(40 of 45 comparisons) from south to north 

 along both shores during periods that 

 rainged variously from May to November in 

 1930 aind 1931. Reductions between the ex- 

 treme regions along the east shore averaged 

 2.0° (3.6° F.) in shallow waters and 3.1° 

 (5.6° F.) in deep waters. Along the west 

 coast corresponding differences averaged 

 4.9° (8.8° F.) and 5.7° (10,3° F.). The 

 regional surface temperatures had decreased 

 to a greater degree along the west than 

 along the east shore. 



Waters of the southernmost areas were 

 first to reach a temperature of 20° C. 

 (68° F.). It was recorded on June 30 and 

 July 1, 1931, but not until the latter half 

 of July in the other regions. This temper- 

 ature had not been recorded at all in the 

 Manitowoc-Two Rivers region. As a matter 

 of fact the 1930 and 1931 peak temperatures 

 decreased from south to north along both 

 shores and ranged from 24.4° (75.9° F.) to 

 18.6° (65.5° F.). 



Comparisons of surface temperatures 

 of opposite regions indicated that all 

 waters off the east coast were generally 

 colder to about the same degree (average 

 1.4° C. ; 2.5° F.) than those off the west 

 coast during the summer months, except the 

 waters off St. Joseph-South Haven where 

 the temperatures were higher (average 2.2* 

 C; 4.0° F.) than those off Waukegan and 

 also off Kenosha-Racine (average 3.7° C; 

 6.7° F.) except early in August. In August 

 cind September the general situation was 

 also reversed in the other regions. The 

 temperatures were then usually about 1° 

 (1.8° F. ) higher off the east shore. 



began to disappear gradually in October 

 and completely in November. An unusual 

 condition existed in the Ludington-F rank- 

 fort region in 1930 when a thermocline was 

 formed on August 5 in shallow water but on 

 none of the other dates of June, July, and 

 August. 



When comparing the deep- and shallow- 

 water therraoclines of the same dates it 

 was discovered that 67 percent had devel- 

 oped in overlapping depths and 33 percent 

 in different strata. Nor did the depths 

 of the thermoclines vary consistently with 

 any of the different regions. The thermo- 

 clines did, however, on the average shift 

 to greater depths with the advance of the 

 season. Excluding the four extreme figures 

 shown in parentheses, the combined ranges 

 and averages of the depths of both years 

 are listed below. 



Number of 

 Month records 



Meter 



Range 



Averages 



Feet 



Meter 



Feet 



June 8 



July 28 



August 28 



September 12 



October 2 



November 1 



4-18(33) 13-59(108) 8.8-12.5 29-41 



6-25 20-82 11.9-17.0 39-56 



5-28(40) 16-92(131) 13.2-17.3 43-57 



11-32 36-105 19.9-23.5 65-77 



21-30 69-98 21.5-28.5 70-94 



40-44 131-144 40.0-44.0 131-144 



Because the ranges of the thermocline 

 depth intervals of a region did not vary 

 consistently with the deep and shallow sta- 

 tions or with the two years, they have 

 been combined. It was then found that the 

 averages of these intervals as well as 

 their thermal gradients were very closely 

 the same in the different regions and ex- 

 actly the same off both shores — 4 meters 

 (13 ft.) and 6° (10.8° F.). 



The differences between the surface 

 temperatures of the regions along the same 

 shore or opposite each other were not the 

 results of the variations of the depths of 

 the water. The other probable causal fac- 

 tors were not investigated, 



Thermoclines did not develop below 

 the Frankfort-Algoma line until the latter 

 half of June in any waters except the St. 

 Joseph-South Haven area where one was re- 

 corded on June 9, 1931, but only in shallow 

 water. The June records were actually 

 isolated incidents. It was not until July, 

 usually the latter half in 1930 and early 

 July in 1931, that the thermoclines per- 

 sisted throughout the season until fall. 

 The availeible data indicated that they 



The 1930 and 1931 thermocline tem- 

 peratures did not appear to differ signi- 

 ficantly on the proximate dates in either 

 the shallow or deep waters of the same 

 region. Only the temperatures of the upper 

 layers were about twice as often somewhat 

 higher in 1931 than in 1930. The data of 

 both years may be combined whenever necessary. 



As expected, the temperatures of 

 nearly all (83 percent) of the deep- and 

 shallow-water thermoclines, observed on the 

 same dates, had overlapped. However, they 

 were three times as often higher in general 

 at the deep than at the shallow stations. 

 In 71 percent of the comparisons the bottom 

 temperatures of the thermoclines were high- 

 er in the deep than in the shallow water 



22 



