380 THE FRESH- WATER LOCHS OF SCOTLAND 
and quite polar lakes, which as a rule are covered with ice throughout 
the year. Between these two extreme limits all conceivable transitions 
occur. 
It may further be mentioned that the annual range of temperature 
variation for all the lakes of the zone is slight, and for many probably 
slighter than in any other zone. In the Scottish mountain lakes at 
the surface the yearly temperature variation is only about 5-13° C. 
(Loch Ness, 41°-5-56°-3 Fahr., Wedderburn, lOOTa, p. 412); for 
certain Norwegian high mountain lakes only about 0-2° (Holmsen, 
1902); Thingvallavatn 1-11°, Myvatn 0-121° (Wesenberg-Lund, 
1906, pp. 1105 and 1140); Mjosen 0-12° (Pettersson, 1902, p. 14); 
Huitfeldt-Kaas, 1905, reports 17° '3, but this temperature hardly 
occurs every year ; Ladoga 0-9° '9 ; Wettern 0-1 3° '32 (Pettersson, 
1902). The ice phenomena of Norway have been specially studied 
by Holmsen in his fundamental work (1902), by Ahlenius (1900, p. 28) ; 
see further Holmsen (1902, pp. 1-15) ; owing to the more special 
character of this exceedingly interesting literature it is merely men- 
tioned here. 
This comparatively low summer temperature is common to all 
the lakes of this zone ; only exceptionally it may probably exceed 
12-14° C. The bottom temperature of many of the deep lakes does 
not sink below 4° C. In the temperate lakes of the northern European 
zone we find two periods of circulation (spring and autumn), separating 
a long winter period of stagnation from a short summer period of 
stagnation ; during the greater part of the year " inverse stratifica- 
tion prevails. In these lakes we meet with the so-called " Sprung- 
schicht," which only exceptionally occurs in the lakes of the arctic 
zone, and at any rate has hitherto hardly been discerned there. 
Ahlenius found it in Saggat lake, about 68° N. lat. (1900, p. 35). 
In many cases, at any rate, we may account for the formation of a 
" Sprungschicht in the following way: — The variations in the 
temperature of the air, day and night, are now so great throughout 
such long periods of the summer half-year that uniform heating of 
the surface water is no longer possible. Owing to the cooling of the 
surface at night and during periods of cold weather, vertical currents 
which equalise the temperature arise ; in different seasons they reach 
different depths. Above these depths a somewhat uniform tempera- 
ture is consequently met with ; below them the temperature slowly 
decreases towards the bottom. The decrease in heat proceeds more 
slowly the deeper the water, most quickly at the upper limit, i.e. nearest 
the lower limit of the upper uniform, warm layer. Here the varia- 
tions in temperature may be so great that they proceed by jumps, 
and therefore this layer, according to the usage introduced by Richter, 
is generally called " Sprungschicht (thermocline by the Americans). 
