ATC Altimeter Settings

"John R. Copeland" wrote in message
...
"Steven P. McNicoll" wrote in message
. net...

"John R. Copeland" wrote in message
...

In the Rockies, there are ground stations at widely varying elevations.
I've guessed that ATC might not use settings from airports in deep
valleys
for enroute settings, wherever more representative values are available.
The terminology I've heard used at specific locations is something like:
"XXX altimeter nn.nn, Area altimeter nn.mm."
I don't really know how the "Area" numbers are established,
but I'd certainly want to use the local airport setting for landings.


Where did you hear "area altimeter nn.nn"?


One place I recall specifically was KBCE.

"John R. Copeland" wrote in message
...

One place I recall specifically was KBCE.


The controller issued the Bryce Canyon altimeter and the "area" altimeter?
Did you happen to ask him how he determined the "area" altimeter?

Roy Smith wrote:

"O. Sami Saydjari" wrote:


So are we are concluding that it is best to switch to ATC altimeter
setting as soon as they give them to you



That's what I'm concluding. You?



and that differences from local airport settings, except when you are on
approach to your destination (where you use that airport's setting).



I'll always favor an altimeter setting ATC gives me over anything I hear on
the ATIS. The ATIS could be up to an hour old; the controller is looking
at the most current reading right now.


And you do not think the controller is getting his information as
reported by that station's ATIS / AWOS/ ETC?

Ross

"Roy Smith" wrote in message
...

I'll always favor an altimeter setting ATC gives me over anything I hear
on
the ATIS. The ATIS could be up to an hour old; the controller is looking
at the most current reading right now.


Which could also be up to an hour old.

"Steven P. McNicoll" wrote in message =
news
=20
"John R. Copeland" wrote in message=20
...

One place I recall specifically was KBCE.

=20
The controller issued the Bryce Canyon altimeter and the "area" =
altimeter?=20
Did you happen to ask him how he determined the "area" altimeter?


It wasn't a controller, it was an FSS specialist, and sadly I asked him =
only
what the "area setting" meant, not how he determined it.
Since nobody here seems to know about it, it may be one of those things
no longer in use. It seemed reasonable to me at the time, though.

"Newps" wrote in message =
...
=20
John R. Copeland wrote:
=20
In the case of non-standard lapse rates, the lower the airport,
the more the altimeter differs from true height across mountain =
peaks.
=20
Lapse rate is temp and has little effect on the altimeter setting.=20
Because you are already separated from the ground by 2000 feet it is=20
much more important that everybody have the same setting. No such =
thing=20
as an area altimeter setting.


I think you could say lapse rate has *no* effect on altimeter setting.
QNH is whatever it takes to make the altimeter read right on the =
airport.
Lapse rate departure from standard, however, is the major contributor
to any difference between true height and altimeter reading aloft.
Humidity is a minor contributor by comparison.

"John R. Copeland" wrote in message
...

It wasn't a controller, it was an FSS specialist,


You said previously it was ATC.



and sadly I asked him only
what the "area setting" meant, not how he determined it.
Since nobody here seems to know about it, it may be one of those things
no longer in use. It seemed reasonable to me at the time, though.


I doubt it was ever in use.

"Steven P. McNicoll" wrote in message =
k.net...
=20
"John R. Copeland" wrote in message
...

It wasn't a controller, it was an FSS specialist,

=20
You said previously it was ATC.
=20

Yes I did, but I absolutely mis-spoke. Sorry.

Lapse rate is lapse rate, whether the station is 2000 feet higher or lower.

"John R. Copeland" wrote in message
...
"Newps" wrote in message
...

John R. Copeland wrote:

In the case of non-standard lapse rates, the lower the airport,
the more the altimeter differs from true height across mountain peaks.

Lapse rate is temp and has little effect on the altimeter setting.
Because you are already separated from the ground by 2000 feet it is
much more important that everybody have the same setting. No such thing
as an area altimeter setting.


I think you could say lapse rate has *no* effect on altimeter setting.
QNH is whatever it takes to make the altimeter read right on the airport.
Lapse rate departure from standard, however, is the major contributor
to any difference between true height and altimeter reading aloft.
Humidity is a minor contributor by comparison.

"Matt Barrow" wrote in message
...

"John R. Copeland" wrote in message
...

In the Rockies, there are ground stations at widely varying elevations.
I've guessed that ATC might not use settings from airports in deep
valleys
for enroute settings, wherever more representative values are available.

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.