CYCLONIC 
The flurries begin shortly after the 
cold air arrives and continue as iong 
as the temperature difference obtains, 
and the upper wind flow carries the 
clouds over the particular location. 
It is sometimes observed that snow 
flurries of this sort occur when the 
wind at the surface is blowing off the 
land. In this case observations reveal 
the wind at levels slightly above the 
surface, say 500 to 1000 m, coming 
from a direction off the water. Ivi- 
dently convection has taken place in 
the off-land air, which is colder than 
the water surface, and the resultant 
clouds and flurries are carried back 
over the land by the upper component 
of the circulation. 
In mountainous country orographic 
effects upon the cold air often lead 
to cloudiness and precipitation. There 
are several factors which enter into 
the problem; only a brief outline can 
be given here. Forced vertical ccn- 
vection, brought about by the rough- 
ness of the surface features, pro- 
duces clouds, if the lifting is suffi- 
cient. But the vertical temperature 
and moisture distribution in the cold 
air may oppose the ascent of the 
air. That is to say, the cold current 
may be dry and stable; the air 
will then tend to flow around obstruc- 
tions rather than over them. Since 
vertical motion depends on the lapse 
rate it is logical to suppose there 
should be a diurnal variation in the 
cloudiness resulting from the above 
cause. During the day time, wren 
the lowest layers are warmest, the 
lapse rate is steepest; on this ac- 
count less force is required to produce 
the vertical displacement necessary 
Wake 
In the preceding two articles of 
this series warm and cold fronts were 
discussed as independent entities. At 
the close of the last article it was 
STRUCTURE 31 
to form snow flurries. At night the 
snow flurries either disappear or dim- 
inish in intensity. 
To the lee of a mountain range 
the air is forced to descend, which 
opposes the formation of lower 
clouds. In these regions, therefore, 
rapid clearing takes place after a 
cold-front passage. The temperature, 
owing to the foehn effect, is also 
higher than that normally prevailing 
at the same level in the cold gir 
mass. 
Sometimes a cold front may de- 
generate into two discontinuities. In 
this case each discontinuity acts as a 
distinct front. the passage being ac- 
companied by fairly definite changes 
in the meteorological elements. These 
secondary fronts are developed when 
the cold front is accelerating, for it 
can be shown that the velocity. field 
of air flow is such that there must be 
a descending current of air some dis- 
tance behind the main front. The 
descending air leads to the formation 
of a new discontinuity between the 
sinking cold air adiabatically warmed, 
and the fresh polar air which is not 
descending. The transition zone of 
descending air frequently appears in 
upper air soundings as a dry and 
stable layer. 
While cold fronts and warm fronts 
have been treated independently thus 
far, they must not be considered 
as separate and disconnected phe- 
nomena. Both are necessary parts of 
the irregular circulation of the mid- 
latitudes. Both are complementary 
parts of the cyclone, the structure 
of which will be treated in the next 
article. 
ELEMENTS OF CYCLONIC STRUCTURE 
pointed out that both are compie- 
mentary parts of the cyclone of the 
surface weather map. The problem 
of the formation and maintenance 
