390 



Alf Trolle. 



On June 3rd 1908, when the investigation was made, the air 

 temperature was negative and the ice had not begun to melt. The 

 thickness of the ice was ca. l^m. and it covered the whole lake. 



There could be no question at this time of heat penetrating from 

 the atmosphere through the ice, as the air-temperature had not yet 

 been positive and the temperatures in the lake should therefore be 

 unchanged the same as in winter. The temperature diagram shows 

 now, that the temperature of the lake from immediately below the 

 ice down to the bottom was ca. + 4° С 



We may picture the following course of events: 



TejnperatuTft г j 6 e +Г г i e в*2'г ♦ & e*-3* ate e+'f' 



6 8+5°2 Ч e e*€' 



1 



VI 



Г 



б 3 



.a 

 a* 



и 5 



.., 



— 1 — 1 — 1 — 1 — 



1 1 ■ t 



— r — 1 — 1 — 1 — 



'111 



Jill 



.._s^ 









Thzchiess^ 



of tJve u:o 









^=o^ 



^ 









































station LVII. 



In the course of the summer 1907 the lake was heated up to about 

 10° С (we have no direct observations in the lake, but the river in 

 its immediate neighbourhood had this temperature) and then in the 

 course of the autumn it became cooled down to + 4° C. (this cool- 

 ing process has probably proceeded with great rapidity, as it gave 

 rise to convection currents in the lake). After the lake had reached 

 + 4° and the water had reached its greatest density, the convec- 

 tion currents ceased and, as the weather was quite calm, only the sur- 

 face layer was cooled any further. An ice-covering was formed, but 

 just under the ice the temperature was constantly + 4° and this con- 

 dition persisted throughout the whole winter. 



Calm freshwater lakes such as that mentioned here thus act as a 

 kind of heat reservoir, storing up a part of the summer heat. The 

 effect of this was, for example, that the lake became earlier free of ice 

 in the spring than the sea and fjords in the neighbourhood. 



