826 
tions. (For further details, see [13; 17; 20, pp. 920— 
923].) For the North American circulation region, Krick 
and Elliott have established a system of weather types.” 
Considering the pressure distribution at the 5-km 
level, we can classify the Grosswetterlagen in all special 
circulation regions of the temperate zone in the North- 
ern Hemisphere according to the following seven funda- 
mental types based on the circulation: 
1. Three types with zonal circulation: 
I. High pressure in the south; low pressure in 
the north; westerlies and jet stream reach 
farther south than normally. 
IJ. High pressure in the south; low pressure in 
the north; northern boundary of high pressure 
either in normal or more northerly position; 
westerlies and jet stream far to the north. 
III. High pressure in the north; low pressure in 
the south. 
2. Four types with meridional circulation: 
IV. Subtropical meridional flow; high pressure 
in the east; low pressure in the west. 
V. Polar meridional flow; high pressure in the 
west; low pressure in the east. 
VI. Meridional ridge. 
VII. Meridional trough. 
These seven types occur in all special circulation re- 
gions of the temperate zone in the Northern Hemi- 
sphere, but with different frequencies. 
Schematic illustrations of the pressure distribution 
and air flow im the middle troposphere occurring with 
these seven types are shown in [21]. 
Annual Variation of Grosswetterlagen. Frequency 
Maxima above Chance Limit (Weather Key-Days). The 
establishment of types of Grosswetterlagen and the de- 
termination of the corresponding weather represent 
only the first steps toward rational macrometeoro- 
logical research. Further steps must include the follow- 
ing: (1) determination of the seasonal trend of Gross- 
wetler types, (2) investigation to determine whether 
certain Grosswetterlagen occur at certain times of the 
year more frequently than according to chance, (3) 
determination of the persistence and repetition ten- 
dency of individual Grosswetter types and their de- 
pendence on the seasons and on the general atmospheric 
circulation, and (4) investigation of the frequency and 
the circumstances of transition from one Grosswetter- 
lage to another. There are many problems to be solved 
which require the closest coordination of theory, synop- 
tics, and statistics based on the probability theory. A 
clear-cut solution of these problems is of mcompa- 
rably greater value for extended-range forecasting, 
particularly for medium-range forecasting, than are 
the many harmonic and rhythm analyses still under- 
taken at several institutes. 
The determination of the annual course of the rela- 
tive frequency of European Grosswetter types in a 63-yr 
period showed [16] that in Europe one or more types of 
Grosswetterlagen occur in 22 intervals of several days 
2. Consult “Hxtended-Range Forecasting by Weather 
Types” by R. D. Elliott, pp. 834-840 in this Compendium. 
WEATHER FORECASTING 
each during the year with so high a relative frequency 
that it cannot be explained as chance fluctuation of a 
uniform distribution over the entire year. The number 
22 represents only the present state of our knowledge; 
it is possible that this number will be increased with 
the further increase of observational data. As an ex- 
ample, Fig. 11 gives a portion of the annual course of 
6 16 26 6 
APRIL 
IS 25 
JULY 
Fie. 11.—Seasonal variation of the relative frequency of 
the HN-situation from April 1 to August 1 for the period 
1881-1943. The dash-dot line represents the annual mean value 
of the relative frequency of the HN -situation among the other 
Grosswetterlagen, the dashed line represents the upper limit of 
the range of chance of this relative frequency, where the range 
of chance is defined in the usual manner as the range which 
contains 99.730 per cent of all values. 
16. 
MAY JUNE 
the relative frequency of the ‘““HN-situation,” a Gross- 
wetterlage which is characterized by a “steering high” 
or “central high”’ over the northeastern Atlantic Ocean 
between 30°W to 5°E and 58°N to 75°N. As can be 
seen from this figure, the relative frequency of occur- 
rence on May 25 and 26, as well as on May 29 to June 
3, exceeds the maximum chance limit. In such a sector 
of better-than-chance occurrence, the day of the great- 
est relative frequency is called a “weather key-day.” 
The general physical explanation of the annual course 
of the relative frequency of the European Grosswetter 
types by means of the annual course of msolation and 
the general circulation of the atmosphere is given else- 
where [17; 20, pp. 920-923]. 
It must be emphasized, however, that the relative 
frequency of a given Grosswetter type, at least in Europe, 
never reaches 50 per cent on any day of the year, and, 
even when several similar Grosswetter types are com- 
bined into one group, it never exceeds 70 per cent (see 
second empirical theorem). 
EreuteH EMerricaL THEOREM: J’he occurrence of cer- 
tain Grosswetter types is connected with the annual 
course of the general atmospheric circulation in such a 
manner that during certain parts of the year certain 
types occur more frequently than according to chance. 
However, they do not occur so frequently that, without 
the aid of other indications, an extended-range forecast 
could be based on them. 
Weather Key-Days and Weather Development. The 
weather key-days indicate, so to speak, the rhythm 
of the normal seasonal trend of the atmospheric cir- 
culation in the special circulation region under con- 
sideration. The term “‘rhythm,’’ however, is not to be 
interpreted as a periodic or quasi-periodic process, but 
its meaning in this connection is only a defined de- 
pendence upon time. The basic cause of this rhythm 
is the annual course of the incoming and outgoing 
