Altimeter: Hot to cold = look out below?!
jcrogin
ambient air pressure. So if you fly into an area of low pressure
(without adjusting the setting), your altimeter will think you're
flying higher - so to "correct" the situation you start flying lower,
closer to all those antennas and mountains and other scary stuff. That
makes sense. But what has never made sense to me is the corresponding
hot to cold rule. I mean, cold air is more dense than hot air. So if
you fly from hot to cold, why doesn't your altimeter think you're
flying lower, rather than higher? What am I missing?
Jonah Rogin
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The fastest and easiest way to search and participate in Usenet - Free!
Roy Smith
> I mean, cold air is more dense than hot air. So if you fly from hot to
> cold, why doesn't your altimeter think you're flying lower, rather
> than higher? What am I missing?
When you say "cold air is more dense than hot air", that's true, but your
altimeter doesn't measure density, it measures pressure.
Think of a column of air that reaches up to the top of the atmosphere. As
you cool it, it contracts, and thus the top of the atmosphere gets lower.
Imagine that a standard atmosphere is, say, 100,000 feet high (I'm sure
that's not the right number, but it'll work for the sake of
illustration). If you're flying at 10,000 feet, you've got 90% of the
atmosphere above you and 10% below you. Now, let's say you cool the
column of air so it contracts enough to make the top of the column only
90,000 feet high. You've now got 11% of the atmosphere below you and 89%
above you. The pressure you feel is the weight of the air above you,
which is less than it was before, so your altimeter reads high.
Eric W. Seelig
finally convinced myself.
High temperature "spreads out" the pressure levels (less dense air).
Draw yourself a graph with location on the x-axis (going from a hot
area to a cold area) and altitude on the y-axis. Now draw in the
isobars (lines of constant pressure). The will slope DOWN as you
go from the hot area to the cold area (and be closer together in
the cold area than the hot area). Now, if you fly along at a
constant indicated altitude (without changing your altimeter
setting) you'll really be flying parallel to those isobars...meaning
you will be descending. Thus...high to low (pressure or temperature)
look out below!
Note that going from an area of high pressure to an area of low
pressure, the isobars will still slope DOWN from the high pressure
area to the low pressure area, but in this case (assuming constant
temperature) they will be parallel, instead of converging as you
get closer to the cold area.
Hope this helps...and I hope I'm correct! If you can't picture the
graph, e-mail me, and I'll draw it up and send you a copy.
Eric
jcrogin wrote:
>
> Ok, we all know that a pressure altimeter works by sensing changes in
> ambient air pressure. So if you fly into an area of low pressure
> (without adjusting the setting), your altimeter will think you're
> flying higher - so to "correct" the situation you start flying lower,
> closer to all those antennas and mountains and other scary stuff. That
> makes sense. But what has never made sense to me is the corresponding
> hot to cold rule. I mean, cold air is more dense than hot air. So if
> you fly from hot to cold, why doesn't your altimeter think you're
> flying lower, rather than higher? What am I missing?
>
Mike Rapoport
only a big problem in very cold temps. In Canada you use charts to adjust
your min altitudes on approaches for temperature.
Mike
MU-2
jcrogin wrote in message <1415c574...@usw-ex0102-011.remarq.com>...
Tony St. Clair
> Ok, we all know that a pressure altimeter works by sensing changes in
> ambient air pressure. So if you fly into an area of low pressure
> (without adjusting the setting), your altimeter will think you're
> flying higher - so to "correct" the situation you start flying lower,
> closer to all those antennas and mountains and other scary stuff. That
> makes sense. But what has never made sense to me is the corresponding
> hot to cold rule. I mean, cold air is more dense than hot air. So if
> you fly from hot to cold, why doesn't your altimeter think you're
> flying lower, rather than higher? What am I missing?
>
> Jonah Rogin
>
> * Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
> The fastest and easiest way to search and participate in Usenet - Free!
There is a relationship between temperature and pressure you may recall as the
"ideal gas law," PV=nRT. No need to get into the details of the equation, but
the important part here is that the pressure is directly proportional to the
temperature. Thus, when you are going from hot to cold (i.e. the T is
decreasing), it is the same as going from high to low pressure (i.e., the P is
decreasing also).
Since your altimeter senses pressure changes, if you do not adjust your
altimeter setting, a lower pressure is sensed as an increase in altimeter
reading (i.e. higher altitude). Therefore, in order to keep your altitude
"constant" you descend to compensate. Oops, now you're closer to the ground
than you really want to be.
--
Tony St. Clair
CFII, AGII
http://tstclair.home.texas.net/flight
Randall Becker
Another way to think of it. Take a whole mess of gas molecules and stick
them in a balloon. Close the balloon. Now, if you heat the balloon, it's
going to expand because of the increase pressure of the *now* faster moving
molecules on the wall of the balloon.
If you have two balloons, both at 29.92 Mb inside pressure, the colder one
will have to have proportionately more molecules inside to exert the same
pressure on the walls as the hotter one. Rephrased, if you had two balloons,
the same size, on the same day at the same altitude, with one hotter than
the other, they'd be at the same pressure, but the colder one would have
more molecules.
<taking off chemistry hat and throwing it out the storm window>
Tony St. Clair <tstc...@texas.net> wrote in message
news:38191938...@texas.net...
Mark Kolber
> I mean, cold air is more dense than hot air. So if
>you fly from hot to cold, why doesn't your altimeter think you're
>flying lower, rather than higher? What am I missing?
I've gotten this one wrong a million times until I read this
(highly simplified) explanation in a magazine:
Imagine your airplane riding on top of a cylinder of air.The cylinder
sits on the ground and its sides are rigid, but it's capable of
expanding vertically.
In general, when air (or any gas) warms, it's volume increases. When
it cools, its volume decreases. What effect will that have on the
cylinder your plane is riding on?
Works the same for low pressure -- generally, low pressure means less
weight -- more expansion -- more volume.
=======================================
______|______ Mark Kolber
\(o)/ Denver, Colorado
o O o www.midlifeflight.com
=======================================
replace "spamaway" with "mlf" for email
Antonio Aponte
> you fly from hot to cold, why doesn't your altimeter think you're
> flying lower, rather than higher? What am I missing?
>
Lets say that you are in an elevator and the pressure inside is 30 inches. It is confirmed
by the elevator altimeter that reads 3500 ft. and you have set the controls to seek out 30
inches, wherever that may be.
