TEMPERATURE OF SCOTTISH LAKES 
107 
waters. It will make what follows more intelligible if the nature of 
the temperature changes taking place in lakes of this class in the 
course of the year is briefly indicated. A similar series of changes 
will take place in temperate lakes during the summer and autumn, 
for in these seasons distribution of temperature is similar to that 
found in tropical lakes. 
The most convenient starting-point from which to consider the 
temperature of such lakes is spring, when the water is all of uniform 
temperature. As summer comes on, the lake gains heat and becomes 
stratified. This is evidenced by a gradual rise in temperature of the 
surface layers, the increase being also noticeable at considerable 
FIRST PHASE SECOND PHASE THIRD PHASE 
TEMPERATURE TEMPERATURE TEMPERATURE 
Fig. 41. Fig. 42. Fig. 43. 
depths ; the falling off in the rate of increase from the surface to the 
bottom is very rapid. At the end of summer there is a short period 
during which there is very little loss or gain of heat, but during 
which important changes take place. Figs. 41 and 42 show the nature 
of the distribution of temperature in spring and summer. The spring 
type is represented by a straight line, the lake being of uniform 
temperature from top to bottom. The summer type is a smoothly 
curved line, showing rapid changes of temperature at the surface and 
very slow changes at greater depths. Fig. 43 shows the autumn type, 
when the discontinuity layer (or Sprungschicht ^) has made its appear- 
ance. There is at the surface a layer of water, say 50 feet in depth, 
1 Spru7igschicht is the word used by Continental writers. I prefer to use the 
word "discontinuity," or "discontinuity layer," to describe the nature of the tempera- 
ture distribution. American writers speak of the ''thermocline." 
