up (July 30, September 10) and usually warm 

 water had descended (August 12, 1930 and 

 1931). This temperature was restricted to 

 depths of 29 to 59 meters (95 to 194 ft.) 

 at the deep stations where it had descended 

 almost always with the advance of the sea- 

 son and exceeded the depths at the shallow 

 stations on corresponding dates except on 

 August 12, 1931. 



Thermoclines had not formed on June 24 

 and July 21, 1930, in either shallow or 

 deep water but did develop in both areas on 

 July 30 and August 12. A thermocline 

 appeared earlier in 1931 thjin in 1930 but 

 not at both stations on June 25 and July 8. 

 Most of the thermoclines occurred at levels 

 between 11 and 18 meters (36 and 59 ft.). 

 An upwelling raised the thermocline to 6 

 meters (20 ft.) on July 30, 1930, only at 

 the shallow station, and on August 12 of 

 both years and October 5, 1931, the descend- 

 ing of a uniform temperature developed a 

 thermocline at depths between 20 and 27 

 meters (66 and 89 ft.) at three stations and 

 between 37 and 40 meters (121 and 131 ft.) 

 at station 134. The deep- cuid shallow-water 

 thermoclines varied on the same dates with 

 respect to depths and/or temperatures, the 

 former usually shallower and their tempera- 

 tures higher. No thermocline had developed 

 on October 15, 1931. The thermal gradients 

 ranged from 2" to 10° (4° to 18° F.) with 

 an average of 5° (9° F.) and covered depth 

 intervals of 2 to 8 meters (7 to 26 ft.) 

 with an average of 3.6 meters (11.8 ft.). 



GENERALIZATIONS CONCERNING THE AREAS 

 BELOW THE FRANKFORT-ALGOMA LINE 



Surface temperatures were usually 

 higher in shallow theui in deep water on the 

 same dates early in the summer. This situa- 

 tion was generally reversed during late 

 July and early August until at least late 

 fall in those regions where comparable data 

 were available. Subsurface temperatures, 

 either of the upper layers or at all depths, 

 followed with few exceptions the trend of 

 surface temperatures and were, therefore, 

 also higher at the shallow than at the deep 

 stations early in the summer but usually 

 lower after late July or early August. 



When the changes of the subsurface 

 temperatures did not extend to the bottom 

 those of the depths below the shifted upper 

 level temperatures increased or decreased 



to the bottom or fluctuated at different 

 depths or remained constant. The maximum 

 depths of these upper temperatures ranged 

 from 3 to 70 meters (10 to 230 ft.) al- 

 though seldom beyond 50 meters (164 ft.). 

 Occasionally upwellings decreased the tem- 

 peratures of the upper strata somewhere 

 between the surface and the 50-meter (164 

 ft.) level during the summer months. On 

 two occasions cold air had also reduced the 

 summer temperatures down to at least 18 

 meters (59 ft.) and perhaps deeper. Sub- 

 surface temperatures usually warmed most 

 rapidly at the shallow than at the deep 

 stations although the temperatures of the 

 latter stations had gradually approached 

 and often exceeded those of the former as 

 the season advanced. 



Surface temperatures of the deep-water 

 stations off both shores averaged higher 

 during the summer months (June-August) in 

 1931 than in 1930. The greatest discrepen- 

 cies occurred in July. Differences between 

 the two years were not consistent in the 

 shallow waters. 



Surface temperatures of the different 

 regions increased with the advance of the 

 season usually reaching the climax late 

 July or early August. Since the monthly 

 trends were similar in 1930 and 1931 and 

 in shallow and deep waters off both shores, 

 all of the data were combined and the 

 averages recorded below. In general the 

 temperature reached its peak in August and 

 thereafter declined gradually but did not 

 yet reach the May average in November. 



Differences between the temperatures 

 of the several regions can be ascertained 

 by comparing records obtained on the two 

 nearest days within a period of four days. 

 Forty-5ix percent of the records were ob- 

 tained on successive days. 



Surface temperatures of both shallow 

 and deep water decreased almost consistently 



21 