The elevator shaft has cold air under it that shrinks when it is cold and expands when
hot, sending the elevator higher and lower with each fluctuation. When the air in the
shaft is cooler, the elevator is closer to the ground floor and vice-versa. If you set the
controls of the elevator to always seek 30 inches of pressure (3500 ft.) it will ride up
and down depending on the temperature of the air in the shaft. Somedays 30 inches will be
located lower than other days; somedays higher. The density of the air where 30 inches
exists changes from day to day. Get it?
Notice that this does not at all conflict with your world view of the density of air and
its relationship to altitude. Good thinking!
Antonio
jcrogin
temperature and pressure are constant across the surface. Or maybe
temparture & pressure are not constant across the surface, but we have
those nifty altimeter settings to compensate. So, correct me if I'm
wrong, but *surface* altimeter settings take low (or high) temperature
into account. And if we could get altimeter settings at 5000 ft AGL,
they would also take nonstandard temperature into account. Thus,
altimeter errors occur not from flying into colder air (as long as it's
getting colder at the surface too) but rather from flying into air that
has a higher than standard lapse rate. Or lower than standard, but
higher is more dangerous since it implies colder air (at altitude) and
thus lower flight.
Ok, I'm flying at 5000 ft over a station at sea level reporting 29.80"
and 5 degrees Celsius. My OAT guage says -5 degrees. Since lapse rate
is standard (even though temperature & pressure are not) my altimeter
is correct, assuming I have it set to 29.80". But if OAT were -10
degrees, then I'd have to start worrying about being too low.
Right?
> Think of a column of air that reaches up to the top of the
> atmosphere. As
> you cool it, it contracts, and thus the top of the atmosphere gets
> lower.
> Imagine that a standard atmosphere is, say, 100,000 feet high (I'm
> sure
> that's not the right number, but it'll work for the sake of
> illustration). If you're flying at 10,000 feet, you've got 90% of
> the
> atmosphere above you and 10% below you. Now, let's say you cool
> the
> column of air so it contracts enough to make the top of the column
> only
> 90,000 feet high. You've now got 11% of the atmosphere below you
> and 89%
> above you. The pressure you feel is the weight of the air above
> you,
> which is less than it was before, so your altimeter reads high.
* Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
Newps
> Ok, I'm flying at 5000 ft over a station at sea level reporting 29.80"
> and 5 degrees Celsius. My OAT guage says -5 degrees. Since lapse rate
> is standard (even though temperature & pressure are not) my altimeter
> is correct, assuming I have it set to 29.80". But if OAT were -10
> degrees, then I'd have to start worrying about being too low.
>
altimeter will read the correct altitude, as long as the setting is
correct. This even though where you are sitting has only one standard
temperature. Yet anytime, winter or summer, that you set your altimeter
correctly it will show you at the proper field elevation. So in
summary, don't worry about the temp when flying, it won't affect your
altitude.
Mike Rapoport
way. If you have the correct local altimeter setting then the altimeter is
ONLY correct at standard temps. You should also know that the 2C/1000 ft is
for calibrating altimeters and has nothing to with the actual lapse rate in
the atmosphere.
Mike
MU-2
jcrogin wrote in message <17599f0b...@usw-ex0102-011.remarq.com>...
>So if I'm understanding this correctly, we are assuming that
>temperature and pressure are constant across the surface. Or maybe
>temparture & pressure are not constant across the surface, but we have
>those nifty altimeter settings to compensate. So, correct me if I'm
>wrong, but *surface* altimeter settings take low (or high) temperature
>into account. And if we could get altimeter settings at 5000 ft AGL,
>they would also take nonstandard temperature into account. Thus,
>altimeter errors occur not from flying into colder air (as long as it's
>getting colder at the surface too) but rather from flying into air that
>has a higher than standard lapse rate. Or lower than standard, but
>higher is more dangerous since it implies colder air (at altitude) and
>thus lower flight.
>
>Ok, I'm flying at 5000 ft over a station at sea level reporting 29.80"
>and 5 degrees Celsius. My OAT guage says -5 degrees. Since lapse rate
>is standard (even though temperature & pressure are not) my altimeter
>is correct, assuming I have it set to 29.80". But if OAT were -10
>degrees, then I'd have to start worrying about being too low.
>
jcrogin
But doesn't the altimeter _setting_ compensate for non-standard
temperature? And even if temperature isn't standard the lapse rate
still might be.
In article <381ba...@news.greatbasin.net>, "Mike Rapoport"
<rapo...@ix.netcom.com> wrote:
> No, your altimeter does not compensate for non-standard
> temperature in any
> way. If you have the correct local altimeter setting then the
> altimeter is
> ONLY correct at standard temps. You should also know that the
> 2C/1000 ft is
> for calibrating altimeters and has nothing to with the actual
> lapse rate in
> the atmosphere.
* Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
Randall Becker
news:1415c574...@usw-ex0101-006.remarq.com...
>
> But doesn't the altimeter _setting_ compensate for non-standard
> temperature? And even if temperature isn't standard the lapse rate
> still might be.
Altimeter setting, from values provided by ATC, AWOS, ASOS, and METARs,
etc., does not in any way take into account non-standard settings. In
Canada, where it *IS* a serious problem in winter, charts are published for
quick reference. I'm providing a sample extract (Do not use this for
navigation):
Ground Height Above Elevation of Ground Source (feet AGL)
Temp 200 500 1000 2000
0C 0 20 40 80
-10C 20 40 80 160
-30C 40 80 160 340
(Reference: Canadian A.I.P. Figure 9.1 - A.I.P. is the Canadian analog to
the US AIM)
ATC will adjust minimum vectoring altitudes (MVA) to compensate for this.
However, we as IFR pilots are required to adjust our IAP altitudes upward
based on the above chart, this includes crossing altitudes, MDA's and DH's.
At -30C temperature on the ground, and the 200 foot level, if you didn't
compensate, you'd be 40 feet lower that you were supposed to be (and
correspondingly closer to the TDZ when you actually looked up at DH. By the
way, you'd be busting regulations if you did that - so be warned: You must
correct for temperature while doing instrument approaches especially in
winter.
Hope this helps,
Randy
jcrogin
that is fundamentally different from the way my altimeter measures
pressure, and therefore the airport barometers don't account for
temperature, and my altimeter does?
Hmmm ...
In article, "Randall Becker" <randall...@sympatico.ca> wrote:
> Altimeter setting, from values provided by ATC, AWOS, ASOS, and
> METARs,
> etc., does not in any way take into account non-standard settings.
* Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
Charlie Wendell
>>Ok, I'm flying at 5000 ft over a station at sea level reporting 29.80"
>>and 5 degrees Celsius. My OAT guage says -5 degrees. Since lapse rate
>>is standard (even though temperature & pressure are not) my altimeter
>>is correct, assuming I have it set to 29.80". But if OAT were -10
>>degrees, then I'd have to start worrying about being too low.
>>
First thing to remember is that your altimeter is calibrated so that
(in the lower part of the atmosphere) it indicates an altitude change of
30 feet for each millibar of pressure change.
Next thing, in warm air ( which is less dense and lighter than cold air)
the pressure will change more slowly with altitude, in other words, you
may find it takes 36 feet to get a millibar of pressure change.
So .. you have gone up 36 feet, but the altimeter indicates a 30 foot
30 foot change because of it's calibration. Now you are higher
than indicated.
Opposite in cold air, you may only need 25 feet to get a millibar
of pressure change. so .. you go up 25 feet, the altimeter indicates
you have gone up 30 feet .. you are not as high as you think.
Re your example .. at 5000 ft, OAT -5 C and ground (sea level) temp
of 5 c. you are flying in an air mass which is colder than
standard. Even with a correct altimeted setting you are below your
indicated altitude .. actually you are at approx 4800 ft above
sea level.
It is probably safe to say that your altimeter only reads the correct
altitude when it is on the runway at the airfield for which the
current altimeter setting is being used. As soon as you take off
you are looking at an indicated altitude which may or may not
be the true altitude. As long as everybody else in your area is using the
same setting, there should be no problems.
anyhow, my 2 cents worth.
--
Cheers, Charlie
Randall Becker
mercury filled tubes). As far as I've been taught, they look at the pressure
value they see on the readout - which gives them a number, provide a
pressure correction between their station and MSL and tell you that number.
They don't go out to the runway and check that the pressure they're
reporting will result in a valid altitude on your airplane. You as the pilot
have to check that. But the variance due to temperature on the ground - hey
this is good - is virtually the same because the altimeters are going to
give you close to the same altitude. Ground readings are not the problem.
Altimeters are calibrated so that changes in pressure (the effect on the
aneroid capsule in your hardware) have corresponding changes in altitude at
standard ISA temperatures. Because you're not at ISA temperatures in the
deep dark cold of winter, **** you can't assume that the calibration is
valid ****. THAT is really the point... the calibration. In my previous
post, I showed a sample correction chart. It's pretty severe.
When flying an airway at 5000 feet (indicated) somewhere near MSP in the
winter (example taken because it's plausible anyway ;-) ), if it's -40 on
the ground, then, if you don't correct for temperature, and you shouldn't in
this case, you're actually at at 3,900 feet (ASL). ATC will correct for this
when you're being vectored and not assign you 5000 feet if that's below
MSA/MVA/MOCA/MEA - so don't panic.
Where it applies is when you're doing an approach and have to worry about
the crossing and decision altitudes that ATC is not assigning. It's your
responsibility to get it right. Remember, radio tower and building tops have
auto-correction for pressure and temperature built in. :-).
Randy
jcrogin <jcrogin...@yahoo.com.invalid> wrote in message
news:0a0133f8...@usw-ex0102-012.remarq.com...
Mike Rapoport
variable is preasure and the lapse rate is irrelevent because you are only
concerned with one elevation. In an airplane the preasure lapse rate AND
the altitude are unknown. The problem is that altimeters measure preasure
and read out altitude. The relationship between preasure and altitude
changes with temperature. Even at ISA, altimeters are in error because
temperatures in the real atmosphere do not lapse 2C/1000'.
Mike
MU-2
jcrogin wrote in message <0a0133f8...@usw-ex0102-012.remarq.com>...
Newps
>
> When flying an airway at 5000 feet (indicated) somewhere near MSP in the
> winter (example taken because it's plausible anyway ;-) ), if it's -40 on
> the ground, then, if you don't correct for temperature, and you shouldn't in
> this case, you're actually at at 3,900 feet (ASL). ATC will correct for this
> when you're being vectored and not assign you 5000 feet if that's below
> MSA/MVA/MOCA/MEA - so don't panic.
Uh...what? We don't correct for anything that's temperature induced.
The MVA/MEA etc, maps are good year round. My MVA maps haven't changed
in years.
>
> Where it applies is when you're doing an approach and have to worry about
> the crossing and decision altitudes that ATC is not assigning. It's your
> responsibility to get it right. Remember, radio tower and building tops have
> auto-correction for pressure and temperature built in. :-).
So you're telling me that when it's -40 here and I put you on the ILS
at 5700 you're really at 4600? That's BS. If you follow the glideslope
down to 200' AGL indicated you will be at about 3850 MSL indicated.
You'll actually be 200 feet AGL. But according to you, you would be
900' underground if you follow your altimeter.
Mike Rapoport
will be lower than indicated on the altimeter. The airplane will not
intercept the GS at the GS intercept point at the intercept point at the GS
intercept altitude (as shown on the altimeter). The error gets smaller as
the airplane gets closer to the altitude of the point where the altimeter
setting is derived from (generally the airport). ATC is not responsible for
changing the altitudes it is the pilots responsibility. You don't see much
affect in the most of the US because the temperatures aren't cold enough,
but in Canada and Alaska it is a big deal. Where in the continental US do
you see -40?
Mike
MU-2
Newps wrote in message <3821BA72...@mcn.net>...
Doug Campbell
Mike Rapoport wrote in message <3821a...@news.greatbasin.net>...
>That is not what anyone has said. The airport elevation is KNOWN. The
only
>variable is preasure and the lapse rate is irrelevent because you are only
>concerned with one elevation. In an airplane the preasure lapse rate AND
>the altitude are unknown. The problem is that altimeters measure preasure
>and read out altitude. The relationship between preasure and altitude
>changes with temperature. Even at ISA, altimeters are in error because
>temperatures in the real atmosphere do not lapse 2C/1000'.
>
>Mike
>MU-2
Well, does the airport report the actual pressure, or the pressure setting
required so the altimeter will read field elevation? This MAY be different,
due to temperature?
Doug
Richard Bollar
front. The temperature drop is a good warning that the barometric pressure
may have changed. And if you're not near a reporting station, you could
easily be much lower than you anticipated.
--
Interested in Personal TV like ReplayTV or TiVo? Subscribe to the Personal
TV mailing list! Write to majo...@swizzle.com with "subscribe personaltv"
in the message body.
rober...@my-deja.com
"The pressure is..." they say
"Altimeter setting...".
The altimeter setting is whatever is necessary to acheive the
field elevation regardless of actual pressure.
