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How Does the Jet Stream Affect Weather?
The Great Flood of 1993, as it is now called, ranks as one of the worst natural disasters
and one of the costliest floods in United States history. Even so, some have called it a
leisurely disaster because it resulted not from a single storm, but from a strange weather
pattern that took shape and remained in place for months during the summer of 1993.
Most thunderstorms form near surface frontal features, such as a cold front, that are
associated with the polar jet stream. This high-speed band of wind travels in the
atmosphere about 10 to 15 kilometers (6 to 9 miles) above Earth. It typically occurs
above the boundary between warm, tropical air masses to the south and cooler air
masses to the north. In the summer, the air mass on the tropical side may be moist and
unstable, thus triggering thunderstorms. These thunderstorms may develop into longlived clusters, or convective systems, near the jet stream. Thus, the position of the jet
stream is a good predictor of where convective systems will form and to where they will
travel.
Typically, the jet stream's course is constantly on the move, changing position over a
period of days or weeks. For much of the summer of 1993, however, the jet stream
remained almost stationary over the Great Plains and upper Midwest. A high-pressure
system to the south provided a constant source of warm, moist air from the Gulf of
Mexico, and evaporation from the saturated soil surface provided even more moisture to
the already unstable air mass. When this moist, unstable air mass intersected with the
cooler, drier air mass to the north, thunderstorms occurred. In some areas, this endless
parade of thunderstorms dumped nearly 100 centimeters (more than 3 feet) of rain
during the month of July. This heavy precipitation fell onto soil already saturated from a
typically wet spring and had nowhere to go but into the rising waters of the Mississippi
River basin.
Watch the video about The Great Flood of 1993 and answer the questions.
1.
What is the pattern of the jet stream during a typical year?
2. How did the jet stream's position in the summer of 1993 affect weather patterns
over the mid-western United States?
3. What do meteorologists suspect caused the jet stream to detour and consistently
flow over the upper Midwest?
4. How would you describe atmospheric conditions (in terms of temperature and
humidity) on either side of the jet stream?
5. Why does this pattern result in powerful storms along the path of the jet stream?
6. How does the jet stream influence the weather where you live?
Jet Stream
Introduction
The jet stream is like a high-speed river of air in the upper atmosphere. It separates
warm and cold regions at the Earth’s surface. It may be several hundred miles across
from north to south. It is very thick (1 to 3 miles). The jet stream is high in the
atmosphere (about 6 to 9 miles high). The jet stream weaves from west to east pulling
pressure systems with it. The jet stream has the strongest wind in the winter, sometimes
up to 300 miles per hour. In summer the jet stream is located closer to the United States
northern boundary allowing warm southern air to reach as high as Minnesota. In winter
it sinks much lower in the United States and it pulls the cold northern air into the lower
states. Each season the jet stream changes position and changes weather patterns
accordingly.
Map Information
The map data was collected using radiosondes from weather balloons as they rose in
the atmosphere. The wind data is plotted when each balloon reached 200mb so all
stations had the same pressure on this map. Equal pressures allow us to just look at
wind data.
Winds are named from where they come from. For example, a west wind (in Figure1)
was reported at Green Bay, Wisconsin at 60 knots. Wind speed is reported in knots (1
knot=1.15mph in an automobile). A feather represents 10 knots, a half feather stands
for 5 knots, and a flag means 50 knots.
Procedure
1. On Figure 1, shade the area where winds are 60 knots or greater. Draw a dark
arrow showing wind direction in this jet stream.
2. Imagine floating through this jet stream on a rug. How fast would you travel?
____________________ List 5 states that you would pass through_________
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3. Even though the wind speed is 60 knots or greater, why would a wind gauge
attached to your rug read 0 knots? _________________________________
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4. Look at winds on either side of the jet stream. The winds on either side are what
speed and direction compared to the jet? (faster/slower) (same
direction/different direction)
5. Shade and draw an arrow showing wind direction on Figure 2. The jet in Figure
2 is traveling 100 knots or higher.
6. In Figure 2 are the temperatures in the western United States warmer or colder
than the temperatures in the eastern United States? Why? _______________
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7. Storms tend to follow the path of the jet stream. In figure 2 where would a storm
in Oklahoma most likely end up…in the Great Lakes OR Florida? ___________
8. Describe airline flight times from Chicago to Los Angeles vs. Los Angeles back to
Chicago. ___________________________________________________