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Name: Arvid
Status: other
Age:  60s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Please explain in detail how the jetstream contributes to create low pressures at the surface. (I have a unversity degree, so don't be afraid of refering to physics or mathematics. I also have a basic knowledge of weather through my private pilot license.)


Replies:
Arvid,

You have asked a fascinating question.

First, there are two jets, normally, the polarfront jet (in temperate latitudes) and the subtropical jet (in subtropical regions well north the equator (it commonly is found hugging the southern coastline of the United States. The polarfront jet tends to vary in latitudinal location more than the subtropical jet. Sometimes, in the summer, only the subtropical jet exists.

The jets form along tropopause breaks. To the north of the break the tropopause is low in the atmosphere and to the south it is higher. This discontinuity results in high wind speeds, significant turbulence (often called Clear Air Turbulence, which is normally found near the edges of the jet), sometimes some rotation of the jet, and significant instability as a result of the very different vertical gradients of temperature of the two tropopauses. Normally, temperatures under the northern tropopause are lower than under the southern tropopause. Surface cold fronts form north of the jet and low pressure areas often form north of the jet and just west of a southern pointing V in the jet (which is where a trough of lower pressure also exists). The V enhances the instability under the jet and results in low pressure areas (instability leads to convection and air mass lifting, which when organized, produces a low pressure area).

You can see this very well in the surface

(www.intellicast.com/LocalWeather/World/UnitedStates/SurfaceAnalysis)

and upper level maps

(www.intellicast.com/LocalWeather/World/UnitedStates/JetStream)

today (Dec. 6, 2000) at Intellicast weather on the Internet.

David R. Cook
Atmospheric Section
Environmental Research Division
Argonne National Laboratory


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