I had an instructor ask me what altitude I would cross the mountains
at night, VFR once. I looked at the highest point and said,
"At least the highest point + X". He had me work out a couple problems
on the Wizwheel accounting for temp. I could easily
have hit a mountain peak!!!
In article <3821a...@news.greatbasin.net>,
"Mike Rapoport" <rapo...@ix.netcom.com> wrote:
> That is not what anyone has said. The airport elevation is KNOWN.
The only
> variable is preasure and the lapse rate is irrelevent because you are
only
> concerned with one elevation. In an airplane the preasure lapse rate
AND
> the altitude are unknown. The problem is that altimeters measure
preasure
> and read out altitude. The relationship between preasure and altitude
> changes with temperature. Even at ISA, altimeters are in error
because
> temperatures in the real atmosphere do not lapse 2C/1000'.
>
> Mike
> MU-2
>
> jcrogin wrote in message
<0a0133f8...@usw-ex0102-012.remarq.com>...
> >So you are saying that airport barometers measure pressure in a way
> >that is fundamentally different from the way my altimeter measures
> >pressure, and therefore the airport barometers don't account for
> >temperature, and my altimeter does?
> >Hmmm ...
> >
> >
> >In article, "Randall Becker" <randall...@sympatico.ca> wrote:
> >
> >> Altimeter setting, from values provided by ATC, AWOS, ASOS, and
> >> METARs,
> >> etc., does not in any way take into account non-standard settings.
> >
> >
> >
> >* Sent from RemarQ http://www.remarq.com The Internet's Discussion
Network
> *
> >The fastest and easiest way to search and participate in Usenet -
Free!
> >
>
>
Sent via Deja.com http://www.deja.com/
Before you buy.
andrew m. boardman
> Uh...what? We don't correct for anything that's temperature induced.
>The MVA/MEA etc, maps are good year round. My MVA maps haven't changed
>in years.
Some Canadian facilities have a separate "cold weather MVA" map. Never
heard of 'em down here, though.
Randall Becker
> So you're telling me that when it's -40 here and I put you on the ILS
> at 5700 you're really at 4600? That's BS. If you follow the glideslope
> down to 200' AGL indicated you will be at about 3850 MSL indicated.
> You'll actually be 200 feet AGL. But according to you, you would be
> 900' underground if you follow your altimeter.
The error gets less and less as you approach the airport, of course. Here's
a quote from the same Canadian regulations (from A.I.P. RAC 9-11) containing
the altitude correction chart. I hope you find this helpful. If the U.S.
regulations are different, please post to that effect.
-- START QUOTE
COLD TEMPERATURE CORRECTIONS
Pressure altimeters are calibrated to indicate true altitude under ISA
conditions. Any deviation from ISA will result in an erroneous reading on
the altimeter. In a case when the temperature is higher than the ISA, the
true altitude will be higher than the figure indicated by the altimeter, and
the true altitude will be lower when the temperature is lower than the ISA.
The altimeter error may be significant, and becomes extremely important when
considering obstacle clearances in very cold temperatures.
In conditions of extreme cold weather, pilots should add the values derived
from the Altitude Correction Chart to the published procedure altitudes,
including minimum sector altitudes and DME arcs, to ensure adequate obstacle
clearance. Unless otherwise specified, the destination aerodrome elevation
is used as the elevation of the altimeter source.
With respect to altitude corrections, the following procedures apply:
1. IFR assigned altitudes may be either accepted or refused. Refusal in this
case is based upon the pilot's assessment of temperature effect on
obstruction clearance.
2. IFR assigned altitudes accepted by a pilot shall not be adjusted to
compensate for cold temperatures, i.e., if a pilot accepts "maintain 3000",
an altitude correction shall not be applied to 3000 feet.
3. Radar vectoring altitudes assigned by ATC are temperature compensated and
require no corrective action by pilots.
4. When altitude corrections are applied to a published final approach fix
crossing altitude, procedure turn or missed approach altitude, pilots should
advise ATC how much of a correction is to be applied.
-- END QUOTE
Newps
rober...@my-deja.com wrote:
>
> This is why on ATIS, they never say
> "The pressure is..." they say
> "Altimeter setting...".
>
> The altimeter setting is whatever is necessary to acheive the
> field elevation regardless of actual pressure.
Uh, Negative. The altimeter setting is the actual pressure at the
station.
Mike Rapoport
"0" at the airport. The preasure is read by setting an altimeter to the
field elavation. The actual preasure here is about 25.1 mm/Hg (4700'
elevation) The altimeter setting is 30.09 at the airport 4 miles away.
Mike
MU-2
Newps wrote in message <38224B3D...@mcn.net>...
Garner Miller
> > The altimeter setting is whatever is necessary to acheive the
> > field elevation regardless of actual pressure.
>
>
> Uh, Negative. The altimeter setting is the actual pressure at the
> station.
Uh, no it's not. If you're at a 5,000-foot-elevation airport on a
standard day, the pressure at that station is most certainly NOT 29.92
inches. It'll be something close to 25 inches. What robertgary said
is right on the mark.
--
Garner R. Miller, CFII/MEI
Pilot, US Airways Express, Portland, ME
http://www.netstreet.net/~garner/
Please don't e-mail me a copy of your usenet reply.
Johannes Czernin
field elevation....... which means WHEN YOU ARE ON THE GROUND RIGHT THERE!
Johannes
Doug Campbell <dou...@netscape.net> wrote in message:
8XkU3.7$RG1....@news.callamer.com...
>
> Mike Rapoport wrote in message <3821a...@news.greatbasin.net>...
> >That is not what anyone has said. The airport elevation is KNOWN. The
> only
> >variable is preasure and the lapse rate is irrelevent because you are
only
> >concerned with one elevation. In an airplane the preasure lapse rate AND
> >the altitude are unknown. The problem is that altimeters measure
preasure
> >and read out altitude. The relationship between preasure and altitude
> >changes with temperature. Even at ISA, altimeters are in error because
> >temperatures in the real atmosphere do not lapse 2C/1000'.
> >
> >Mike
> >MU-2
>
rober...@my-deja.com
altimeter setting. The altimeter setting you
get off ATIS guarantees you that a good altimeter
will read field elevation when on the ground
(at the airport reference point).
It's just unfortunate that they use numbers that
look like pressure. If they created a whole new
measurement this confusion would be less.
The reason you worry when going from hot to cold
is that you may not have a more current altimeter
setting. When you go from hot to cold, just find
a current altimeter setting from approach and you
no longer have to worry.
This is important in California when you cross the costal mountains
in the summer. It may be 70 degress on the ground on the coast side,
and 110 on the valley side.
