Shepherds tend their flocks on the 
treeless, almost barren plateau. In the 
background are the Transhimalaya 
Mountains of central Tibet. 
seasonal variations in weather patterns 
would follow the same course year after 
year. But recent climate anomalies 
show that this is not the case. Obvious- 
ly, other factors must be involved in 
generating large meanders in the jet 
stream flow. The prime candidate in ef- 
fecting these changes is the process that 
supplies the upper atmosphere with 
heat energy from below. 
When the air layers on their east- 
ward flow rise over the Rocky Moun- 
tains, they are squeezed vertically by 
the stable stratosphere above them and 
are curved anticyclonically, that is, in a 
clockwise direction. This establishes a 
high-pressure, warm-air system over 
the Rockies and the Great Basin. In the 
lee of the Rockies, the warm air ex- 
pands, creating a low-pressure cy- 
clonic, or counterclockwise, system 
containing cooler air masses. Jet 
streams are affected by the horizontal 
contrast of temperature and pressure 
below them. Therefore, this combina- 
tion of anticyclonic and cyclonic air- 
flow in the lower atmosphere alters the 
path followed by the high-traveling jet 
streams. 
In the case of the Tibet plateau, tem- 
perature again seems to be the control- 
ling variable. The plateau is colder in 
winter and warmer in summer than the 
“free” atmosphere at the same eleva- 
tion above sea level over the adjacent 
land areas and oceans. A cold Tibetan 
plateau is frequently invaded and domi- 
nated by cold, low-pressure systems 
that are associated with cyclonic, or 
counterclockwise, airflow. The juxta- 
position of cold air over Tibet and 
warmer air over India, to the south- 
west, represents a horizontal tempera- 
ture contrast between the two regions 
and results in horizontal pressure con- 
trast, with low pressure over Tibet and 
high pressure over India. This pressure 
differential tends to be at a maximum 
near the tropopause level of the atmo- 
sphere (at approximately 30,000 to 
40,000 feet), where a strong jet stream 
skirts the Himalayas. 
A law of physics dictates that in the 
Northern Hemisphere, when the wind 
is at your back, low pressures with cold 
air must be to your left and high pres- 
sures with warm air to your right. (This 
rule, derived originally from observa- 
tions of low-level airflow, also applies 
to the jet streams.) Accordingly, the 
streams flowing around the Tibet pla- 
teau in winter move in a west-to-east di- 
rection. But when the sun starts to heat 
the plateau in spring, and the air over 
Tibet becomes warmer than the air at 
the same altitude over India, the pla- 
teau ceases to serve as a “refrigerator," 
causing a cyclonic curvature of the jet 
stream, and begins to act like a giant, el- 
evated “hot plate.” 
During the spring and summer, heat 
from the land rises and is mixed into the 
turbulent planetary boundary layer of 
the atmosphere. (This layer is the part 
of the atmosphere, about three-quar- 
ters of a mile thick, on the average, that 
lies immediately above the earth’s sur- 
face.) The layer tends to be thicker over 
warm surfaces than over cold ones. 
Hence a warm region acts on the flow 
of the lower atmosphere in much the 
same way that a wide mountain range 
acts on the jet streams, causing the air- 
flow to skirt anticyclonically — in a 
clockwise-curving arc. 
As the heating of the high Tibetan 
landmass continues, the horizontal 
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