Download Jet Stream Overview

WEATHER/
By Jack Williams
WILEY POSTS high-altitude flights
get some credit for discovering the
jet stream.
THEAIRUPTHERE
JET STREAMS AFFECT WEATHER WHERE YOU FLY
hen you're watching the weather on local television—which helps
you see the big picture before you obtain a preflight weather briefing—the meteorologist is likely to show a simplified upper air map
to help explain the current or expected weather while pointing to
the "jet stream" on the map.
W
JETSTREAM
WIND SPEEDS
VARY WITH
"JETSTREAKS"
OF FASTER WINDS
EMBEDDED
IN SLOWER
WINDS.
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FLIGHTTRAINING.AOPA.ORG
Often the map's "jet stream" doesn't meet the
strict definition: "a relatively narrow river of
very strong horizontal winds (usually 50 knots
or greater) embedded in the winds that circle
Earth aloft." That's OK. For your purposes, all
you need to know is generally which way winds
are blowing high in the atmosphere.
This is because high-altitude winds follow
the boundaries between deep layers of warm
and cold air. These boundaries are fronts at the
surface and are usually locations of potentially
dangerous weather.
If the "jet stream" you see on television is
running generally west to east w i t h maybe a
few shallow waves, the weather should remain
generally calm for at least the next day or two.
Since temperature of the air between the
surface and any particular level i n the upper
atmosphere determines patterns of winds
aloft, high-altitude winds that turn toward
the south indicate cold air is moving south.
The jet stream is above the warm-cold air
boundary below. W h e n a jet stream that's
been heading toward the south turns to head
north, it shows that warm air below is moving
toward the north.
On the other hand, upper-level winds blowing generally west to east indicate that large
Figure 2 is a three-dimensional view
of a jet stream. As you see i n Figure 2, jet
streams are somewhat like tubes within
tubes—although the "tubes" in real jet
streams aren't as perfectly round as those
in the drawing. The tube at the center in
Figure 2 could represent a core of winds
as fast as 300 knots, although highest
speeds are more often i n the 150-knot
range. The outer tube in the figure
represents 50-knot winds.
THE WEATHER BELOW. Jet streaks are one
important way in w h i c h jet streams affect
the weather down to the surface. A t the
beginning of a jet streak, such as off the
FIGURE 1.
300-millibar map
showing wind speeds
and direction.
coast of Baja California, Mexico (Figure
1), the air is speeding up. At the end of a
streak, such as over the Southeast, the
air is slowing down. Where the wind is
FIGURE 2. 3-D view of a jet stream.
speeding up the air is spreading out as
l0
o-4°°
1,000-7,000 feet thick
• 1,000-3,000 miles long •
masses of cold air are not moving toward
The map's slightly lighter lines—such
fast-moving air leaves slower-moving air
the south and masses of warm air aren't
as those making circles off the Southern
moving north. The absence of such move-
California coast and over the Gulf of
Such divergence encourages air to rise
ments stifles storms because they form
Mexico—are contours of equal pressure
from the ground somewhat like air rush-
when masses of cold and warm air come
heights.
ing to fill a vacuum. As air rises it forms or
into conflict.
The map shows a "polar jet" along and
mostly north of the United States-Canada
behind. Meteorologists say it's "diverging."
deepens areas of low atmospheric pressure at the surface.
A CLOSER LOOK AT JET STREAMS. Figure
border; and a "subtropical jet" across the
At the end of a jet streak, where the
1 is a simplified 300-millibar map of
south and running up the Atlantic Coast.
wind is slowing down, the air is piling
North America showing only w i n d
Such a pattern is common across North
up—converging—which forces air down to
speeds and directions roughly 39,000 feet
America during the winter.
form or strengthen an area of high pres-
above sea level. The slightly darker black
You'll see that an area of 50 knots plus
sure at the surface or to weaken an area of
lines w i t h numbers at the ends are iso-
connects the parts of the subtropical jet
tachs, w h i c h connect equal w i n d speeds.
winds that drop below 78 knots above
The numbers at the end of the line show
the Southeast and resume off the North
cause air to diverge or converge with simi-
wind speeds i n knots. If you look closely
Carolina coast. The polar jet's dip over
lar effects on atmospheric pressures at the
at these lines you don't see any numbers
the Great Lakes shows that cold air has
surface. These effects of jet streams are
below 50.
moved into this area.
one of the biggest ways i n which winds
The lighter red areas are winds
The map clearly shows that jet stream
low pressure on the surface below.
The curving paths of jet streams also
aloft influence the weather at the surface
between 75 and 100 knots and the dark
wind speeds vary with "jet streaks" of
and the lower levels of the atmosphere
red indicates winds faster than 100 knots.
faster winds embedded in slower winds.
where most general aviation aircraft fly.
OCTOBER 2012 / FLIGHT TRAINING
/ 41
WEATHERH
FIGURE 3. Jet
stream winds and
surface storm
system.
SURFACE WEATHER. If you look at a
surface weather map that shows winds,
you'll see air is moving in counterclockwise swirls (in the Northern Hemisphere)
as it flows into low-pressure areas. A i r
flowing into a low-pressure area would
eventually increase the pressure inside
the area to match pressures outside and
wind would stop blowing. Meteorologists
say the low has filled.
On the other hand, if air is flowing up
from the surface low faster than surface
air is arriving, the pressure in the low
w i l l drop. Meteorologists say the low has
deepened; it is stronger and winds flowing into it w i l l blow faster.
If converging air aloft is forcing air
down into the surface low, the low w i l l fill
and winds flowing into it w i l l slow down.
Converging air aloft can also strengthen
an area of high pressure at the surface if
it more than makes up for air that's flowing out of the surface high at the surface
in a clockwise swirl (in the Northern
Hemisphere).
Figure 3 shows a typical relation
between jet stream winds and a surface
storm system with a low pressure center,
marked with the small " L " over the area
where Oklahoma, Kansas, Missouri, and
Arkansas meet. The curving black lines
are pressure contours at the 300-millibar
level.
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FLIGHTTRAINING.AOPA.ORG
The large " L " at the top of the page
represents the lowest altitude of the
300-millibar pressure on this map. The
dashed line from it down over Texas is
the axis of a trough aloft, an elongated
area of low pressure.
As shown here such high-altitude
troughs often help set the stage for
storms.
If you look at the three contour lines
centered on the surface low, you see
they are diverging. In other words,
the 300-millibar winds are spreading
out here. The surface low formed and
strengthened as air rose to replace the
diverging air aloft.
The relations among upper air and
surface weather features, like most other
aspects of meteorology are complex,
but being a weatherwise pilot doesn't
require mastering meteorology's
complexities.
Nevertheless, you should go beyond
what most self-study, pilot-training
materials offer. Consider a good, collegelevel beginning text. One of the best is
Meteorology Today: Art Introduction to
Weather, Climate, and the Environment by
C. Donald Ahrens.
Jack Williams is an instrument-rated private pilot.
His latest book is The AMS Weather Book: The Ultimate
Guide to America's Weather.