TEMPERATURE OF SCOTTISH LAKES 
99 
in which the lake trends — north and south, or east and west. The 
orientation of a lake is important, as it determines whether the 
prevailing winds sweep along the whole length of the lake, or whether 
they merely blow across it. The effect of winds on a lake will be 
discussed in considerable detail later, but it may be stated that 
winds are of great importance in determining the temperature 
distribution, for they are the means of mixing the w^aters at the 
surface, which have been heated or cooled, with the deeper layers of 
water, thus helping to make temperature changes felt to a greater 
depth than would be possible by conduction or radiation. If the 
prevailing winds blow along the length of the lake they have a much 
greater mixing effect than if they merely blow across it. 
(5) Effect of Surroundings. — It is at once evident that the natural 
surroundings of a lake are very important. If the lake is surrounded 
by high hills, as is the case with many of our Scottish lakes, the pre- 
vailing winds will nearly always be deflected to blow along the lake, 
thus exercising a greater mixing effect than if they blew across it or 
at an angle. Again, one lake may be greatly sheltered from winds by 
surrounding hills and woods, while another, similar as regards size and 
shape, may be exposed to every wind that blows. The presence of hills 
also has its effect in directly cutting off sunshine from reaching the sur- 
face of the lake, and in the formation of clouds which obscure the sun. 
(6) Effect of General Shape. — It is evident that the shape of the 
basin of a lake has a considerable effect in determining its temperature 
conditions. The effect of the wind is different in deep and shallow 
lakes, as already indicated. Lakes with shelving shores also behave 
differently in some respects from lakes whose shores are steep. 
Narrow lakes in confined glens cannot be compared with broad and 
open sheets of water. 
These and many other points will be dealt with later, but I may 
state here that I have been criticised for generalising from a lake' 
such as Loch Ness, which is deep and narrow compared with other 
lakes. The criticism is fair, but it only shows the necessity for 
complete observations in lakes of all kinds. Forel insists that each 
lake has its own individuality, and that only by a careful and 
complete study of each lake can we approach the truth. Complete 
observations in a great number of lakes are not available, but it seems 
improbable that the temperature conditions observed in Lochs Ness 
and Garry should not have their counterpart in all lakes. 
Methods by which a Lake Gains or Loses Heat 
What are the methods by which a lake can gain or lose heat ? 
Some methods may be mentioned merely to be set aside as being too 
