Birge—Heat Budgets of American and European Lakes. 169 
fairly be compared with each other on the basis of their wind- 
distributed heat as well as on that of the annual heat budget. 
As shown by Table A, the conclusions to be drawn from the two 
types of budget are closely similar. 
This statement holds for lakes which lie so far to the south 
that the temperature of 4° is reached early in the open season. 
If a lake lies far to the north it may not reach that temperature 
until a date so late as seriously to affect the remainder of its 
budget. This is clearly the case with Ladoga (See Table A, Fig. 
1, and p. 45), in which 15,000 cal., or nearly one-half of its bud¬ 
get, were required to raise the water from the winter tempera¬ 
ture to 4°. 
This is in itself a large amount, although it is not so large as 
that demanded by Mjosen in 1901, and it is only slightly larger 
than that called for by Skaneateles lake in 1911. But the water 
of Mjosen reaches 4° early in June and the New York lakes 
reach that temperature early in May or even in April. Homen 
states (’02, p. 3) that Ladoga does not warm to 4° until the end 
of July. If this is the case, it is surprising that Ladoga accumu¬ 
lates so much heat above 4° as 18,000 cal. per sq. cm., since Ho¬ 
men also states that the temperature of the surface begins to fall 
by the end of August. In any case, it is clear that the summer 
heat-income of Wettern, Mjosen, and Ladoga is reduced by the 
late date at which the temperature of 4° is reached. It is also 
clear that the summer heat-income of these lakes cannot fairly 
be compared with that of lakes further south. 
But the summer heat-income of lakes which reach 4° in April 
or early May in but little, if at all, affected by the amount of heat 
necessary to raise the water to 4°. The gains of heat may go on 
well into August, or even later, and so much heat is delivered to 
the lake which cannot be absorbed in any case, that in these lakes 
the summer heat-income is practically independent of the date 
when the temperature of 4° is reached, or of the amount of heat 
needed to warm the water to that point. 
Thus, the size of each of the two members of the annual heat 
budget—-that below and that above 4°—is independent of that 
of the other in many lakes. The variations in the lower part 
of the budget are due to winter conditions and are especially 
dependent on irregular accidents of weather rather than on the 
average conditions of the season. This part of the budget, 
therefore, introduces a considerable element of variation into 
