A LIMNOLOGICAL STUDY OF THE FINGER LAKES. 
547 
2. This condition recurs annually, unless under extraordinary conditions of weather, 
and the annual differences are not great enough to invalidate or seriously weaken general 
conclusions based on a single year. 
3. A series of temperature observations taken in summer under good conditions of 
weather, and near the center of oscillation of a lake of regular form, gives a fair idea of 
the mean temperature of the water of the lake. 
4. In lakes of this type, all heat gained which is above 4° and which is found below 
a depth of 5 meters, has been conveyed there by mechanical agencies; by currents due, 
directly or indirectly, to wind. Such heat may be called wind-distributed heat. The 
same is true of most of the heat found between the depth of i meter and 5 meters. There 
is as yet no clear evidence that thermal convection currents aid appreciably in carrying 
heat downward. 
5. The thickness of the epilimnion in lakes of different size and otherwise comparable 
is a fair measure of the relative efficiency of the wind in distributing heat. 
SUMMER TEMPERATURES. 
As a result of the normal conditions of weather acting on the lake, it divides in 
summer into three well-known thermal regions. 
1. The epilimnion, a stratum in which the temperature is nearly uniform. The 
surface is usually the warmest part of the stratum. Under summer conditions the fall 
of temperature in this region varies from a small fraction of a degree to several degrees. 
The amount varies chiefly with the temperature of the surface and is therefore subject 
to diurnal variation. It is greatest in the afternoon of a hot, calm day; least in the early 
morning, when the surface may be cooler than the stratum immediately below it. 
2. The thermocline, the stratum of rapid cooling, whose limits are Somewhat arbi- 
trarily fixed as those of the region in which the fall of temperature equals or exceeds 
I degree per meter. Its upper limit is usually fairly definite, but below it grades off 
into the third region and its lower limit is often somewhat arbitrary. From 60 to 70 
per cent, or even more, of the fall in temperature is usually found here. The thermo- 
cline is subject to variation in thickness under the action of wind and of oscillations due 
to temperature seiches. At the center of the lake where these influences are least felt, 
it is still subject to oscillations of considerable amount. These may cause an increase 
or decrease of the thickness of the thermocline, and at its bottom isotherms may be 
drawn in or excluded by its extension or contraction. 
3. The hypolimnion, the region below the thermocline and extending to the bottom 
of the lake. In this region the temperature falls slowly, the temperature curve soon 
approaching a straight line. The amount of fall in this region varies greatly according 
to the conditions of the spring warming. It may be less than 3°, even in a layer more 
than 60 meters thick, or it may be as much as 6°. It may be as little as 1 1 per cent of 
the total fall in temperature or it may be nearly 40 per cent. 
