[轉錄]Why is 850MB so important?

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發信人: [e-mail] (Daryl), 看板: Meto_e
標  題: Why is 850MB so important?
發信站: http://groups.google.com/ (Fri Dec 20 17:03:35 2002)
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Origin: 65.160.5.100

This probably looks like a stupid question, but as I've been reading
more detailed forcasts (especially in relation to lake effect snow).
I see references to the 850MB readings.  While I understand WHAT the
850 MB reading is,  I have yet to find a good explaination as to WHY
850 is so important.  What happens at 850 MB that does not happen at
700MB or 900MB that makes this reading so critical?

I get the feeling this is common knowledge so I apologize for wasting
the bytes here.

-Daryl

> -------------------------------------------------------------------------- <

發信人: [e-mail] (夢白), 看板: TY_Research
標  題: [轉錄]Re: Why is 850MB so important?
發信站: 大氣與海洋專業BBS站 (Tue Dec 24 04:35:18 2002)
轉信站: Ptt!warpnews!itcz

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發信人: [e-mail] (I R A Darth Aggie), 看板: Meto_e
標  題: Re: Why is 850MB so important?
發信站: Texas A&M University - Tallahassee (Fri Dec 20 17:40:25 2002)
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Origin: aggie.stat.fsu.edu

On 20 Dec 2002 14:03:35 -0800,
Daryl <[e-mail]>, in
<[e-mail]> wrote:
+> This probably looks like a stupid question, but as I've been reading
+> more detailed forcasts (especially in relation to lake effect snow).
+> I see references to the 850MB readings.  While I understand WHAT the
+> 850 MB reading is,  I have yet to find a good explaination as to WHY
+> 850 is so important.  What happens at 850 MB that does not happen at
+> 700MB or 900MB that makes this reading so critical?

Ummm...the 850 mb level is a mandatory level, so you're always
guaranteed of getting data there. That's part of the magic. The other
is that lake effects tend to be shallow, so 700mb (also a mandatory
level) is probably too high to give insight into lake effects. 850
probably provides enough insight to be useful, at least in some (many?
most?) circumstances.

1000mb is also mandatory, but it may not exist, or is very close to
the surface data.

Does that help?

James

> -------------------------------------------------------------------------- <

發信人: [e-mail] (夢白), 看板: TY_Research
標  題: [轉錄]Re: Why is 850MB so important?
發信站: 大氣與海洋專業BBS站 (Tue Dec 24 04:35:30 2002)
轉信站: Ptt!warpnews!itcz

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發信人: Icebound <[e-mail]>, 看板: Meto_e
標  題: Re: Why is 850MB so important?
發信站:  (Fri Dec 20 19:34:02 2002)
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Origin: 24.100.76.111

Daryl wrote:
> ... snip ... What happens at 850 MB that does not happen at
> 700MB ... [snip]?
> ...

What "happens" at each level is quite different and any *one* level can
be an indicator which becomes part of the overall puzzle.

As James pointed out, 850 is a mandatory level that is probably pretty
close to the mid- or upper- levels of most lake-effect cloud, and is
very useful in determining the extent of the moisture at those levels.

Also, the wind patterns at this level can help identify  areas of
convergence at low levels (which in turn imply ascent, a usual
contributor to convection).  This level can also be useful in
identifying frontal zones, especially those that may be masked by
nighttime temperature at the surface, or by other surface effects.

> -------------------------------------------------------------------------- <

發信人: [e-mail] (夢白), 看板: TY_Research
標  題: [轉錄]Re: Why is 850MB so important?
發信站: 大氣與海洋專業BBS站 (Tue Dec 24 04:35:38 2002)
轉信站: Ptt!warpnews!itcz

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發信人: "R. Martin" <[e-mail]>, 看板: Meto_e
標  題: Re: Why is 850MB so important?
發信站: R. L. Martin and Associates (Fri Dec 20 22:40:04 2002)
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Origin: 64.24.90.122

Daryl wrote:
>
> This probably looks like a stupid question, but as I've been reading
> more detailed forcasts (especially in relation to lake effect snow).
> I see references to the 850MB readings.  While I understand WHAT the
> 850 MB reading is,  I have yet to find a good explaination as to WHY
> 850 is so important.  What happens at 850 MB that does not happen at
> 700MB or 900MB that makes this reading so critical?
>
> I get the feeling this is common knowledge so I apologize for wasting
> the bytes here.
>
> -Daryl

