824 
II, and to the northwest for region IV, thus producing 
a new high-pressure cell in region V, which, according 
to the investigations of Palmén and Rossby [53], re- 
mained there until July 6. 
ae ene 
Fie. 7.—Centers of anticyclones (@), ridges (©), and cy- 
clones (++) at the 5-km level from June 21 to July 5, 1949. 
Assuming that the meridional heat transport is the 
main effect of the general circulation, we can dis- 
tinguish the following principal forms of the general 
circulation [8; 17, pp. 49-55]. 
40° 
30° 20° 
Fic. 8.—Centers of anticyclones (@) and ridges (©) at the 
5-km level from July 23 to August 8, 1949. 
1. An unordered heat exchange. This occurs in west- 
east-travelling highs and lows of the lower troposphere 
with isobars running approximately from west to east 
at the 5-km level (zonal circulation). In this case, the 
southern portion of the troposphere (in the Northern 
Hemisphere) is warm, the northern portion is cold. As 
long as the flow equilibrium between warm and cold 
air remains undisturbed, a “frontal zone”’ ascending 
poleward exists in the troposphere with a strong merid- 
WEATHER FORECASTING 
ional temperature gradient [47, 52, 53]. Above this 
frontal zone, the isobars of the upper troposphere are 
crowded in a comparatively narrow belt, so that the 
west-east current reaches very high speeds (jet stream). 
The maximum of this zonal movement is found where 
the tropopause slopes down most strongly toward the 
pole. This form of circulation (Fig. 5) rarely develops 
as an upper circumpolar low as far south as the 55th 
parallel, as is so in the case illustrated in Fig. 5, and, 
according to our present experience, it usually de- 
velops in only one or two quadrants, and never in the 
entire temperate zone around the whole hemisphere. 
Even in the rare case represented in Fig. 5, the jet 
stream does not run uninterruptedly around the entire 
hemisphere, but parts of it are dissipated by divergent 
currents. 
2. The meridional circulation strips. An ample heat 
exchange is effected in these strips; quasi-stationary 
warm tropospheric anticyclones that reach up to the 
stratosphere and are displaced far poleward alternate 
with cold tropospheric troughs that reach far equator- 
ward and likewise extend to high altitudes (Fig. 6). 
Whereas these two principal forms occur during all 
seasons, the two following secondary forms occur only 
in certain seasons: 
3. The winter anticylcone in the lower troposphere 
[52]. Below the cireumpolar low at high altitudes there 
often exists during the winter an anticyclonic circula- 
tion system in the lower troposphere over the interior 
of the Arctic. When these arctic air masses penetrate 
to the south, additional cooling occurs through radia- 
tion over the continent, and wide regions of stationary 
cold anticyclones are thus formed. The frontal zone 
between these cold air masses and warm air to the 
south, and the resulting westward drift, are displaced 
far toward the equator. In Europe this displacement 
extends to the Mediterranean area, in North America 
to the northern edge of the Gulf of Mexico. 
4. The arctic anticyclone of the stratosphere in mid- 
summer. According to Baur and Philipps [23], the _ 
total insolation during the summer solstice is strongest 
at the pole, and thus the ultraviolet radiation is most 
likely also to be a maximum there. For thisreason, in the 
upper stratosphere a pressure gradient from the pole to 
the equator exists during this short season, in strong 
contrast to midwinter conditions. 
Figure 9 shows the average topography of the 41-mb 
surface (at approximately 22-km height) as it exists, 
according to Scherhag [47], in July over the Northern 
Hemisphere. In the Arctic this high pressure extends 
in some years to the earth’s surface, as in Europe from 
June 5 to June 13, 1947 (Fig. 10) before the extremely 
dry summer. 
Firra EmprricaL Turorem: On the average for all 
meridians and over long periods of time, the zonal and 
meridional components of the atmospheric circulation im 
the lower troposphere are nearly equal in middle latitudes 
of the Northern Hemisphere. 
SixtH EmprricaL Tarorrem: In the lower and middle 
troposphere on both hemispheres, there are always long 
