818 
lies still farther to the south, and the daily insolation 
is still increasing, an overheating of central Europe 
causes an irreversible pressure fall over the continent 
and a corresponding pressure rise over the North At- 
lantic. In July, however, when the Azores High is far- 
WEATHER FORECASTING 
ture in the first half of December in central Europe was 
more than 3C above normal, the following midwinter 
(January and February) was too warm in all 13 cases. 
However, if the first half of December was below 
normal by just as many degrees, the following mid- 
(a) 
ther to the north and the daily insolation is decreasing, 
the pressure fall caused by overheating is no longer 
irreversible and, during the following August and Sep- 
Taste I. Larce Positive TrmMenratuRE ANOMALIES IN 
BERLIN IN JUNE AND SUBSEQUENT PRECIPITATION AND 
TEMPERATURE ANOMALIES IN CENTRAL HUROPE 
Precipitation depar- | Temperature depar- 
Temperature depar- |ture incentral Europe|turein central Europe 
Year ture in Berlin for June| for the following for the following 
1-15 (cent. deg.) midsummer (average midsummer 
of 16 stations) (mm) (cent. deg.) 
1855 +3.1 +34 —0.1 
1858 +4.5 +43 —0.2 
1866 +3.0 +26 —1.5 
1877 +3.3 +42 +0.4 
1889 +6.5 +16 —0.9 
1896 +3.3 +25 —0.7 
1897 +2.1 +22 0.0 
1910 +5.2 +44 1.3 
1915 +3.4 +17 —1.4 
1917 2.7 +16 +0.2 
1930 +3. 1 +60 —0.6 
1937 +4.1 —16 +0.2 
1940 +2.8 +43 —1.5 
1948 +3.9 +45 —0.4 
tember, it is possible for high-pressure cells to split off 
repeatedly from the Azores High and to migrate toward 
central Europe. 
Also of importance for the physical explanation is 
the fact that in many cases a repetition or change 
tendency exists only for anomalies of one sign. During 
the period 1848-1947 for instance, when the tempera- 
(b) 
Fra. 1.—Isopleths of the correlation coefficients between the mean barometric pressure of the preceding 10-day period for 
various places in Europe and the mean barometric pressure at Potsdam for the following 10-day period for the years 1893- 
1932 (aumber of cases NV = 40 X 10 = 400), (a) when the ten preceding 10-day periods end on the days between June 15 and 
June 24, (6) when the ten preceding 10-day periods end on the days between July 26 and August 4. 
winter was too cold in only 4 out of 14 cases; in 10 
cases, the midwinter was warmer than normal. 
The considerations of this section can be summarized 
as follows: 
Turrp EmprricaL THnorEeM: The repetition tendency 
of weather anomalies varies with the seasons and, as a 
rule, 1s not the same for positive and negative anomalies. 
Correlations between Successive Weather Anomalies in 
Distant Regions. A large part of the nonsimultaneous 
correlations of weather anomalies in distant regions 
can be traced to the persistence tendency of the gen- 
eral atmospheric circulation, as has been shown by 
Schell [46]. However, these correlations are, for the 
larger part, significant only in the tropics and sub- 
tropics and in the Southern Hemisphere. In the tem- 
perate zone of the Northern Hemisphere only a few 
exceed the limit of chance. Here, those correlations 
that point toward a change of the circulation are more 
important; in this respect, the ‘general circulation” 
is less important as a rule than the “special large-scale 
circulations” into which the general circulation of the 
Northern Hemisphere resolves (see p. 825). 
Particularly, the atmospheric circulation over the 
North Atlantic is, to a great extent, a closed system 
that follows certain rules of its own (see p. 817). Gen- 
erally, the pressure gradient between the Azores and 
Iceland determines the strength of the zonal circula- 
tion over the North Atlantic Ocean. According to 
Baur [7], however, the preceding zonal pressure gra- 
