[轉錄]Re: Why is 850MB so important?
※ 本文轉錄自 [Meto_e] 看板
發信人: imweather@hotmail.com (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" <russell.martin@wdn.com> wrote in message news:<3E03E4A0.7992@wdn.com>...
> 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
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