31 



In using an isotherm chart the fact that the water temperatures 

 and currents are changing from day to day must be kept constantly 

 in mind. An idea of the change can be gotten from comparing pre- 

 vious charts with the latest one available. The charts should be con- 

 sidered with the usual drifts of bergs about the Grand Banks in mind. 

 The main tracks that bergs usually take in the southern part of the 

 Labrador current are shown in Figure 30 on page 69 of Coast Guard 

 Bulletin 16. This chart sums up the berg drift information gathered 

 by the ice patrol up to the time of its publication in 1927. Also the 

 observed drifts of bergs during the current season should not be lost 

 sight of, though at times one can not assume that two bergs even 

 separated by only a few miles with no apparent difference in w^ater 

 or current will drift alike. The case cited above proves this. 



Another thing that affects the surface isotherms is the wind. If a 

 strong breeze has been blowing from one direction for several days, it 

 blows the surface water from one stream and covers the waters from 

 the other. During the past season after continued south winds a 

 marked increase in surface temperatures were noticed for perhaps 30 

 or more miles from the cold wall. Several daj^s after the wind died 

 down the surface temperatures would return to normal. So after a 

 continued unidirectional wind the surface temperature does not give 

 a true indication of the water that is affecting the movement of the 

 ice, and therefore can not be taken as a guide to berg drifts. 



While the distance between the isotherm lines gives some idea as 

 to the speed of the current there is no accurate method even to ap- 

 proximate the velocity of the current from an isotherm chart. The 

 velocity must be judged from past daily drifts or taken from the 

 dynamic current chart, if one is available. 



With all the available means of predicting the drifts of icebergs, 

 past knowledge of ice drifts, isotherm charts, and dynamic charts, the 

 ice observation officer believes that while this information is in- 

 valuable to the ice-patrol vessel in locating ice lost during a fog or 

 bad weather and in deciding the areas to be scouted for approaching 

 ice, the information is not accurate enough to be used in broadcasting 

 the probable positions of bergs not sighted recently due to fog or other 

 reasons. If an infallible current chart could be constructed fre- 

 quently enough to predict the positions of bergs not sighted for a week 

 or longer, the greatest hindrance to the ice patrol — fog — would be 

 beaten. 



