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tions. The navigator should therefore always be on guard against such 

 a possibility whenever there is a chance that a temperature inversion 

 may exist. If the motion of the ship permits use of a bubble sextant, it 

 can be quickly determined whether the apparent horizon is appreciably 

 displaced. 



PILOTING 



Piloting in polar regions is very uncertain. There are few es- 

 tablished aids to navigation and almost no cultural features. More- 

 over, the land areas shown on many charts are not dependable either 

 as to location or feature. Perpetual snow and ice cover many of 

 the features and result in a monotonous sameness of the visible land- 

 scape over large areas. On approach to waters where ic€ has been 

 observed in past years, lookouts should be stationed on forecastle head 

 to watch for growlers or drift ice. 



On dark, clear nights, icebergs may be seen at a distance of 1 to 2 

 miles, appearing either as white or black objects. Under such con- 

 ditions of visibility, growlers are a greater menace to vessels, and speed 

 should be reduced and a sharp lookout maintained. The moon may 

 be an aid or hindrance to ice detection, depending upon its age and 

 bearing. With the moon ahead, bergs are difficult to see; with the 

 moon astern, a blink is thrown up by the bergs rendering them visible 

 from a great distance. A clouded sky at night, through which the 

 moon appears and disappears, renders ice detection difficult; heavy 

 passing clouds may dim or obscure an object sighted ahead. Fleecy 

 cumulus and cumulo-nimbus clouds often give the appearance of blink 

 from icebergs. 



Radar equipment can easily pick up large bergs in ample time to 

 avoid collision. However, small bergs or growlers capable of in- 

 flicting serious damage to vessels may go undetected even with moder- 

 ate conditions of wind and sea. As the state of the sea increases, so 

 does the minimum size of berg that can be detected. On very rough 

 seas, bergs as high as 50 feet cannot always be detected in the sea 

 return. Only in exceptionally smooth seas can radar be depended 

 upon to pick up growlers. It is, therefore, unsafe for any vessel, 

 because of radar, to assume immunity to ice hazards. 



Air temperature is not a reliable guide to the presence of icebergs; 

 nor can sea temperatures be depended upon to give warning of their 

 approach. It is true that a small increase in water surface tem- 

 perature can usually be detected within a mile or so of an iceberg. 

 This increase is due to the freshening of the surface layer of the sea 

 by the melting ice, causing a reduction in vertical mixing as a result 



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