Jet Streams in the Northern Hemisphere 
Joe LeMonmer 
The normal jet stream pattern, upper drawing, differs from the strong wavy 
pattern, lower drawing, that correlates with regional temperature anomalies. 
Ridges tend to have warmer than normal temperatures (red), while troughs of 
the jet stream match regions of cold temperature anomalies (blue). 
recent weather anomalies in the United 
States. For example, during the winter 
of 1980-81, the jet streams, instead of 
moving south, remained along or to the 
north of the Canadian border in the 
Rocky Mountain region; to the east, 
they strayed over the New England re- 
gion. That brought to Colorado and 
neighboring states a lack of snow, se- 
vere drought conditions, an abundance 
of sunshine, and unseasonably warm 
weather, while the northeastern sea- 
board states were swept by arctic air 
masses and snowstorms. This situation 
was nearly reversed in the winter of 
1979-80, \yhen New York State’s Lake 
Placid, scene of the Winter Olympics, 
was starved of snow but record 
amounts fell in the Rocky Mountains 
under that winter’s main jet stream 
flow. 
The changeable behavior of the jet 
streams from one year to the next is not 
confined to the United States. Way- 
ward shifts of the jet streams have also 
been associated with crop failures in the 
USSR and droughts in China during 
the past four to five years. These events 
raise the question of what causes jet 
stream aberrations. 
Despite the great altitudes at which 
they circulate, jet streams are them- 
selves affected by terrestrial features, 
such as mountain ranges and high land- 
masses, and by such variables as 
changes in land- and sea-surface tem- 
peratures. In the Northern Hemisphere 
the principal geographical features ulti- 
mately responsible for jet stream mean- 
ders are the Rocky Mountains, the 
Himalayas, and the Tibet plateau in 
Southeast Asia. 
Global atmospheric airflow at mid- 
latitudes in and below the jet streams 
has a west-to-east orientation. The 
Rocky Mountain chain, whose highest 
peak is 20,320-foot Mount McKinley in 
the Alaska Range, extends some 3,000 
miles from northwest Alaska south- 
ward to central New Mexico. The 
Rockies lie almost exactly perpendicu- 
lar to the airflow pattern, presenting a 
formidable barrier. Because the moun- 
tains are so extensive, the air cannot 
easily move around them, and so it goes 
over them. The Tibet plateau, however, 
is a different kind of obstacle. On the 
south, the plateau is sheltered by the 
Himalayas, which extend in a broad 
curve some 1,500 miles and include 
peaks that rise more than 25,000 feet; 
on the north it is sheltered by the simi- 
larly long Tien Shan mountain chain, 
with peaks exceeding 20,000 feet. The 
average elevation of the plateau itself is 
more than 14,750 feet. 
Regardless of its name, the Tibet pla- 
teau is not an expanse of level land but a 
corrugated area of mountains and val- 
leys, containing the headwaters of sev- 
eral large rivers and many saltwater 
and freshwater lakes. The minimum el- 
evation of the valleys is 12,000 feet. De- 
spite the enormous height of the 
Himalayas and of the plateau itself, 
these barriers, unlike the Rockies, do 
not lie perpendicular to the airflow pat- 
tern nor are they insuperably extensive. 
Air can therefore flow around them. 
But this deflection of airflow gives rise, 
in turn, to large meanders in the global 
jet streams. And these meanders set the 
stage for the “Tibet connection” with 
weather and climate in the Northern 
Hemisphere, a phenomenon of far- 
reaching implications that can affect 
Indian monsoons, cause North Ameri- 
can summer heat waves, and that may 
be linked to the unusually cold North 
American winters of 1976 through 
1979. That such a linkage exists is evi- 
dent from weather maps. 
If daily tabulations of global atmo- 
spheric airflow and pressure distribu- 
tions for any month in any year are 
averaged, the “wiggles” associated with 
traveling waves in the airflow and per- 
turbations that cause the daily variabil- 
ity of local weather disappear. What is 
left is the stable wave pattern caused 
primarily by the topography of the 
earth’s surface. In regions without sig- 
nificant mountains, that is, over the At- 
lantic and Pacific oceans and over 
Europe, the observed mean monthly 
flow patterns are probably due to reso- 
nance in the effects of the two afore- 
mentioned major mountain obstacles. 
If the physical barriers of mountain 
ranges and a high plateau were the only 
factors acting on atmospheric airflow, 
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