EXTENDED-RANGE WEATHER FORECASTING 
zones, extending over many meridians, in which warm 
subtropical air borders on cold polar air (polar front). 
In the upper troposphere above these zones, there is always 
a narrow belt with extremely strong westerly winds (jet 
stream), which reach their maximum velocity at the tro- 
popause level. However, neither the polar front nor the 
jet stream runs uninterruptedly around the entire North- 
ern Hemiphere. 
SreventH Errrican THrorem: A zonal circulation 
exists only on rare occasions over all meridians of the 
middle latitudes. In most cases, regions with a prevailing 
zonal circulation and regions with a prevailing meridional 
circulation exist simultaneously. 
Types of Grosswetter. Since in middle latitudes of the 
Northern Hemisphere the character of the circulation 
is seldom uniform over the entire hemisphere, it is 
necessary to consider a division into separate circula- 
tion cells, which Seilkopf refers to as “‘special circula- 
tions” [52]. Furthermore, it is necessary to define an 
observational unit which lies between the daily weather 
situation of a smaller region and the world weather 
situation over the Northern Hemisphere or even the 
entire earth. This unit is the Grosswetterlage. Because 
of the existence of two continents (one of which has a 
large longitudinal extent) and two oceans on the North- 
ern Hemisphere, there usually exist four or five troughs 
and the same number of ridges in the Northern Hemi- 
sphere. Therefore, the most suitable regional delinea- 
tion of Grosswetterlagen results from a division of the 
total circulation of the Northern Hemisphere into five 
(partly overlapping) special circulation regions [17, pp. 
58-62]: 
North America (160°W-60°W) 
North Atlantic ( 70°W-0°) 
( 30°W-45°R) 
( 45°B-150°E) 
North Pacifie (150°E-135°W). 
At some future time it might perhaps be useful to 
combine the overlapping North Atlantic and the Euro- 
pean regions into a single circulation region. 
The temporal limits of a Grosswetterlage result from 
the following definition [13]: A Grosswetterlage is the 
mean pressure distribution (at sea level) for a time 
interval during which the position of the stationary 
(steermg) cyclones and anticyclones and the steering 
within a special circulation region remain essentially 
unchanged. Here ‘steering’? means the direction of 
propagation of 24-hr isallobars. The reference to the 
pressure distribution at sea level is necessary in order 
to enable one to apply this definition to observations of 
earlier years when there were no daily aerological ob- 
servations. On the other hand, by requiring constancy 
of the steering, the pressure distribution in the middle 
troposphere (5-km level) is indirectly taken into con- 
sideration. 
The mean pressure distribution during an arbitrary 
five-day period cannot be considered as a Grosswetter- 
lage. Rather, it is possible that by such an arbitrary 
formation of means, two different Grosswetterlagen 
might cancel each other. 
825 
Baur [17; 20, pp. 920-923] established 17 types of 
Grosswetterlagen for the circulation region of Europe— 
25 types, if finer subdivisions are included. For the cir- 
ENA 
Saree’: ae 
oN 
oN 
rs) 
iN 
x2 
ae 
ELE By 
Fie. 9 —Average topography of the 41-mb surface for July 
in the Northern Hemisphere. (After Scherhag [47]). 
culation region of the North Atlantic he introduced 16 
types of Grosswetterlagen. The 25 European Grosswetter 
types can be grouped into the followmg divisions: 
1. A high in the northwest region (between Green- 
land and Scandinavia). 
cp = 
Paes 
Fie. 10.—Mean pressure distribution (in mb) at sea level 
in the north polar region for June 4-12, 1947. 
2. A high in the west and southwest (between the 
Azores and England). 
3. A continental high. 
4. A high in the southeast and south. 
5. Westerly cyclonic flow. 
6. Central cyclonic situations. 
The 16 types of the North Atlantic Grosswetterlagen 
are grouped into zonal, meridional, and mixed situa- 
