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Alexandre E Eremenko wrote:
>
> But then I need a barometer, or to correct
> the pressure from the Weather channel for
> my altitude, because,
> if I understand correctly, the pressure broadcast
> for the weather prediction is reduced to the sea level.

The "barometric pressure" reported on TV in the U.S. is really the
altimeter setting. Even the National Weather Service does that on their
web site. For example, look at the current weather report at the Rapid
City airport:
http://weather.noaa.gov/weather/current/KRAP.html

The "pressure" they give here is the altimeter setting in inches Hg
(currently 30.07) and hectopascals (1018). On the line below that is the
raw observaton from the airport. As I write this it says:

KRAP 311852Z 01004KT 10SM CLR 01/M17 A3007 RMK AO2 SLP225 T00061172

The "A3007" means altimeter setting 30.07 inches. The "SLP225" is sea
level pressure, i.e., barometer reduced to sea level. In this case it's
225, meaning 1022.5 hPa. (To save space, these reports use a 9xx.x or
10xx.x format and give only the xx.x digits. Since the pressure is
always near 1000, the missing 9 or 10 is obvious.)

Note that the 1022.5 hPa sea level pressure is different from the 1018
hPa altimeter setting. That's because the formula to convert station
pressure (the actual air pressure) to altimeter setting requires only
the altitude of the station. On the other hand, converting station
pressure to sea level pressure takes temperature into account too.

In the U.S. you'll normally have the altimeter setting, so the problem
is to convert that to station pressure. If you're using Windows and can
link to a DLL, my SOFA/JPL DLL has the needed routines. One of the
examples at my web site demonstates their use as it computes a lunar
distance:
http://home.earthlink.net/~s543t-24dst/sofajpl/lunardist.html

Its output is verbose because I wanted to check intermediate results
against MICA 2.0 as much as possible. (All checks were perfect.) There's
a lot of code, but much of it just formats the results for output. Calls
to DLL functions do all the hard work. These are recognizable because
the function names start with jpl_ (calling the JPL ephemeris), iau_
(calling a routine in the SOFA library), or psh_ (the routines that I
wrote).

The station pressure functions are in the source code at this site, but
you'd have to download the whole package. If you wish I can extract just
that part of the code. There's not much to it -- the basic air pressure
function is only 13 lines of code. Even if you don't know C++, I'm sure
the algorithm would be understandable.

I don't want to just give you the formula in normal mathematical
notation because I might make a mistake when attempting to reconstruct
it from the source code.


To find your coordinates and altitude in the U.S., try The National Map
operated by the USGS:

http://nationalmap.gov/index.html

After the startup map loads, click the Zoom In tool. Click and drag
your mouse to box the area you want to view. Then click the Elevation
tool and click a point on the map. It'll display coordinates in several
formats, plus elevation.

If you know your coordinates, you can go right to the location by
clicking the Find Place tool. That opens a coordinate input dialog.

On the right are menus which open when clicked, revealing check boxes
which turn "layers" of the map on or off. For example, you can
turn off shaded relief here. If you're on a dialup connection that will
save a *lot* of load time.

If you have popups blocked, a lot of the features at this site
won't work. I think you need Javascript enabled too.

--
I block messages that contain attachments or HTML.







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