103 
1908-9.] Temperature Observations in Loch Garry. 
less than over Loch Ness. The period of observation in Loch Garry was 
particularly sunless. 
Some interesting observations on the freezing of the lake were made 
during the spring of the year. It does not follow that because a lake is 
of the temperate class that it will become frozen over every year. The 
classification depends solely upon the temperature of the water reaching* 
the maximum density point. When that point is reached the lake is in a 
suitable condition for freezing, for water which is cooled at the surface by 
conduction from the atmosphere or by radiation remains at the surface and 
does not sink. In point of fact, however, the water in a lake is usually 
cooled considerably below the maximum density point before freezing occurs. 
There is usually an interval of time between the date at which the 
temperature of the water in a lake reaches the maximum density point 
and the date at which the temperature of the atmosphere falls sufficiently 
to cause freezing ; and during this time the water all through the lake is 
falling in temperature, owing to the circulation of the water produced by 
winds ; and though, as will be seen from Table II. , the surface temperature 
fell as low as 37 F., freezing all over the lake did not take place. On several 
occasions, however, notably in the month of March, during frosty, clear 
nights large irregular patches of ice were formed in a single night, with a 
thickness in one case of as much as half an inch. Isolated patches of ice 
were formed in mid lake, and when observed in the early morning these 
appeared not to be connected with the shallow shore waters, where one 
would naturally expect freezing to take place most readily. These patches 
rapidly disappeared whenever a breeze sprang up to mix the thin surface 
layer, which had been rapidly cooled during the night, with the warmer 
waters below. It is thought that the irregularity of the patches of ice 
formed may be due to differences of surface tension in the lake, which 
would have an effect on the ripples produced by light winds, and thus on 
the mixing of the surface layers of the lake. Local differences at the 
surface may also cause differences in the rate of radiation from the lake and 
evaporation at the surface, and further contribute to the irregular formation 
of ice. In lakes with shallow shores and bays, freezing would probably 
occur first by the formation of a shore fringe of ice, gradually extending* 
out into the lake ; but the shores of Loch Garry are for the most part steep, 
and it is probable that when the lake becomes covered with ice (as it does 
in any ordinary winter) it will be covered all over by a thin sheet of ice in 
a single night in calm weather, and that if the weather continues calm this 
sheet will gradually grow thicker by conduction. For further discussion 
of the observations on formation of ice in Loch Garry, reference is made to 
