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Q: Weather Patterns ( Answered,   0 Comments )
Subject: Weather Patterns
Category: Science
Asked by: whether-ga
List Price: $4.00
Posted: 23 Jul 2003 16:03 PDT
Expires: 22 Aug 2003 16:03 PDT
Question ID: 234356
In the continental United States, why are air masses from the north
better defined, bigger, and more frequent in winter than in summer?
(I know it has something to do with the position of the jet stream, but why does
that change with the seasons?)
Subject: Re: Weather Patterns
Answered By: andrewxmp-ga on 24 Jul 2003 08:01 PDT
Hi whether,

This is quite an interesting phenomenon!  First, there is the basic
principle behind the effect a jet stream has on weather at a
particular location:

"The polar-front jet stream exists along the region where cold polar
air and warm air masses are in contact. Hence, weather is relatively
cold when the polar-front jet stream is south of a particular location
and relatively warm when the jet stream is north of the location. "

This seasonal shift in jet stream location is described:

"The northward shift of the westerlies over eastern North America is
most pronounced, with the maximum westerlies shifting from the
Southeast US during January-March to southern Canada during July and
August. The upper-tropospheric circulation over North America switches
from cyclonic during winter to anticyclonic during summer, associated
with the seasonal variation in thermal forcing over the continent. "

"The polar front jet streams are embedded in the upper level
westerlies, snaking north and south daily and seasonally with the
movement of the polar front depressions. They exist because of the
strong thermal gradient in that area and they are regions of maximum
upper-level air mass transport. As they meander polewards and
equatorwards with the general upper air waves they tend to steer, by
their sheer mass, the movement of major low-level air masses,
encouraging development of surface pressure features, and
intensification of pre-existing features, by the concentrated
convergence/divergence within the jet stream. The jet streams are
stronger in the winter when the polar front is closest to the
[] [SECTION 4.8]

You should check out [
] and go to the last section on the page, especially the last two
links "Winter" and "Summer".  One can clearly see the difference in
wind-speed gradient, as well as volume of the jet streams.  So in
summary, it seams that annual temperature changes due to the seasons
changing alter the temperature of resident air masses, which move jet
streams in an annual cycle.  The relative position of the streams to
any location is what result in the weather phenomenon you described.

I hope this helped!  Before rating this answer, please feel free to
ask for any clarifications necessary.  Thank you for bringing your
question to us!

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