A jetstream is a narrow band of strong winds that encircles the Earth
in the mid-latitudes, containing regions where locally strong pressure
gradients produce exceptionally strong winds, called jetstreaks, which migrate through curved flow patterns. A jetstreak
is located in a region of strong pressure gradient and is indicated by the
large values of isotachs (lines of constant wind speeds) and the close spacing
of the pressure or height contours. As air moves through the jetstreak, air
parcels are displaced northward in the entrance region and southward in the
exit region. Divergence occurs in the right entrance region (looking in the
direction of the flow) while convergence in the left entrance region displaces
air from the right (south) to the left (north) side of the jet. Divergence
aloft will result in lower surface pressure, whereas convergence will result in
higher surface pressure.
Although the jetstream is not always
a single ribbon of fast-moving air encircling the pole. In nature, a single
jetstream can split into two branches and then merge again at a downstream
location. In fact, the most extreme low pressures associated with cyclones in
the middle latitudes usually occur when two (or even three) jetstreaks, each
embedded in a different branch, interact with one another as their parent
jetstreams merge.
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