Basic Answers Page
1) A jet stream is an intense narrow wind that is located in the upper
atmosphere at altitudes of 10 to 15 km above sea level.
2) There are two main jet streams found in North America, the subtropical
jet which is located around 30 degrees latitude and the polar jet which
is located between 40 and 60 degrees latitude.
3) The polar jet is the stronger of the two jets because there is greater
contrast in temperature causing higher winds inside the jet. This
strong temperature contrast creates strong pressure differences between
warm and cold air.
4) The greater the pressure gradient, the stronger the winds will be,
which is why the polar jet stream wind speeds varies in different locations.
Therefore, the polar jet stream shifts its location from winter to summer,
depending on the temperature differences which cause strong pressure gradients.
5) The greatest temperature gradients in the winter reach farther south
meaning the polar jet stream extends farther south during winter months.
In the summer, temperature differences aren’t as great and are farther north
(in Canada) so the polar jet stream is usually farther north. The large
contrast in temperature in the winter also causes the polar jet stream to
be stronger in the winter months than summer.
6) The location and strength of the polar jet stream is very important
in predicting the weather. Jet streams often steer storms and also play
a role in the development of storms. If the polar jet stream is to the south
of a location, often colder and drier air is accompanied. This usually
occurs in the upper-level trough, which for this module is the eastern part
of the country. However, if the polar jet is to the north of a location,
then warm and moist conditions are often experienced and is usually associated
with an upper-level ridge (like we see in the western states in our IDV
bundle).
7) The region of maximum winds in the core of the jet stream is called
a jet streak. A jet streak tends to form in the trough of the jet
stream where the pressure gradient is greatest.
8) The speed of the jet streak compared to the slower winds around them,
along with the curve of the jet, play an important role in the formation
of storms.
9) When a parcel of air enters an area
of slower moving winds and a weaker pressure gradient, this area is called
an exit region. At an exit region, the pressure gradient acceleration
becomes smaller than the Coriolis acceleration. This causes an area
of strong upper-level convergence (where air piles up) to the right of
the exit region and upper-level divergence (where air spreads out) to the
left of the exit region. When a parcel of air enters an area of faster
moving winds and a stronger pressure gradient, this area is called an entrance
region. At an entrance region, the pressure gradient acceleration
becomes larger than the Coriolis acceleration .
This causes an area of strong upper-level convergence
(where air piles up) to the left of the entrance region and upper-level
divergence (where air spreads out) to the
right of the entrance region.
10) Divergence aloft allows air to rise (upward motion) and favors the
development of cyclones to the right of the entrance region and left of
an exit region. In this module, the area of upward motion over the
Gulf of Mexico is an example of this.