I really should know this, but I was wondering if someone could tell me what
causes the jet stream to occur? We seem to be blaming the jet stream for our
below average end of June / beginning of July because it is so far south,
but why does the jetstream occur at all and why does it (usually) retreat
north in the summer?

Thanks,

Jeremy

"Jeremy Handscomb" wrote
I was wondering if someone could tell me what
causes the jet stream to occur?
http://www.classzone.com/books/earth...1906page01.cfm

Useful - I will be doing some study myself.

Jack

"Jeremy Handscomb" wrote in message
...
I really should know this, but I was wondering if someone could tell me what
causes the jet stream to occur? We seem to be blaming the jet stream for our
below average end of June / beginning of July because it is so far south,
but why does the jetstream occur at all and why does it (usually) retreat
north in the summer?

Thanks,

Jeremy

Here is an answer, which even if not correct you may find interestinggrin

In Martin Rowley's FAQ he starts his description of the Polar Front Jet [the
one which causes our weather] with "As its name implies, this jet stream is
associated with the marked discontinuity found at the boundary of well defined
air masses - polar to the north/sub tropical to the south (in the northern
hemisphere), conventionly found at the polar front." In fact the Polar Front
Jet is associated with only one air mass which diverges into two climate
cells; Ferrel and Polar. The cells can be imagined as rectangular les viewed
zonally, which fit beneath the tropopause.

There is of course, a third, better known cell - the Hadley cell. Since the
tropopause is higher above the equator and lower at the poles, the rectangles
decrease in height, with the highest being the Hadley and the lowest being the
Polar. Where the Polar and Ferrel cells fit together there is a step, and it
is this severe baroclinicity which induces the strong wind at right angles to
the gradient. A similar step also occurs between the Ferrel and Hadley cells
which induces the the SubTropical Jet. Here two air masses do meet, mix, and
subside into the Horse Latitudes to form one air mass. Beneath the Polar
Front Jet is the Polar Front where the Polar and SubTropical air masses do
meet, mix and rise. This process is characterised by the cyclones
(depressions) which form under the jet, and provide the upwelling air which
powers it.

HTH,

Cheers, Alastair.