The Great Plain (central basin) now had an aver- 

 age surface temperature of 17° C. , with cooler waters 

 on the north shore than on the south shore. The cen- 

 tral part of the plain was covered with bottom water 

 of from 9.4° to 10°, while the 12° bottom isotherm 

 included nearly all of the plain except the southern 

 marginal zone from Sandusky to Ashtabula, where 

 17° bottom water was encountered 2 miles offshore 

 from both Lorain and F airport. 



Thus June found about 80 percent of the whole 

 lake area with bottom water under 12°. Surface 

 warming had not yet been sufficient to build up a 

 definite thermocline over any extensive area. 



Cruise 3. --At the termination of cruise 2 and 

 before the next regular cruise (4), the physical ob- 

 servations were confined to the Deep Hole area 

 where the 6° C. bottom isotherm was determined. 

 The horizontal shifting of the subsurface, cold- 

 water mass of this area was observed in the 1928 sur- 

 vey. It was therefore ccnsidered advisable this year 

 to determine the limits of 6° bottom water as fre- 

 quently as the survey program would permit. The 

 results of these determinations are discussed under 

 "Water movements. " 



The mean surface temperature from 13 stations 

 observed on cruise 3 was respectively 50 percent 

 and 12 percent greater than the means of the same 

 stations on cruises 2 and 4. As these cruises were 

 progressively 2 weeks apart, it would be logical to 

 expect that the mean for cruise 3 would be about 

 equal to the average of the means of cruises 2 and 4, 

 since the maximum surface temperature for the lake 

 as a whole was not reached until August (table 4). 

 But when one turns to the weather conditions, the 

 reason for the excessive temperatures of cruise 3 is 

 apparent. We find that on cruise 3 the stations were 

 occupied during generally calm weather and the sur- 

 face temperatures were not lowered by deep vertical 

 mixing, whereas on cruise 4 they were occupied im- 

 mediately following 3 days of stormy weather, and 

 the surface temperature was lowered temporarily by 

 mixing with the cold subsurface waters of this Deep 

 Hole area. 



An inspection of the means of the bottom tem- 

 peratures of these stations gives a clearer picture of 

 the seasonal warming. Here we find the mean for 

 cruises 2, 3, and 4 to be 5. 1°, 6. 0°, and 7. 8° C. 

 respectively. 



Cruise 4. --In July the mean lake tempera- 

 tures were: surface 18. 9° and bottom 12. 8°. 

 Table 4 shows that the maximum mean air temper- 

 ature was reached during this month, although the 

 surface water temperature continued to rise to a max- 

 imum in August. 



The coldest surface temperatures were in Long 

 Point Bay, where 13. 3° C. water was encountered, 

 with bottom readings of 11°. The cold bottom waters 

 of the adjacent Deep Hole had flowed into this shal- 

 low bay, and the storms which occurred just prior to 

 the occupation of these stations lowered the surface 

 temperature considerably by vertical mixing. This 

 condition is temporary as this bay is subject to wide 

 thermal fluctuations, due to the movements of the 

 cold-water mass of the Deep Hole. Probably the 

 mean surface temperature for the whole month was 

 lower in the Deep Hole region than it was in this 

 area; but, due to the impracticability of frequent 

 and simultaneous observations over the whole lake, 

 the situation could not be verified. 



The surface water of the eastern area was about 

 2° C. warmer on the southern and eastern sides than 

 in the northern zone where there was 17° water. The 

 surface water of the Deep Hole was 16° (fig. 6). 



Two new and interesting thermal conditions were 

 observed in the Great Plain on this cruise. As they 

 still prevailed on cruise 6 (August), they are prob- 

 ably normal summer phenomena and must be of con- 

 siderable biological importance. The first is the 

 presence of comparatively cold surface water in the 

 Port Stanley region; the second is the thin cold bot- 

 tom layer extending over the greater portion of the 

 Plain. The isotherms on figures 6 and 7 clearly 

 show these conditions. 



Although comprehensive temperature studies of 

 the whole lake had not been made prior to the sur- 

 vey, it was generally beUeved that the waters of the 

 Great Plain were nearly homothermous, as the great- 

 est depth is only 25 meters and the area is frequently 

 swept by strong winds. 



The prevailing wind direction for July at Cleve- 

 land was south. One might expect the leeward shore 

 to have higher surface temperatures, but observations 

 showed the situation to be reversed. Temperatures 

 were 17° C. off Port Stanley and 22° off Fairport 

 and Ashtabula. On the day that these stations were 



41 



