EXTENDED-RANGE FORECASTING 
clones which for certain periods happen to find some 
favorite life history and path. 
In later years fresh emphasis on the importance of 
the centers of action in governing general weather con- 
ditions for long periods was afforded by Baur, whose 
work forms the basis for another article on extended- 
range forecasting elsewhere in this volume. Baur recog- 
nized certain broad-scale (macroscopic) features of the 
general circulation which were effective in “steering” 
the individual cyclones and anticyclones along fairly 
well determined paths. He referred to these as Gross- 
wetterlagen, a term now known the world over as de- 
noting the broadscale weather situation as distinct from 
the day-to-day snapshot of the weather map. More- 
over, it was Baur who was largely instrumental in asso- 
ciating the large-scale weather situation observed at 
the surface with the flow patterns in the middle and 
high troposphere and in ascribing to this latter flow 
pattern the property of steerimg. Contemporaneously, 
the Russian meteorologist, V. P. B. Multanovsky, had 
also discovered rather indirectly the large-scale weather 
situation in “natural periods.” During these periods 
cyclones and anticyclones tend to follow in much the 
same tracks as their predecessors, that is, a series of 
cyclones (or anticyclones) during a natural period ap- 
pear to move within a restricted region. 
The work of these two investigators was certainly not 
unique in the meteorological world, for there may be 
found in the literature of the past half century con- 
siderable evidence that many weather forecasters had 
been toying with these ideas. But Multanovsky and, 
particularly, Baur became the most persistent advo- 
cates of the large-scale approach. 
Despite the fact that the concept of Grosswetter- 
lagen enjoyed increasing attention prior to World War 
II, no one had succeeded in developing a technique for 
predicting the strange and different large-scale weather 
situations which unquestionably showed up in the form 
of the natural periods of Multanovsky, in the steering 
patterns of Baur, or in the multitudinous systems of 
map typing proposed by various groups throughout 
the world. 
Indeed it was not until the late 1930’s that a physical 
theory of the evolution of different forms and positions 
of the great centers of action was proposed. This ap- 
peared in two papers: one by C.-G. Rossby and col- 
laborators, ‘‘Relations between Variations in the In- 
tensity of the Zonal Circulation and the Displacement 
of the Semi-Permanent Centers of Action” [17], and 
another by J. Bjerknes, ‘‘The Theory of Extratropical 
Cyclone Formation” [3]. From these epoch-making 
papers and considerable subsequent work, particularly 
by the American school of meteorologists, it has become 
increasingly clear that while earlier investigators were 
correct in stressing the broad-scale aspects of long-range 
forecasting, the problem probably involves an even larger 
scale than they had assumed, perhaps world-wide inter- 
actions. The recognition of the global nature of the 
1. Consult ‘‘Extended-Range Weather Forecasting” by F. 
Baur, pp. 814-833. 
803 
problem arose essentially from two facts which in the 
past ten years have been documented with considerable 
empirical evidence: (1) that the behavior of any center 
of action depends upon that of others in the same hemi- 
sphere, regardless of how remote; and (2) that the speed 
of propagation of planetary wave energy from one sys- 
tem to another often takes place at a rate appreciably 
greater than the actual speed of the air particles in any 
observable atmospheric layer. The credit for the first 
discovery belongs largely to Rossby as indicated in his 
1939 article [17] and in subsequent papers. To him also 
belongs much of the credit for proposing on theoretical 
grounds the second atmospheric characteristic of “‘dis- 
persion,” although this had been discovered earlier in 
forecasting practice by the author [14]. 
To the knowledge of the present writer there exists 
only one method of attack, to be described later, which 
utilizes directly these two concepts through the medium 
of hemispheric surface and upper-level charts. This 
method, first practiced in the United States, is now 
gaining ground in certain European countries, and dur- 
ing the past twelve years hundreds of related papers 
have appeared, some of which are summarized else- 
where in this Compendium (e.g., see articles dealing 
with the general circulation). 
Yet it must not be assumed that this vast research 
has led to amazing success in the prognostication of 
weather for long periods. To be sure, prediction is the 
ultimate goal here just as it is in all sciences. The pomt 
is that these global methods for the first time enable us 
to explain on a rational physical basis the evolution of 
large-scale weather types (Grosswetterlagen). The degree 
to which this “explanation” can be expressed in quanti- 
tative terms will determine the predictive skill of the 
method. 
Once the two phenomena, hemispheric teleconnection 
of the centers of action and the dispersive character of 
planetary atmospheric waves, are recognized as facts, 
it becomes obvious that any system of extended-range 
forecasting based solely upon the data for a limited 
area of the hemisphere is bound to have imperfections. 
The degree of imperfection will presumably increase as 
the area considered in preparing the prediction is di- 
minished. For this reason certain forecasting methods 
to be described and compared suffer inherent difficulties 
which place upon their maximum possible degree of 
skill a ceiling which is well removed from perfection. 
Among the methods to which these restrictions par- 
ticularly apply are those involving symmetry points, 
singularities, regional weather types, and trends (kine- 
matics). 
The foregoing paragraph is not meant to indicate that 
these methods are of no value. Indeed, in our present 
imperfect state of knowledge of extended-range fore- 
casting these tools, used judiciously, can assist ma- 
terially in obtaining forecasts of some degree of skill. 
But these tools in themselves, without consideration of 
interhemispheric reactions, can hardly be expected to 
raise forecasting skill to a satisfactory level. 
