TEMPERATURE OF SCOTTISH LAKES
103
(4) Convection. — Radiation and conduction are the principal
ultimate sources of gain or loss of heat. Currents produced by
convection and by wind are the principal causes of the temperature
distribution found in lakes. In spring and summer, when the surfacewater is being rapidly heated up, there forms at the surface a layer of
water of much higher temperature than the layers immediately below
it, and convection currents are set up which equalise the temperature
near the surface. I have observed at Dores, on Loch Ness, in perfectly
calm weather and out of reach of river influences, the surface
temperature to change as much as 6° Fahr. in two minutes. Again,
in calm frosty weather there is a similar action of convection currents,
the surface-water is rapidly cooled down, and the resulting difference
of temperature between the water at the surface and the lower layers
is equalised or minimised by convection currents. These currents
were clearly demonstrated by the electrical recorder used on Loch
Ness. Even the cooling which takes place in the course of cold spring
evenings is followed by rapid convection currents, which were at
times shown in a startling manner by the recorder.
Convection currents are, however, not limited to the surface.
When the discontinuity layer (see page 117) and the temperature
seiche are in evidence, there are convection currents set up in the
neighbourhood of the discontinuity.
It is inevitable that there
should be such currents where there are two layers of water of
widely different temperatures in contact, especially as the temperature seiche and wind currents cause relative motion between the
two layers.
(5) Wind.—The effect of wind in determining the temperature
changes in lakes has never, I think, been given its due place. To the
effect of winds more than to any other thing I attribute not only the
cycle of changes which occurs in a lake, but also the absolute quantity
of heat which becomes stored up in it. If one could imagine a lake
which is never troubled with winds, and the waters of which are ever
calm, the problem of its temperature changes would be very easy of
solution—there would be no wind currents to carry warm water to
its depths and no temperature seiche. There would be no sudden
differences in temperature from top to bottom, but a gradual change
throughout. The transference of heat from top to bottom would take
place almost entirely by conduction, and, as has already been seen, the
transference of heat by conduction is very slow.
The detailed examination of the effect of winds will be more conveniently dealt with after an explanation of some of the phenomena
which appear. Mention is only made of it at this point for the sake
of completeness.