TEMPERATURE OF THE HYDROSPHERE 197 



to disappear in the latter, only to appear in reversed order as win- 

 ter approaches, when the polar type of reversed stratification is 

 pronounced. This will disappear again in spring and give place to 

 the tropical type. The minimum and maximum surface tempera- 

 tures of fresh-water lakes are : for tropical lakes -]- 4° C. and 

 -|- 25° C. to -+-30° C. ; for polar lakes 0° and +4° C. ; and for 

 temperate lakes below + 4° i" winter, and above + 4° in summer, 

 seldom rising, in the deeper lakes of the temperate zone, above 25° 

 C. In all cases the temperature of the surface waters is lower than 

 that of the air immediately above it in summer, and higher in 

 winter. The depth to which the seasonal variation penetrates is 

 about 100 meters. 



Differences of temperature also exist between the littoral and 

 open lake or pelagic district, the former being warmer in summer 

 and colder in winter than the latter. As has already been indi- 

 cated, the bottom temperatures of tropical lakes are generally above 

 4° C, though those of subtemperate ones may be at times as low as 

 4° C. Temperate lakes, even those inclining toward either extreme, 

 i. e., subtropical and subpolar, have a normal bottom temperature of 

 4°, though the former may at times be greater than 4° and the lat- 

 ter less. Polar lakes have a normal bottom temperature lower than 

 4° C, though the subtemperate ones may occasionally have as high 

 a bottom temperature as 4°. Lake Geneva, having a depth of 309 

 m. (surface elevation 375 m.), is beyond the influence of seasonal 

 variation (100 m.), nevertheless a difference of from 0.1° to 0.3° 

 between seasons has been observed in the deepest layers. This is 

 regarded as chiefly due to the sinking down of sediment-bearing 

 waters of higher temperature. The actual temperatures at the bot- 

 tom of this lake are from +4° to + 5° C, while the temperature 

 of the littoral belt varies from -j- 15° to -j- 25° C. in summer, that 

 of the inflowing Rhone being + 10° to -(- 15°. In winter the tem- 

 perature of the littoral region sinks to +4.5° or to +5.5° in dif- 

 ferent years ; only near the shore, in the shallow littoral region, 

 does the temperature fall below 4° and may fall to 0° with the 

 formation of ice. This results in the formation of a reversed ther- 

 mal stratification, whereas that of the littoral region is normal or 

 direct, as shown in the accompanying diagram (Fig. 30), copied 

 from Forel. The line along which the densest water of 4° tem- 

 perature reaches the surface has been called the thermal barrier. 



Freezing of Lakes. Where reversed thermal stratification ex- 

 ists the temperature on the surface may sink to 0°, and ice will be 

 formed. This is normal in polar lakes, usual during the colder 

 months in temperate lakes, and may occur in the shallow littoral belt 



