and in 8 percent they were about equal in 

 both waters. However, the upper tempera- 

 tures were as often higher at the shallow 

 as at the deep stations and were about 

 equal in 8 percent of the comparisons. 



Along both shores the thermocline 

 temperatures generally decreased progres- 

 sively from the southern to the northern 

 areas. Comparisons were made on dates that 

 were no more than three days apart — 72 

 percent on successive days. The records 

 also indicate that these temperatures were 

 nearly always higher along the west oppo- 

 site the east shore in the Grand Haven and 

 the Kenosha and Milwaukee regions. In 

 order to obtain sufficient data the compar- 

 able records involved a maximum of four 

 days apart. Not enough information was 

 available to compare the thermocline tem- 

 peratures of the other opposite areas. The 

 data of the various regions must not be 

 considered as a unit. 



In all regions the thermocline tem- 

 peratures of both the shallow and deep 

 stations generally increased progressively 

 from June to August and then declined there- 

 after. Since the trend was the same in 

 all areas and depths the combined data can 

 be employed to indicate the general nature 

 of the seasonal shift in the entire area 

 below the Frankfort-Algoma line. The com- 

 bined temperatures are shown below. 



* Only of deep waters. 



General conclusions can also involve 

 the actual and interpolated calculated 

 depths of the post-spring cold waters, 

 especially those of 5° (41° F.) and less. 

 It was first determined that the 1930 and 

 1931 depth records of the 5-degree water 

 could be combined. Comparisons had been 

 made between the individual records of 

 shallow and deep stations, of the months, 

 and of all regions. The general depth at 

 all stations averaged 34 meters (112 ft.) 

 in 1930 and 33 (108 ft.) in 1931. 



Depths of the 5-degree (41° F.) 

 water also fluctuated irregularly from 

 month to month in all waters off both 

 shores, so that the records of a single 

 month may be combined for all regions. 

 The average depths were practically the 

 same during the months of May-September 

 varying at random from 33 to 37 meters 

 (108 to 121 ft,). In October and November, 

 however, the depths avereiged 51 meters 

 (167 ft.). 



The estimated depths of the 5-degree 

 water did not vary consistently between 

 the shallow and deep stations on the same 

 dates. Only about half (55 percent) of 

 the records indicated that this water had 

 descended lower at the deep stations, down 

 to an average of 35 meters (115 ft.) in 

 comparison with 32 meters (105 ft.) in 

 shallow waters. 



In both the shallow and deep waters 

 off both shores the depth of the 5-degree 

 water decreased progressively from the 

 southern to the northern areas. Along the 

 east shore the average ranged from 42 

 meters (138 ft.) to 33 (108 ft.) and off 

 the west shore from 39 (128 ft.) to 30 

 (98 ft,). 



The bottom temperatures of 1930 and 

 1931 were compared for each month from 

 June to August of the shallow and deep sta- 

 tions (excluding those in depths of 189-250 

 meters, i.e., 620-820 ft.) of each region. 

 Since these detailed comparable records 

 indicated no significant differences they 

 were combined for all areas for each year. 

 The monthly averages never varied more 

 than O.r (0.18° F.) between the three 

 months of the same year or the same month 

 of both years. The combined records of all 

 months were identical in the two years for 

 the same waters— 4.3° (39.7° F.) in the 

 shallow and 4.0° (39.0* F.) in the deep 

 waters. The grand averages of all data 

 were identical (4,1° C; 39,4° F.) in 1930 

 and 1931, 



Bottom temperatures of a region did 

 not increase progressively in all waters 

 with advance of the season during May- 

 September, The few records indicated that 

 in general the temperatures did rise in 

 October and November, The combined data 

 of each month also revealed no progressive 

 trend through September in either the shal- 

 low or deep waters along the east or west 



23 



