘as fresh as rain water’ until it is two or morę ycars old.

When sca icc thaws in such a way that thcrc arc puddles on top of it these are fresh enough for cooking, provided there are no cracks or holes connecting them with thc salt water under the floes. The water can therefore be pumped into a ship from the icc through a hose, which was ordinary scalcr and whalcr practice. However, water should not be pumped from a puddle that is so ncar to thc edge of a floc that spray has bcen mixed with it. Whalcrs preferred to go about 10 metres or morę from the edge of a floc to find a puddle from which to pump.

Sea icc is divided into two main typcs according to its mobility. One is pack ice, which is rcasonably free to move under thc action of wind and current; the other is fast ice which does not movc. Ice first forms ncar coasts and spreads seawards. A ccrtain width of fairly lcvel icc, depending on thc depth of thc water, becomcs fast to the coastline and is immobile. The outer edge of thc fast ice is often locatcd in thc vicinity of thc 25-mctre contour. A reason for this is that wcll-hummockcd and ridged ice may ground in these depths and so form ofT-shorc anchor-points for thc ncw season’s icc to bccome fast. Beyond this ice lies thc pack icc which has formed, to a smali but important extent, from pieccs of ice which have broken off from the fast icc. As these spread seawards they, together with any remaining old ice floes, facilitate the formation of new, and later young ice in thc open sca. This ice, as it thickcns, is constantly broken up by wind and waves so that the pack ice consists of icc of all sizes and ages from giant floes of several ycars’ growth to thc scveral forms of new ice whose life may be measured in hours.

Deformation of Ice

Under thc action of wind, currcnt and internal stress thc pack ice is constantly in motion. Where the ice is subjected to pressure its surface becomes deformed. In new and young ice this may rcsult in rafting as an ice sheet overrides its ncighbour; in thicker ice it leads to the formation of ridges and hummocks according to the pattern and strcngth of thc convergcnt forccs.

FORMATION AND CHARACTER OF ICE

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During thc process of ridging and hummocking, when large pieccs of ice are piled up abovc the generał ice level, vast quantities of ice are foreed downward to support the weight of ice in the ridge or hummock. The downward extension of icc below a ridge is known as an ice keel, and that bclow a hummock is callcd a bummock. The total vertical dimensions of these features may reach 55 metres, approximately 10 metres showing above sea levcl. In shallow water ice floes piled up against the coastline may reach 15 metres above mean sea level.

cracks, leads and polynyas (open areas within an icc field) may form after pressurc within the icc has been relaxed. When these openings occur in winter they rapidly bccome covcrcd by new and young icc which, givcn sufficient time, will thicken into first-ycar ice and cement the older floes together. Normally, howevcr, bcforc the first-ycar stage is reached, thc younger ice is subjccted to pressure as the older floes move together, resulting in thc deforma-tion features alrcady dcscribcd.

Off-shorc winds drive the pack ice away from thc coastline and open up a shore lead, which is a navigablc passage bctwccn thc main body of thc pack icc, and the shore. In somc regions where off-shore winds persist through the ice season, localizcd movcmcnt of shipping may be possiblc for much of the winter. Where there is fast ice against thc shore, off-shore winds develop a lead at thc boundary, or flaw as it is known, between the fast ice and thc pack icc: this opening is called a flaw lead. In both types of lead, ncw-ice formation will be considerably impeded or even prevented if thc off-shore winds are strong. On most occasions, howcver, new or young ice forms in the leads and when winds become on-shore the re-frozen lead closes up and thc younger ice is complctely deformed. For this rcason the flaw and thc coast, cspecially when on-shore winds prcvail, are usually marked by tortuous ice conditions.

Clearance of Ice

The clcarancc of icc from a given arca in summer may occur in two different ways. The first, applicable to pack ice only, is the direct rcmoval of the ice by wind or currcnt. The sccond method is by melting in situ which may be achievcd in scvcral ways. Wind again plays a part in that where the ice is wcll broken (open icc or lesser concentrations) wavc action will cause a considcrablc amount of melting cvcn if thc sca temperaturę is only a littlc abovc the freczing point. Where pack ice is not wcll broken or where there is fast icc the melting process is dependent on incoming radiation.

In thc Arctic in winter the ice becomes covered with snów to a depth of about 30 to 60 ccntimctrcs. Whilc this snów covcr persists, almost 90% of the incoming radiation is rcflccted back to space. Eventually, howcvcr, the snów begins to melt as air temperatures risc above o°c in carly summer and the resulting fresh water forms puddles on thc surface. These puddles now absorb about 60% of the incoming radiation and rapidly warm up, steadily enlarging as they melt the surrounding snów and, later, the ice. Eventually thc fresh water runs off or through thc icc floc and, where the concentration of the pack icc is high, it w^rill scttlc between thc icc floes and thc undcrlying sca water. At this stage the temperaturę of thc sca water will still be below o°c so that the fresh water frcezes on to thc under-surface of thc ice, thus temporarily reducing thc melting ratę. Mcanwhile, as the temperaturę within thc ice rises, thc ice becomcs riddled with brine pockets, as described earlier. It is considerably weakcncd and offers little rcsistance to the destructive action of wind and waves. At this stage the fast icc brcaks into pack ice and eventually thc icc floes, when