970 CLIMATOLOGY 
the manner which Bergeron, Linke, and Dinies en- 
visaged, though it must be remembered that they are 
typical values and not means or normals. 
TaseE II. Comparison or TEMPERATURES OVER THE AZORES 
ence of atmospheric stability in the more recent studies 
of microclimate (e.g., Pasquill’s discussion of the dif- 
fusion of water vapor and heat near the ground [28]). 
WITH THOSE oF AIR REACHING BRITAIN FROM THE AZORES 
Summer 
BGAN; GOD)..ccceccossevdrscsnsdosbue 900 800 700 
Tropical air arriving over Azores (°C). 16 13 8 
Tropical air arriving over Britain from 
thevAzoresi(C) a ashe eee 14 11 5 
Maritime polar air arriving over Bri- 
tain from the Azores (°C)........... 11 4 —2 
Winter 
600 500 900 800 700 600 500 
1 =7/ 10 7 1 —6 —15 
=2 =ilil 8 4 =2 =) —18 
—9 =19) 4 —3 = —17 =27 
TaB.eE III. Comparison OF TEMPERATURES OVER ICELAND WITH THOSE oF AIR REACHING BRITAIN FROM IcELAND 
Summer Winter 
lalgnedmh (Gans), 06 nesadeesenonnusasapono. 900 800 700 600 500 900 800 700 600 500 
Polar air arriving over Iceland (°C).. 1 —6 —12 —18 —28 —13 —19 —26 —33 —43 
Polar air reaching Britain from Den- 
Taark Strait (CC), oe ee 8 1 SW ei fh 80 = =) |) a1) | 3 || =< 
Polar air reaching Britain from Jan 
Misyent(t@) ieee erase eae 5 —1 —7 —16 —26 —6 —12 —20 —29 —39 
Difficulties of Air-Mass Climatic Description 
The primary difficulties in using air masses in clima- 
tological work are (1) the difficulty of a definition of air 
masses which can have world-wide application, (2) 
the difficulty of dealing with frontal weather, and (3) 
the complications introduced by vertical motion in 
anticyclones and wedges. 
To some extent the first difficulty is also encountered 
by the synoptic meteorologist, but in practice the 
forecaster for a particular region concentrates on the 
air-mass types which are most likely to occur in that 
region. It is of far less importance to him that the 
definitions in the more remote parts of his chart are not 
precisely in accordance with the physical conditions 
there, since before the air from those remote regions 
has entered into his working area, it will have become 
modified to a great extent. In his original formulation 
Bergeron [7] set out a comparatively simple classifica- 
tion, but as is pointed out by Willett [33] this classifica- 
tion was made primarily from the study of the air masses 
which affected northwestern Europe and though it was 
possible to apply it on world-wide scale, it did not suit 
the local conditions of the North American Continent. 
To a very considerable extent the difficulties in adopt- 
ing a world-wide classification are difficulties of scale. 
The broad view which encompasses the world will fail 
if attention is fixed on one continent alone, because of 
necessity the detail considered will be greater. In the 
same way the classification suitable to the continent 
will be less applicable to a smaller unit, state or coun- 
try. It would be logical to apply the same reasoning 
right down to microclimatie studies, since the essence 
of air-mass analysis is the vertical structure, and it is 
possible to conceive a discussion of microclimate by 
means of micro-air-mass analysis. Indeed that is in fact 
the method which leads to the insistence on the influ- 
In the second place a difficulty, which has no counter- 
part for the synoptic meteorologist, arises in the use of 
air masses in climatology, namely the difficulty of the 
expression of the effects of frontal passages. It may be 
comparatively simple to give a generalised picture of 
the weather which is likely to occur in the cores of the 
air masses, even when they are suffering marked modi- 
fications, but the generalisation of the weather at frontal 
zones in all their manifold diversities is most complex. 
A smoothing of the elements, which is the result of the 
climatology of the mean, masks much that is essential, 
but how should frontal zones be shown and classified 
in air-mass climatology? The plotting of the normal 
position of deformation fields and fronts such as is 
shown by Bergeron [7] over the Pacific reveals much 
more, particularly in the tropics, but it is not an ideal 
generalisation of the arctic front nor of the polar fronts 
in higher latitudes, because of the wide range through 
which those fronts are likely to oscillate. 
The third difficulty is the greatest of all. Air masses 
are not only transformed by the surfaces over which 
they move and the radiation which they receive; they 
are also profoundly influenced by subsidence and at 
present we are not in a position to estimate the magni- 
tudes of those effects. All that seems possible, therefore, 
is to show how air masses have in the past been affected 
by moving over different surfaces with a proviso that 
no account is taken of the presence or absence of sub- 
sidence. 
The Use of Synoptic Maps in Climatology 
From what has been said above it is clear that for 
certain purposes the climatologist’s task and that of 
the synoptic forecaster were becoming increasingly 
dependent on the same technique. Indeed the synoptic 
forecaster from his memory of his maps could write 