In article <3821c...@news.greatbasin.net>,
"Mike Rapoport" <rapo...@ix.netcom.com> wrote:
> Actually its not BS. If the temperature is below standard then the
airplane
> will be lower than indicated on the altimeter. The airplane will not
> intercept the GS at the GS intercept point at the intercept point at
the GS
> intercept altitude (as shown on the altimeter). The error gets
smaller as
> the airplane gets closer to the altitude of the point where the
altimeter
> setting is derived from (generally the airport). ATC is not
responsible for
> changing the altitudes it is the pilots responsibility. You don't see
much
> affect in the most of the US because the temperatures aren't cold
enough,
> but in Canada and Alaska it is a big deal. Where in the continental
US do
> you see -40?
>
> Mike
> MU-2
>
> Newps wrote in message <3821BA72...@mcn.net>...
> >
> > Uh...what? We don't correct for anything that's temperature
induced.
> >The MVA/MEA etc, maps are good year round. My MVA maps haven't
changed
> >in years.
> >
> >
> >>
> >> Where it applies is when you're doing an approach and have to worry
about
> >> the crossing and decision altitudes that ATC is not assigning. It's
your
> >> responsibility to get it right. Remember, radio tower and building
tops
> have
> >> auto-correction for pressure and temperature built in. :-).
> >
> > So you're telling me that when it's -40 here and I put you on the
ILS
> >at 5700 you're really at 4600? That's BS. If you follow the
glideslope
> >down to 200' AGL indicated you will be at about 3850 MSL indicated.
> >You'll actually be 200 feet AGL. But according to you, you would be
> >900' underground if you follow your altimeter.
>
>
Everett M. Greene
[snip]
> When flying an airway at 5000 feet (indicated) somewhere near MSP in the
> winter (example taken because it's plausible anyway ;-) ), if it's -40 on
> the ground, then, if you don't correct for temperature, and you shouldn't in
> this case, you're actually at at 3,900 feet (ASL). ATC will correct for this
> when you're being vectored and not assign you 5000 feet if that's below
> MSA/MVA/MOCA/MEA - so don't panic.
5000 feet should be safe in the MSP area at any temperature.
I doubt you can find much of anything above 2000 feet within
500 miles of there in any direction. 8-)
[snip]
----------------------------------------------------------------------
Everett M. Greene (The Mojave Greene, crotalus scutulatus scutulatus)
Ridgecrest, Ca. 93555 Path: moj...@IWVISP.com
Everett M. Greene
> Newps wrote in message <3821BA72...@mcn.net>...
> >
> > Uh...what? We don't correct for anything that's temperature induced.
> >The MVA/MEA etc, maps are good year round. My MVA maps haven't changed
> >in years.
> >>
> >> Where it applies is when you're doing an approach and have to worry about
> >> the crossing and decision altitudes that ATC is not assigning. It's your
> >> responsibility to get it right. Remember, radio tower and building tops
> >> have auto-correction for pressure and temperature built in. :-).
> >
> > So you're telling me that when it's -40 here and I put you on the ILS
> >at 5700 you're really at 4600? That's BS. If you follow the glideslope
> >down to 200' AGL indicated you will be at about 3850 MSL indicated.
> >You'll actually be 200 feet AGL. But according to you, you would be
> >900' underground if you follow your altimeter.
>
> will be lower than indicated on the altimeter. The airplane will not
> intercept the GS at the GS intercept point at the intercept point at the GS
> intercept altitude (as shown on the altimeter). The error gets smaller as
> the airplane gets closer to the altitude of the point where the altimeter
> setting is derived from (generally the airport). ATC is not responsible for
> changing the altitudes it is the pilots responsibility. You don't see much
> affect in the most of the US because the temperatures aren't cold enough,
> but in Canada and Alaska it is a big deal. Where in the continental US do
> you see -40?
Minnesota, North Dakota,... in January.
Mike Rapoport
It is done by setting an altimeter to the airport elevation. If it was
measuring actual preasure the numbers at mountain airports would be below
25". At the airport the altimeter will be accurate. In nonstandard
conditions it will ONLY be accurate at the airport.
Mike
MU-2
Doug Campbell wrote in message <8XkU3.7$RG1....@news.callamer.com>...
Mike Rapoport
low is -33F. There is no place in the lower 48 that gets -40F regularly.
That is why the FAA doesn't issue the altimeter correction chart and Canada
does (The USAF does also). See http://weather.yahoo.com/almanac/
Mike
MU-2
Everett M. Greene wrote in message
<19991105.7...@mojaveg.iwvisp.com>...
Mike Rapoport
altimeter setting with the rate that preasure lapses with altitude. There
is an article in last months "Business and Commercial Aviation" (NBAA
magazine) and fairly recently in IFR and the topic is covered in Barry
Schiff's books. The issue is almost irrelevent in CA since the temp is
almost always well above ISA, so you are higher than indicated.
Mike
MU-2
rober...@my-deja.com wrote in message <7vvpmv$fj2$1...@nnrp1.deja.com>...
>I think you are confusing pressure with
>altimeter setting. The altimeter setting you
>get off ATIS guarantees you that a good altimeter
>will read field elevation when on the ground
>(at the airport reference point).
>
>It's just unfortunate that they use numbers that
>look like pressure. If they created a whole new
>measurement this confusion would be less.
>
>The reason you worry when going from hot to cold
>is that you may not have a more current altimeter
>setting. When you go from hot to cold, just find
>a current altimeter setting from approach and you
>no longer have to worry.
>
>This is important in California when you cross the costal mountains
>in the summer. It may be 70 degress on the ground on the coast side,
>and 110 on the valley side.
>
>
>In article <3821c...@news.greatbasin.net>,
> "Mike Rapoport" <rapo...@ix.netcom.com> wrote:
>> Actually its not BS. If the temperature is below standard then the
>airplane
>> will be lower than indicated on the altimeter. The airplane will not
>> intercept the GS at the GS intercept point at the intercept point at
>the GS
>> intercept altitude (as shown on the altimeter). The error gets
>smaller as
>> the airplane gets closer to the altitude of the point where the
>altimeter
>> setting is derived from (generally the airport). ATC is not
>responsible for
>> changing the altitudes it is the pilots responsibility. You don't see
>much
>> affect in the most of the US because the temperatures aren't cold
>enough,
>> but in Canada and Alaska it is a big deal. Where in the continental
>US do
>> you see -40?
>>
>> Mike
>> MU-2
>>
>> Newps wrote in message <3821BA72...@mcn.net>...
