"Icebound" wrote in message
...

"Matt Barrow" wrote in message
...

"Icebound" wrote in message
...
How would a station be more representative if it was/wasn't in a
valley
or
on a hill top?



This is an obstruction-clearance issue in very cold weather.

If the altimeter setting came from a station in the valley 5000 feet in
true
height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height
from
the
aircraft, the difference is likely less than 300 feet. (Source:
Canadian
AIP)

If you are choosing flight altitudes without much margin for obstacle
clearance, you may want to ask for somewhat higher altitudes if you
will
be
using settings from valley stations.


Mind telling me how that variation (of that magnitude) could come
about?



Its been discussed here many times, but I will review:

We know that an altimeter setting by definition means that, set to the
station-determined value, our INDICATED altitude will match the REAL
ELEVATION, when we are parked on the threshold of the station. Bascially,
the "altimeter setting" which we set in the Kollsman window, is what
accomplishes that correction-for-station-elevation, so that we get MSL and
our INDICATED readout.

Beyond that, when we fly at some indicated altitude other than that
exactly
equal to the station's, we are following a constant air pressure... a
pressure which is some fixed amount LESS (if we are higher) than the
actual
air pressure at our station. The altimeter, converts this
pressure-difference (between the pressure at the station, and the actual
pressure at the airplane)... into an altitude readout. Adjusted, of
course,
by adding the elevation of the station to get MSL... as I have already
mentioned, by means of the altimeter-setting adjustment.

But because the altimeter has no knowledge of the ambient temperature, it
makes this pressure-difference conversion assuming the "standard"
atmosphere. In our very cold conditions, the air is much denser that the
"standard", and thus the pressure levels are much closer together... in
other words: as we climb away from the altimeter-setting-station, the
pressure decreases much more, in 1000 feet of REAL altitude, than it does
in
"standard" temperature conditions. Therefore our altimeter will read
higher
than the REAL altitude, because it only knows about the lower pressure,
and
nothing about the colder temperature. The altimeter reads higher than
real,
real elevation is lower than indicataed, obstruction clearance may be an
issue.

Naturally, the further we climb from our altimeter-setting-station, the
greater the error. That is why we can reduce this error if we can get the
altimeter setting from the station closest to our real altitude.

Remember that this has nothing to do with assigned altitudes. Those are
flown according to the INDICATED altitude whatever it happens to be, based
on whatever approved official altimeter setting you have. The Canadian
rules
specifically say:

quote:
IFR assigned altitudes accepted by a pilot shall not be adjusted to
compensate for cold
temperatures, i.e., if a pilot accepts “maintain 3 000”, an altitude
correction shall not be
applied to 3 000 ft.
:unquote

BUT IF there a concern about clearing obstacles or terrain, a knowledge of
the probable error between indicated and actual may cause you to file
and/or
request a higher INDICATED altitude.

The actual numbers can be calculated from knowledge of "air constants",
and
temperature-density relationships, etc., but since we fly in very cold
weather up here quite a bit, Transport Canada has done the math and
published some figures for us in the Canadian equivalent of the AIM.

Fore example, at 40 below, 5000 feet away from altimeter-setting-station,
error is published as 1210 feet. At 50 below it is 1500 feet. 1000 feet
away from altimeter-setting-station, error is published as 240 feet at 40
below, 300 feet at 50...

The comment is often made: "Well if the air is cold, and cold air is
dense,
then the pressure that I am flying at should be higher", and not lower as
I
have shown above. But that statement ignores the fact that air is
compressible and thus more of that cold density is BELOW you. The
pressure
at your altitude is produced only by the weight of the air ABOVE you.



You're bringing in a lot of irrelevant material. The original point was
"Area" altmiter settings and another point made that a station in
mountainous areas would be more accurate if it was located on the mountain
top at higher altitude than a station at lower altitude in the same area.

You're still not explaining how a ground station at, say, 6000' MSL would
be have a more accurate baro reading than one down in a nearby valley at,
say, 2000' MSL. That is the point of the thread.