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current of rapidly moving air
The jet stream is a current of fast moving air found in the upper
levels of the atmosphere.
This rapid current is typically thousands of kilometers long,
a few hundred kilometers wide, and only a few kilometers thick.
Jet streams are usually found somewhere between 10-15 km (6-9 miles) above the
The position of this upper-level jet stream denotes the location of
the strongest SURFACE temperature contrast (as in the diagram below).
[Image: jet stream schematic (28K)]
During the winter months, Arctic and tropical air masses
create a stronger surface temperature contrast resulting in a strong
jet stream. However, during the summer months, when the surface
temperature variation is less dramatic, the winds of the jet are weaker.