>> >
>> > Uh...what? We don't correct for anything that's temperature
>induced.
>> >The MVA/MEA etc, maps are good year round. My MVA maps haven't
>changed
>> >in years.
>> >
>> >
>> >>
>> >> Where it applies is when you're doing an approach and have to worry
>about
>> >> the crossing and decision altitudes that ATC is not assigning. It's
>your
>> >> responsibility to get it right. Remember, radio tower and building
>tops
>> have
>> >> auto-correction for pressure and temperature built in. :-).
>> >
>> > So you're telling me that when it's -40 here and I put you on the
>ILS
>> >at 5700 you're really at 4600? That's BS. If you follow the
>glideslope
>> >down to 200' AGL indicated you will be at about 3850 MSL indicated.
>> >You'll actually be 200 feet AGL. But according to you, you would be
>> >900' underground if you follow your altimeter.
>>
>>
>
>
William W. Plummer
temperatures?
Also, -40C is also -40F. Cool.
--Bill
Mike Rapoport wrote in message <382b0...@news.greatbasin.net>...
>The RECORD January low for Fargo, ND is -35F and for Missoula, MT the
RECORD
>low is -33F. There is no place in the lower 48 that gets -40F regularly.
>That is why the FAA doesn't issue the altimeter correction chart and Canada
>does (The USAF does also). See http://weather.yahoo.com/almanac/
>
>Mike
>MU-2
>
>
>
>
>
>
>
>
>Everett M. Greene wrote in message
><19991105.7...@mojaveg.iwvisp.com>...
>>In article <3821c...@news.greatbasin.net> "Mike Rapoport"
><rapo...@ix.netcom.com> writes:
>
>>> Actually its not BS. If the temperature is below standard then the
>airplane
>>> will be lower than indicated on the altimeter. The airplane will not
>>> intercept the GS at the GS intercept point at the intercept point at the
>GS
>>> intercept altitude (as shown on the altimeter). The error gets smaller
>as
>>> the airplane gets closer to the altitude of the point where the
altimeter
>>> setting is derived from (generally the airport). ATC is not responsible
>for
>>> changing the altitudes it is the pilots responsibility. You don't see
>much
>>> affect in the most of the US because the temperatures aren't cold
enough,
>>> but in Canada and Alaska it is a big deal. Where in the continental US
>do
>>> you see -40?
>>
Randall Becker
until July :-)
William W. Plummer <wplu...@V-sign.com> wrote in message
news:s2ms7a...@corp.supernews.com...
Mike Rapoport
May! That was at 17,200'
Mike
MU-2
William W. Plummer wrote in message ...
Mark Mallory
rober...@my-deja.com wrote:
> The reason you worry when going from hot to cold
> is that you may not have a more current altimeter
> setting. When you go from hot to cold, just find
> a current altimeter setting from approach and you
> no longer have to worry.
No, it has nothing to do with how "current" the altimeter setting is.
Because cold air is denser than warm air, pressure decreases faster with
height if the air is cold. For a given increase in acutal altitude, on
a cold day the altimeter will see a greater pressure decrease and thus
indicate a larger altitude increase.
The altimeter setting is that which will cause the altimeter to read the
correct altitude ON THE GROUND, at the location of the reporting
station. If an airplane takes off from the reporting station and climbs
to 1000 AGL, the altimeter will change by MORE than 1000 ft if the
temperature is belowl standard, and (if set to the current altimeter
setting) will thus indicate HIGHER than 1000 agl. The acutal altitude
is LOWER than the indicated altitude on a cold day.
rober...@my-deja.com
that the altimeter will indicate incorrect (even
with a current alt. setting of the airport directly under you).
If you are using a change in altitude temp. to try to determine
changes in lapse rate then I can agree with you.
In article <382C68F4...@earthlink.com>,
Mark Mallory
>
> I understand that if the lapse rate is not standard
> that the altimeter will indicate incorrect (even
> with a current alt. setting of the airport directly under you).
>
> If you are using a change in altitude temp. to try to determine
> changes in lapse rate then I can agree with you.
It's not Lapse Rate; it's TEMPERATURE. On a cold day, the temperature
can be BELOW standard even if the lapse rate IS standard (1.98 degC/1000
ft).
Randall Becker
the reference source at the nearby airport. Mine reading wrong. Instead of
being on the 11th floor, I think I'm on the 13th floor. It's a good thing,
because rent is higher on higher floors, so I feel like I'm getting a deal.
Really, I'd get a weather balloon and do a skew-t from the ground to get
lapse rates up to my balcony, if you want, but I'm sure also, that it's
temperature that's solely at issue here.
Big smiles and blue skies,
Randy
Mark Mallory <mar...@earthlink.com> wrote in message
news:383083A6...@earthlink.com...
rober...@my-deja.com
change is accounting for in the altimeter setting and
does not change with altitude.
We can express our disagreement in a simple formula.....
y=Ax + B
where...
y = true altitude as read by a military GPS or GOD
x = the pressure felt on the static port by the altimeter
A,B = some factors that we will disagree about...
My position: temp affects only B (assuming constant lapse rate)
Your position: temp affects A (otherwise altitude would not affect
and altimeter setting would account for temp at all altitudes).
We can probably agree that an altimeter reads A as a constant
(there is no special mechanism smart enough to make the altimeter
calculate a non-constant B).
We know that factor A includes our lapse rate (with or without other
stuff).
If what you are saying is correct the factor A MUST vary by temperature.
This doesn't make any sense though since even density altitude
varies DIRECTLY with temp.
In article <383083A6...@earthlink.com>,
Mark Mallory <mar...@earthlink.com> wrote:
> It's not Lapse Rate; it's TEMPERATURE. On a cold day, the temperature
> can be BELOW standard even if the lapse rate IS standard (1.98
degC/1000
> ft).
>
>
rober...@my-deja.com
The threads seems to have already agreed that altimeter
setting is not barametric pressure, but whatever is necessary
to show field elevation on the field. I'm sure if you walked
over to your airport you would find that the altimeter setting
being reported is NOT the same as the pressure shown on your
barameter. My position is that
the temperature difference that is already built into the altimeter
setting (not barameteric pressure) accounting for field temperature,
works at ALL altitudes (assuming standard lapse rate).
If this were not true, the altimeters would be worthless above
10,000 feet since even during a standard lapse rate of 2degrees/1,000
feet
would already be off 20 degrees!
In article <AI2Y3.54763$up3....@news21.bellglobal.com>,
"Randall Becker" <randall...@sympatico.ca> wrote:
> It snowed here in Toronto today <brrrrrr>. My barometer is 500 feet
above
> the reference source at the nearby airport. Mine reading wrong.
