134 THE FRESH-WATER LOCHS OF SCOTLAND 
Effect of Lakes on the Climate of Their Surroundings 
The temperature of the atmosphere is one of the factors determining 
the temperature distribution in lakes ; but lakes, especially those of 
considerable size, also have an effect on the temperature of the 
atmosphere. 
In the case of Loch Ness and Loch Garry rough estimates were 
made of the quantity of heat given out by the lakes during autumn 
and winter. The estimates were based on a calculation of the 
maximum and minimum quantities of heat in the lakes in summer 
and winter respectively, as determined from actual temperature 
observations. Similar observations were made by Forel in the Lake 
of Geneva. 
The results were as follows : — 
Lake. 
Total Quantity 
of Heat set free. 
Gram Calories 
Quantity set 
free per Square 
Centimetre of 
Surface. 
Gram Calories. 
Geneva . 
43-6 
75,000 
Ness 
1-9 
34,000 
Garry 
* 
20,000 
The quantity of heat set free per unit area by the Lake of Geneva is 
very much greater than that set free by Loch Ness, and that set free 
by Loch Ness is greater than that set free by Loch Garry. It would 
appear, then, that the larger a lake is, the greater the quantity of 
heat it is able to set free in autumn and winter, not only as a whole, 
but per unit surface. The reason of this is probably that in large 
lakes the period at which heat begins to pass from the lake occurs 
later than in small lakes, whose temperature rises comparatively 
rapidly. In them the point at which there is equality between 
the surface temperature and the average temperature of the 
atmosphere occurs earlier than in large lakes, and consequently the 
point up to which the latter gain heat by conduction from the 
atmosphere is later in the year, and a greater quantity of heat is 
stored up per unit area. 
The difference in latitude between the Lake of Geneva and Loch 
Ness, and the climatic conditions, also account in part for the 
difference between the two lakes. 
The quantity of heat set free is enormous in the case of the I^ake 
of Geneva. According to Forel, it is equal to the heat set free by 
the combustion of 54 million tons of coal. A similar calculation 
