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Proceedings of the Royal Society of Edinburgh. [Sess. 
the discontinuity in temperature can be explained. It is clear that the 
method of the formation of stratus described above represents a dividing 
line between the two methods already described, — i.e. the arrival of warmer 
air at the higher level in mild westerly conditions, and the intrusion of a 
cold surface current in easterly conditions — and that every intermediate 
method of formation may take place. In fact, all three methods of forma- 
tion may be considered as due to the increase of the westerly component 
of the wind velocity above the level where the clouds form. A cold north- 
easterly type not infrequently gives way to a milder north-westerly type, 
with an increase in the quantity of stratus. 
Other Factors in the Problem. 
The reversed temperature gradient, which may be produced by one of 
the methods described above, is certainly the primary cause of the formation 
of anticyclonic stratus, but other factors have also to be considered. Among 
these are humidity, the difference in wind velocity above and below the 
clouds, and radiation. In damp weather the stratus cloud may have a 
thickness of 1500 feet or more, and low cloud masses underneath may be 
joined to it. Occasionally these may even cause showers of rain under the 
stratus; in France this sometimes happens both with north and with west 
winds, both of which are from the sea. 
It is undoubtedly the rapid increase in the wind velocity just above the 
stratus in westerly conditions which produces the wavy effect so commonly 
seen. Difference in wind velocity may also sometimes cause clouds to form, 
when otherwise the cold air would lie undisturbed underneath the warm air. 
In winter, if a frost prevailed, clouds forming in this way would usually 
soon cause a thaw, as they would prevent radiation from the ground, but 
allow some of the sun’s heat to pass through them. The frost at the 
beginning of February 1911 appeared to end in this way; on the night of 
February 2, stratus appeared simultaneously over the greater part of 
Britain, with a light northerly wind, and a thaw followed next day. 
Judging from autumn observations, I should say that in winter anti- 
cyclonic frosts there are probably often two quite distinct reversed gradients, 
the lower one within 1500 feet of the ground. If stratus clouds formed 
below the upper reversed gradient, the lower one would to a large extent 
die out. 
In summer the most important effect of radiation is the absorption of 
the sun’s heat by the clouds, which results in their gradual dissolution. 
The clouds do not always disappear altogether, as the effect of “ cooling by 
warming ” may maintain patches of the clouds ; cumuli may also rise from 
