understanding 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'.
Nowadays jet 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.
They 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
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 global circulations...
- 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 climates.
- 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.
- 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
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 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.