Instead of
> being on the 11th floor, I think I'm on the 13th floor. It's a good
thing,
> because rent is higher on higher floors, so I feel like I'm getting a
deal.
>
> Really, I'd get a weather balloon and do a skew-t from the ground to
get
> lapse rates up to my balcony, if you want, but I'm sure also, that
it's
> temperature that's solely at issue here.
>
> Big smiles and blue skies,
>
> Randy
>
> Mark Mallory <mar...@earthlink.com> wrote in message
> news:383083A6...@earthlink.com...
> > rober...@my-deja.com wrote:
> > >
> > > I understand that if the lapse rate is not standard
> > > that the altimeter will indicate incorrect (even
> > > with a current alt. setting of the airport directly under you).
> > >
> > > If you are using a change in altitude temp. to try to determine
> > > changes in lapse rate then I can agree with you.
> >
Mark Mallory
rober...@my-deja.com wrote:
> y=Ax + B
> where...
> y = true altitude as read by a military GPS or GOD
> x = the pressure felt on the static port by the altimeter
> A,B = some factors that we will disagree about...
>
> My position: temp affects only B (assuming constant lapse rate)
> Your position: temp affects A (otherwise altitude would not affect
> and altimeter setting would account for temp at all altitudes).
>
> We can probably agree that an altimeter reads A as a constant
>
(pressure). However, for small *changes* in x, the function can be
assumed to be linear with A = constant.
> (there is no special mechanism smart enough to make the altimeter
> calculate a non-constant B).
>
Actually, B is what you change when you adjust the altimeter setting.
> We know that factor A includes our lapse rate (with or without other
> stuff).
>
Suggest you forget about Lapse Rate; it only introduces unnecessary
confusion. For this example, assume that the Standard Atmosphere Lapse
Rate is ZERO (as it is in the stratosphere) and that the altimeter is
calibrated to read accurately in such an atmosphere.
> If what you are saying is correct the factor A MUST vary by temperature.
>
CORRECT! This is *precisely^ the problem: the altimeter responds to
Pressure ONLY; it has no mechanism for using different values of "A"
based upon temperature. However, this is exactly what it would have to
do in order to give an accurate reading in non-standard temperatures.
> This doesn't make any sense though since even density altitude
> varies DIRECTLY with temp.
>
Sounds like you've got it. The RATE at which Pressure decreases with
Altitude (your factor "A") is directly proportional to DENSITY, which
(as you note) varies with temperature.
Mike Rapoport
sense preasure and read out in altitude. They do this by assuming that a
given increase in altitude results in a specific decrease in preasure. The
underlying assumption is that the air has a specific density. Since cold
air is denser than warm, the altimeter is ALWAYS in error unless the
atmosphere is at ISA temperature and ISA temperature lapse rate. If you
don't think that the density of the fluid affects the rate at which preasure
changes with depth, then take you altimeter scuba diving. You will become a
believer.
Mike
MU-2
rober...@my-deja.com wrote in message <80s0fg$5j1$1...@nnrp1.deja.com>...
>Yes, but my position is that that temperature
>change is accounting for in the altimeter setting and
>does not change with altitude.
>
>We can express our disagreement in a simple formula.....
>
>y=Ax + B
>where...
>y = true altitude as read by a military GPS or GOD
>x = the pressure felt on the static port by the altimeter
>A,B = some factors that we will disagree about...
>
>My position: temp affects only B (assuming constant lapse rate)
>Your position: temp affects A (otherwise altitude would not affect
>and altimeter setting would account for temp at all altitudes).
>
>We can probably agree that an altimeter reads A as a constant
>(there is no special mechanism smart enough to make the altimeter
>calculate a non-constant B).
>
>We know that factor A includes our lapse rate (with or without other
>stuff).
>
>If what you are saying is correct the factor A MUST vary by temperature.
>
>This doesn't make any sense though since even density altitude
>varies DIRECTLY with temp.
>
>
>In article <383083A6...@earthlink.com>,
> Mark Mallory <mar...@earthlink.com> wrote:
>
rober...@my-deja.com
Let me phrase my belief in another way....
"It seems silly to me that anyone would design an altimeter to
simply record a 100 foot difference for each 0.10 difference
in pressure felt on the static port, to add a small skew to assume that
its 20 degrees colder
at 10,000 feet would be trivial".
If we assume that the original altimeters were designed empiricly
and not mathematicly, we would expect this.
Someone in the earily days designing an altimeter would probably have
climbed to a know height and ensured his altimeter was adjusted
to read correctly (accounting for all sorts of "mysterious" differences
without even realizing it).
Since density altitude varies predictably with temperature
change, this seems easy.
Notice that I'm NOT saying the altimeter senses temperature,
I'm suggesting that it may ASSUME temperature change with altitude.
Since the temperature is already reflected in the ground based
altimeter setting, this assumption would carry that reflection
up with altitude.
I've acutaly just sent off an email to an altimeter manufacturing
company to ask if there is any such skew built in.
Mark Mallory wrote:
>
> rober...@my-deja.com wrote:
> > y=Ax + B
> > where...
> > y = true altitude as read by a military GPS or GOD
> > x = the pressure felt on the static port by the altimeter
> > A,B = some factors that we will disagree about...
> >
> > My position: temp affects only B (assuming constant lapse rate)
> > Your position: temp affects A (otherwise altitude would not affect
> > and altimeter setting would account for temp at all altitudes).
> >
> > We can probably agree that an altimeter reads A as a constant
> >
> It's not a constant, but rather a *non-linear* FUNCTION of x
> (pressure). However, for small *changes* in x, the function can be
> assumed to be linear with A = constant.
>
> > (there is no special mechanism smart enough to make the altimeter
> > calculate a non-constant B).
> >
> Actually, B is what you change when you adjust the altimeter setting.
>
> > We know that factor A includes our lapse rate (with or without other
> > stuff).
> >
> Suggest you forget about Lapse Rate; it only introduces unnecessary
> confusion. For this example, assume that the Standard Atmosphere
Lapse
> Rate is ZERO (as it is in the stratosphere) and that the altimeter is
> calibrated to read accurately in such an atmosphere.
>
> > If what you are saying is correct the factor A MUST vary by
temperature.
> >
> CORRECT! This is *precisely^ the problem: the altimeter responds to
> Pressure ONLY; it has no mechanism for using different values of "A"
> based upon temperature. However, this is exactly what it would have
to
> do in order to give an accurate reading in non-standard temperatures.