Precisely 850 mb is by and large just an artifact I think.  AFAIK
there is just a little fundamental reason for this.  I would guess
what fundamental reason there is to be that the 850 mb level is high
enough to be generally above the boundary layer, temperature inversions
near the surface, etc., and yet is low enough that its properties, such
as temperature, are fairly well related to those of the surface, on
average.  The artifact portion comes about because 850 mb is a
mandatory level for reporting radiosonde data.  Thus over the years
when people have studied the weather the 850 mb data was almost
always available if any upper air data was, while conditions 10 or
20 mb higher or lower were not recorded and were just interpolated
between the mandatory levels (unless something interesting like a
sharp change was seen).  Such study found useful rules of thumb for
forecasting surface conditions related to the 850 mb conditions.
In part the reason such rules could be discovered goes back to the
fundamentals of the situation mentioned above, but if 846 mb had for
some reason been a mandatory level we'd all be talking about 846 mb
readings.  All of the above doesn't matter when the surface altitude
is such that the surface pressure is less than 850 mb, of course. ;-)

Regards,
Russell

> -------------------------------------------------------------------------- <

發信人: [e-mail] (夢白), 看板: TY_Research
標  題: [轉錄]Re: Why is 850MB so important?
發信站: 大氣與海洋專業BBS站 (Thu Dec 26 05:52:16 2002)
轉信站: Ptt!warpnews!itcz

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發信人: [e-mail] (Greg), 看板: Meto_e
標  題: Re: Why is 850MB so important?
發信站: http://groups.google.com/ (Wed Dec 25 01:45:52 2002)
轉信站: itcz!news2.wam.umd.edu!ra.nrl.navy.mil!dca6-feed2.news.algx.net!allegia
Origin: 67.115.84.82

yeah, why i think 850 is one of the most important charts
reason 1. 850 is the surface in many locations
       2. 850 reflection of the surface chart
       3. 850-1000mb low-lvl thkns composite
       4. 850 is used to track weak frontal boundries ovr shallow cool pools
       5. 850 for ll jet placement in svr
       6. 850 filling lows, distal v proximal
       7. 850 slope of front, rise over run
       8. 850 is 050ft the 1st 1/4 of where weather happens
       the list goes on WHY it is not 925 or 700

"R. Martin" <[e-mail]> wrote in message news:<[e-mail]>...
> Daryl wrote:
> >
> > This probably looks like a stupid question, but as I've been reading
> > more detailed forcasts (especially in relation to lake effect snow).
> > I see references to the 850MB readings.  While I understand WHAT the
> > 850 MB reading is,  I have yet to find a good explaination as to WHY
> > 850 is so important.  What happens at 850 MB that does not happen at
> > 700MB or 900MB that makes this reading so critical?
> >
> > I get the feeling this is common knowledge so I apologize for wasting
> > the bytes here.
> >
> > -Daryl
>
> Precisely 850 mb is by and large just an artifact I think.  AFAIK
> there is just a little fundamental reason for this.  I would guess
> what fundamental reason there is to be that the 850 mb level is high
> enough to be generally above the boundary layer, temperature inversions
> near the surface, etc., and yet is low enough that its properties, such
> as temperature, are fairly well related to those of the surface, on
> average.  The artifact portion comes about because 850 mb is a
> mandatory level for reporting radiosonde data.  Thus over the years
> when people have studied the weather the 850 mb data was almost
> always available if any upper air data was, while conditions 10 or
> 20 mb higher or lower were not recorded and were just interpolated
> between the mandatory levels (unless something interesting like a
> sharp change was seen).  Such study found useful rules of thumb for
> forecasting surface conditions related to the 850 mb conditions.
> In part the reason such rules could be discovered goes back to the
> fundamentals of the situation mentioned above, but if 846 mb had for
> some reason been a mandatory level we'd all be talking about 846 mb
> readings.  All of the above doesn't matter when the surface altitude
> is such that the surface pressure is less than 850 mb, of course. ;-)
>
> Regards,
> Russell