in the greater depths during the period 

 from June 24 to August 27, 1931. The low 

 temperatures near and at the bottom of 

 these deep stations were no doubt the re- 

 sult of increased pressure that decreased 

 the temperatures of maximum density. 



Not until July did the deep-water 

 temperatures of 1930 approached and finally 

 surpassed the shallow-water temperatures 

 at various levels including all depths on 

 the last two paired dates. In 1931 the 

 temperatures were lower at the deep than 

 at the shallow stations at all depths in 

 mid-May but only down to depths of 8 to 25 

 meters (26 to 82 ft.) through August 18. 

 On the last three paired dates the deep- 

 station temperatures were higher down to 

 levels of 12 to 15 meters (39 to 49 ft.) 

 and to the bottom on August 27. The upper 

 waters, including the surface, were warmer 

 during the summer months of 1931 than of 

 1930, 



Five-degree (41" F.) water was gen- 

 erally restricted during the summer of 1930 

 to depths of 30 to 37 meters (98 to 121 

 ft.) at the shallow stations and 35 to 49 

 meters (115 to 161 ft.) at the deep sta- 

 tions, except on June 18 and 27 when it was 

 found at a level of sibout 15 meters (49 

 ft.) at a deep station. In 1931 this tem- 

 perature, which did not appear until early 

 June, was recorded at depths of 20 to 32 

 meters (66 to 105 ft.) in shallow water and 

 25 to 50 meters (82 to 164 ft.) in deep 

 water. 



The bottom temperature of the shallow 

 stations reached a maximum of 4.6° on 

 June 27, 1930, and 4.5° on August 26, 1931, 

 (40.3° and 40.1° F.) but on other dates 

 fluctuated from 4.0° to 4.3° (39.2° to 39.7° 

 F.) and 3.8° to 4.2° (38.8° to 39.6° F.) 

 in 1930 jmd 1931 respectively. At the deep 

 stations it ranged from 3.9° to 4.0° (39.0° 

 to 39.2° F.) in 1930 and from 3.8° to 4.0° 

 (38.8° to 39.2° F.) in 1931, except the 

 single maximum of 4.2° (39.6° F.) and the 

 temperatures of 3.6° and 3.7° (38.5° and 

 38.7° F.) recorded at depths of 220 to 250 

 meters (722 to 820 ft.). 



A thermocline had not been observed 

 in 1930 until August 5 but only at the 

 shallow station at depths of 17 to 20 

 meters (56 to 66 ft.). The first thermo- 

 cline of 1931 was recorded on June 23 at 

 depths of 10 to 12 meters (33 to 39 ft.) 



at a deep station. It probably was also 

 present at the same level on June 24 at 

 the 220-meter (722 ft.) station. Not un- 

 til July 6 did a thermocline persist to 

 the last date at all stations, in contrast 

 to the situation in 1930. Except on 

 August 27, the thermocline was always at 

 a greater depth in the shallow than in the 

 deep areas on the same dates. In both 

 waters the thermocline fluctuated between 

 depths of 5 and 28 meters (16 and 92 ft.). 

 The ranges of the thermocline depth inter- 

 vals and temperatures were very closely 

 the same in both waters. They varied 

 from 2 to 7 meters (7 to 23 ft.) and 2° to 

 13° (4° to 23° F.) with general averages 

 of 4 meters (13 ft.) and 7° (13° F.). On 

 September 4 two thermoclines hsid formed 

 between depths of 15 and 23 meters (49 and 

 76 ft.). 



STATIONS OFF WAUKEGAN 



Temperatures off the Waukegan shore 

 were obtained only on November 11 in 1930 

 and on five dates in 1931 from May 11 to 

 November 2 (table 5). The 1930 station 

 was 34 meters deep (112 ft.). The depths 

 of the three 1931 stations ranged from 50 

 to 88 meters (164 to 289 ft.) and averaged 

 70 meters (230 ft.). 



The surface temperatures taken on 

 the same day varied very little with the 

 depths. They increased from 4.7° (40.^ F.) 

 on May 11 to 20.9° (69.6° F.) on June 30 

 and decreased to 11.0° (51.8° F.) on Novem- 

 ber 2. On November 11, 1930, the tempera- 

 ture of 5.8° (42.4° F.) had extended from 

 top to bottom. On May 26 asid June 30, 

 1931, 5-degree (41° F.) water was confined 

 to depths of 30 to 40 meters (89 to 131 

 ft.) but was lowered to 47 to 52 meters 

 (154 to 171 ft.) in the fall. The 1931 

 bottom temperatures ranged from 4.0° in 

 the spring to 4.8° in the fall (39,2° to 

 40.6° F,), 



The subsurface temperatures increased 

 at all depths on May 26 to nearly the same 

 degrees at both stations but only down to 

 25 meters (82 ft.) in June when they were 

 higher at the deeper station at a level of 

 10 meters (33 ft,) and beyond. In October 

 and November 1931 the waters had cooled 

 off down to about 10 meters (33 ft.) but 

 were still warmer than those of June at 

 the lower levels. The thermoclines were 



13 



