OBSERVATIONAL STUDIES OF GENERAL CIRCULATION PATTERNS 
mean charts may be no more serious than on daily 
charts. In the following pages it will be seen that mean 
circulations behave as though they were true physical 
entities. 
Normal Circulation of the Troposphere and Lower 
Stratosphere. The prevailing motion of the greatest 
mass of the earth’s atmosphere is in general eastward. 
The most recent normal maps prepared by the U. 8. 
Weather Bureau [30, 50] and by the British Meteor- 
ological Office [9] effectively bring to light the existence 
of a vast circumpolar whirl which, while somewhat 
obscured at the earth’s surface, becomes stronger with 
elevation until near the base of the stratosphere it 
assumes the characteristics of a sharp maximum in 
speed. This narrow band of high speed, called the jet 
stream, and illustrated by Willett [56] and Hess [24] 
in north-south mean wintertime sections (not repro- 
duced), seems to be found at most meridians around the 
Northern Hemisphere. What meager data are available 
in the Southern Hemisphere (worked up by Flohn [22]) 
also suggest a jet stream there. A series of unpublished 
vertical cross sections for each 20° of longitude around 
the Northern Hemisphere have been prepared at the 
U.S. Weather Bureau as part of the work in construct- 
ing normal monthly maps at various levels up to 19 km 
[80]. From these cross sections, showing the geostrophic 
zonal wind component as a function of elevation, the 
latitude, strength, and horizontal shear across the jet 
stream have been entered and analyzed around the 
Northern Hemisphere as shown in Figs. 3 and 4 [88]. 
Fig. 3.—Average position and strength of the jet stream 
for January, prepared from eighteen hemispheric meridional 
cross sections. Heavy solid lines indicate geostrophic wind 
speed in miles per hour at the level of maximum speed. Heavy 
aa represent axis of the jet. (After Namias and Clapp 
The elevation of the jet, not shown on these figures, is 
everywhere in the range between 11 and 14 km. The 
material presented in these figures must, of course, be 
503 
taken with some reservation, and one cannot scale off 
from them numerical values upon which to draw re- 
fined conclusions. Besides, they represent only the zonal 
component of flow; but this is by far the largest com- 
ponent, and an analysis of the total flow (not shown) 
gives a quite similar picture of the normal character of 
the jet stream. 
The characteristic features of the normal jet stream 
seem to be: 
1. Its nature is essentially circumpolar in both sum- 
mer and winter. 
2. It is considerably farther north and weaker in 
summer than m winter—from a surprisingly low lati- 
tude of about 20°N to 35°N in winter, to perhaps 35°N 
to 45°N im summer, and with roughly double the speed 
in winter. 
3. The location of the jet stream appears to coincide 
with the strongest meridional temperature gradient 
just below it in the mid-troposphere. 
4. The jet stream is found where the tropopause has 
its greatest change in elevation. 
5. The ideal circumpolar appearance of the jet stream 
is marred by appreciable longitudinal differences in 
strength. Thus in winter (Fig. 3) the jet stream reaches 
its maximum strength along and just off the east coasts 
of Asia and North America and decays in the eastern 
parts of the oceans. Another maximum appears over 
Africa. 
6. The distribution of wind speed across the jet 
stream shows very strong lateral shear both to the north 
and to the south; the vertical component of the total ab- 
solute vorticity north of the jet stream (not illustrated 
here) suggests approximate constancy with latitude. 
To a considerable extent charts constructed in mid- 
troposphere (Figs. 5 and 6) bear a striking resemblance 
to those at the level of the jet stream. This might be 
