TEMPERATURE OF SCOTTISH LAKES
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depends solely upon the water reaching its maximum density point ;
but when that point is reached, the water is in a condition which
makes freezing possible. Before that point is reached, water cooled by
contact with the air or otherwise is heavier than the water immediately
below it, and accordingly it sinks until it is mixed with the water
through which it falls, or until it reaches water of equal density. In
this way warm water is always brought to the surface, and freezing is
practically impossible.
But when the maximum density point has been reached, the water
which is cooled at the surface no longer sinks. Its tendency is to
remain at the surface, and as the rate of conduction in water is small,
freezing will readily take place in water at and below the maximum
density point.
It is possible for some of the water in a lake to be above the
maximum density point while the remainder is below. Fig. 40 shows
the temperature distribution to be found in such a case. There are
not many actual observations of such a temperature distribution
recorded, but it is a matter of common observation that shallow bays
in a lake, which as a whole is very seldom covered with ice, may freeze
over readily ; and in most cases this means that, while the bulk of the
water in the lake is above the maximum density point, there is water
round the shores that has been cooled below that point.
As a matter of fact, however, the water in a lake is usually cooled
considerably below the maximum density point before freezing takes
place. There is usually a lapse of time between the date at which
the water in the lake has been cooled to the maximum density point
and the date at which the air temperature falls below freezing point.
During this time the water in the lake is gradually falling in temperature, and owing to the circulation of the water produced by the wind,
the whole body of water contained in the lake, and not merely the
surface layer, falls in temperature. Thus Buchanan found that the
mean temperature of the water under the ice in Loch Lomond was
34° Fahr., or about 5° below the maximum density point, and in
Linlithgow Loch 37° Fahr.
Freezing takes place most readily in cairn weather, as the effect
of winds and storms is to mix the water cooled at the surface with
the water below it. At times the freezing takes place all over the
surface of the lake in one night, while at other times it begins at the
shores, and the ice gradually creeps in to the centre of the lake. This
latter is the manner in which lakes with shallow shores freeze. As
previously explained, the water round about shallow shores cools more
rapidly than the water in the centre of the lake, and so freezes more
readily. When once a fringe of ice has formed round the shores,
there are set up convection currents from the shore towards the
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