※ 本文轉錄自 [Meto_e] 看板
發信人: "R. Martin" <russell.martin@wdn.com>, 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)
轉信站: itcz!news2.wam.umd.edu!nntp.abs.net!news.maxwell.syr.edu!feed3.newsread
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