98 THE FUESH- WATER LOCHS OF SCOTLAND 
lake has a considerable influence in determining the depth to which 
wind-produced currents are felt ; and it is found that comparatively 
deep lakes which are small always have a greater range of temperature 
from top to bottom than longer lakes of equal depth. 
(2) Effect of Altitude. — The effect of altitude on the temperature 
of a lake requires no elaboration. Altitude enters into the tempera- 
ture of the atmosphere, and so into the temperature of the lake, for 
conduction from the atmosphere is one of the methods in which heat 
is communicated to a lake. Rarefaction of the atmosphere has also a 
small effect on the rate of conduction to or from a lake. Lakes at 
considerable altitudes have usually small drainage areas, and this too 
must be taken into account ; sometimes the quantity of water caught 
in the drainage area of a lake in the course of a year is sufficient to 
fill the lake-basin several times over, and it is evident that such a lake 
cannot be considered a stagnant body of water. The effect of water 
flowing into a lake, however, is chiefly felt at the surface, and the 
bottom temperature is not greatly affected, for, in general, drainage 
water is in summer warmer than the bulk of water in the lake, and 
in winter colder, and therefore remains on the surface.^ 
(3) Effect of Latitude. — In the same way latitude is a factor 
determining the temperature of a lake, owing to the difference in 
atmospheric conditions, and in the incidence of the sun's rays 
brought about by difference in latitude. Difference in latitude 
produces great changes in the range of temperature to be found in 
lakes. Thus in the Lake of Geneva the mean surface temperature in 
the harbour of Geneva for the years 1853 to 1875 for the month of 
February was 40° "9 Fahr., and for the month of August 65° -6— a 
range of nearly 25°. In Loch Ness, the means at Fort Augustus for 
these months for two consecutive years were respectively 41°*9 and 
54° -9 Fahr. — a range of only 13°, or little more than half the range 
in Lake Geneva. And it is not only the yearly range that diminishes 
with latitude, but also the temperature gradient in the lake. In low 
latitudes the heating is comparatively rapid, so that much greater 
temperature gradients are found. Thus in the Wolfgangsee (which 
is a lake with a maximum depth of 375 feet) from the surface to a 
depth of 70 feet there was a fall in temperature of 30° Fahr., whereas • 
in Loch Ness a fall [of 10° or 11° in a similar distance was all that 
was found, and the greatest range observed in any Scottish lake was 
21° -2 in Loch Achilty (see page 137). As will be seen in the 
sequel, large temperature gradients such as these have an important 
bearing on the temperature changes which occur in lakes. 
(4) Effect of Orientation. — The effect of orientation of a lake is 
not at first sight so apparent. By orientation is meant the direction 
^ But see pages 100-101. 
