Jet streams are relatively narrow
bands of strong wind in the upper levels of the atmosphere. The winds blow from
west to east in jet streams but the flow often shifts to the north and south.
Jet streams follow the boundaries between hot and cold air.
Since these hot and cold air
boundaries are most pronounced in winter, jet streams are the strongest for
both the northern and southern hemisphere winters.
Why do the jet stream winds blow
from west to east? Recall from the previous section what the global wind
patterns would be like if the earth was not rotating. (The warm air rising at
the equator will move toward both poles.)
How the earth's rotation the effects the west to east direction of the jet stream |
We saw that the earth's rotation
divided this circulation into three cells. The earth's rotation is responsible
for the jet stream as well.
The motion of the air is not
directly north and south but is affected by the momentum the air has as it
moves away from the equator. The reason has to do with momentum and how fast a
location on or above the Earth moves relative to the Earth's axis.
Your speed relative to the
Earth's axis depends on your location. Someone standing on the equator is
moving much faster than someone standing on a 45° latitude line. In the graphic
(above right) the person at the position on the equator arrives at the yellow
line sooner than the other two.
Someone standing on a pole is not
moving at all (except that he or she would be slowly spinning). The speed of
the rotation is great enough to cause you to weigh one pound less at the
equator than you would at the north or south pole.
The momentum the air has as it
travels around the earth is conserved, which means as the air that's over the
equator starts moving toward one of the poles, it keeps its eastward motion
constant. The Earth below the air, however, moves slower as that air travels
toward the poles. The result is that the air moves faster and faster in an
easterly direction (relative to the Earth's surface below) the farther it moves
from the equator.
North hemisphere cross section showing jet streams and tropopause elevations. |
In addition, with the three-cell
circulations mentioned previously, the regions around 30° N/S and
50°-60° N/S are areas where temperature changes are the greatest. As the
difference in temperature increases between the two locations the strength of
the wind increases. Therefore, the regions around 30° N/S and 50°-60° N/S are
also regions where the wind, in the upper atmosphere, is the strongest.
The 50°-60° N/S region is where
the polar jet located with the subtropical jet located
around 30°N. Jet streams vary in height of four to eight miles and can reach
speeds of more than 275 mph (239 kts / 442 km/h).
The actual appearance of jet
streams result from the complex interaction between many variables - such as
the location of high and low pressure systems, warm and cold air, and seasonal changes.
They meander around the globe, dipping and rising in altitude/latitude,
splitting at times and forming eddies, and even disappearing altogether to
appear somewhere else.
Jet streams also "follow the
sun" in that as the sun's elevation increases each day in the spring, the
average latitude of the jet stream shifts poleward. (By Summer in the Northern
Hemisphere, it is typically found near the U.S. Canadian border.) As Autumn
approaches and the sun's elevation decreases, the jet stream's average latitude
moves toward the equator.
Jet streams are often indicated
by a line on a weather map indicating the location of the strongest wind.
However, jet streams are wider and not as distinct as a single line but are
actually regions where the wind speed increases toward a central core of greatest
strength.
One way of visualizing this is to
consider a river. The river's current is generally the strongest in the center
with decreasing strength as one approaches the river's bank. Therefore, it is
said that jet streams are "rivers of air".
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