the jet streams
'Jet streams' were
first discovered during the second world war. Pilots were regularly
flying between Britain and the United States of America and they
noticed that it was quicker to fly to the United Kingdom and
reported tailwinds of over 100 miles per hour. These winds blew in
narrow ribbons and were named 'jet streams'.
streams are closely monitored and forecast. Pilots want to know
where to find them as their added push will save them time and fuel,
and therefore money. But jet streams are not only important to
pilots. When Breitling Orbiter 3 became the first balloon to fly
non-stop around the world it used knowledge of the position of the
jet streams to speed up its flight.
are relatively narrow bands of strong wind in the upper levels of
the atmosphere. The winds blows 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
Since the earth
rotates, the axis is tilted, and there is more land mass in the
northern hemisphere than in the southern hemisphere, there are three
cell - Low latitude
air movement toward the equator that with heating, rises
vertically, with poleward movement in the upper atmosphere. This
forms a convection cell that dominates tropical and sub-tropical
cell - A mid-latitude
mean atmospheric circulation cell for weather named by Ferrel in
the 19th century. In this cell the air flows poleward and eastward
near the surface and equatorward and westward at higher levels.
cell - Air rises,
diverges, and travels toward the poles. Once over the poles, the
air sinks, forming the polar highs. At the surface air diverges
outward from the polar highs. Surface winds in the polar cell are
easterly (polar easterlies).
The earth's rotation
is also responsible for the jet stream to move from West to East.
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 (left) 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.
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 between the two locations
increase, 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. The actual appearence 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 jet streams shifts north moving into Canada by
Summer. As Autumn approaches and the sun's elevation decreases, the
jet stream moves south into the United States helping to bring
cooler air to the country.