>
> > This doesn't make any sense though since even density altitude
> > varies DIRECTLY with temp.
> >
> Sounds like you've got it. The RATE at which Pressure decreases with
> Altitude (your factor "A") is directly proportional to DENSITY, which
> (as you note) varies with temperature.
jcrogin
> which (as you note) varies with temperature.
Ah, but this brings us back to the original paradox - the fact that
cold air is MORE dense than warm air. And you seem to be saying that
more density = higher pressure, thus hot to cold = clear the sky. I'm
still in a fog.
jcrogin
> It snowed here in Toronto today <brrrrrr>. My barometer is 500
> feet above
> the reference source at the nearby airport. Mine reading wrong.
> Instead of
> being on the 11th floor, I think I'm on the 13th floor. It's a
> good thing,
> because rent is higher on higher floors, so I feel like I'm
> getting a deal.
> Really, I'd get a weather balloon and do a skew-t from the ground
> to get
> lapse rates up to my balcony, if you want, but I'm sure also, that
> it's
> temperature that's solely at issue here.
* Sent from RemarQ http://www.remarq.com The Internet's Discussion Network *
Mike Rapoport
altimeter is calibrated for ISA and therefore does account for a 2C/1000'
lapse. So if it was -40C at an airport on the south col of Everest at
27,500' (which is ISA), the altimeter would read correctly above the
airport (if the altimeter was perfect mechanically). Ideally the altimeter
correction tables would take the field elevation into account but this would
require a three dimensional chart (or a bunch of charts). Since there are
no airports with instrument approaches above 10,000' where it ever gets
to -40C, the charts assume the airport is at sea level. The point is that
if the temperature is below ISA then the altimeter will read high when it is
above the elevation where the altimeter setting is produced.
Mike
MU-2
rober...@my-deja.com wrote in message <80ulb0$1sd$1...@nnrp1.deja.com>...
>> Sounds like you've got it. The RATE at which Pressure decreases with
>> Altitude (your factor "A") is directly proportional to DENSITY, which
>> (as you note) varies with temperature.
>
>
Mike Rapoport
Mike
MU-2
jcrogin wrote in message <0a0133f8...@usw-ex0101-006.remarq.com>...
Mark Mallory
> > which (as you note) varies with temperature.
jcrogin wrote:
> Ah, but this brings us back to the original paradox - the fact that
> cold air is MORE dense than warm air. And you seem to be saying that
> more density = higher pressure, thus hot to cold = clear the sky. I'm
> still in a fog.
>
Which is where you're likely to remain until you learn to READ. Nowhere
above did I say more density = higher pressure.
And how do you get "hot to cold = clear the sky"? Ever heard of DEW
POINT?
Mark Mallory
>I'm suggesting that it may ASSUME temperature change with altitude.
>
corresponding to the STANDARD ATMOSPHERE. Again, the problem is that if
the temperature is BELOW standard (even if the Lapse Rate IS standard)
the air will be DENSER than what the altimeter is calibrated for, and
will show a GREATER change in altitude (for a given ACTUAL change in
altitude) than is actually the case.
>Since the temperature is already reflected in the ground based
>altimeter setting, this assumption would carry that reflection
>up with altitude.
>
WRONG. Temperature is NOT "reflected" in the ground-based altimeter
setting. You CANNOT infer the surface Temperature from the surface
Pressure.
The altimeter "assumes" that the temperature is equal to the STANDARD
VALUE, at the measured AMBIENT PRESSURE. For example, at Sea Level
(pressure = 29.92 inHg), this temperature is 15 degC. However, suppose
the Sea Level TEMPERATURE were to drop to -10 C, but the Sea Level
pressure REMAINED at 29.92 (with Lapse Rates the SAME in both cases).
In BOTH of these cases the altimeter setting is 29.92, and in BOTH of
these cases the altimeter would give an ACCURATE reading at SEA LEVEL,
but in the latter (colder) case an altimeter at 1000' AGL would read
HIGHER than the actual altitude, because the air is DENSER than what the
altimeter is calibrated for.
Mark Mallory
rober...@my-deja.com wrote:
> "It seems silly to me that anyone would design an altimeter to
> simply record a 100 foot difference for each 0.10 difference
> in pressure felt on the static port,
Nobody does. The relationship between Pressure and Altitude in the
Standard Atmosphere is NON-LINEAR. Below 36,089 ft (11,000 m) the
relationship is given by:
_ _
| .190165 |
A = 145,366 * | 1 - ( P / 29.92 ) |
|_ _|
where A = indicated altitude (feet)
P = ambient pressure (inches Hg)
An altimeter set to 29.92, if properly calibrated, will indicate in
accordance with this relationship. The *complete* relationship
(including the Kollsman window setting K) is:
_ _
| .190165 .190165 |
A = 145,366 * | ( K / 29.92 ) - ( P / 29.92 ) |
|_ _|
jcrogin
enough to snow inside his office, so wouldn't the 'meter read
differently than if it were outside?
> It doesn't matter since the pressure will equalize.
> Mike
> MU-2
> jcrogin wrote in message:
jcrogin
temperature are (drumroll) the result of the _nonlinear_ function
between pressure and altitude. Cause if it WAS linear, then the
altimeter setting would be able to compensate for nonstandard temps, no
matter whether you were at airport elevation or 10,000 ft above.
So to bring lapse rate into the equation, it would seem that a higher
than standard temperature lapse would tend to EXACERBATE altimeter
error, and lower than standard would tend to MITIGATE it. Thus, to
complete the spiel, we could say: hot to cold and well above the
reporting station with a higher than standard lapse rate = look out
below.
I can visualize the iso-pressure lines stacking up in the atmosphere
(at cold temperature) and causing the altimeter to "think" it is higher
than it is, but it still seems weird that density and pressure move in
inverse directions. Ah well. I'm a pilot, not a physicist.
Mike Rapoport
preasure drops with increasing altitude is greater when the air is denser.
Mike
MU-2
jcrogin wrote in message <17599f0b...@usw-ex0101-006.remarq.com>...
>> Sounds like you've got it. The RATE at which Pressure decreases
>> with Altitude is directly proportional to DENSITY,
>> which (as you note) varies with temperature.
>
>Ah, but this brings us back to the original paradox - the fact that
>cold air is MORE dense than warm air. And you seem to be saying that
>more density = higher pressure, thus hot to cold = clear the sky. I'm
>still in a fog.
>
>
Mike Rapoport
and pressures. However temperature lapse rates don't vary so much that you
could be way off when below 10,000'.
Mike
MU-2
jcrogin wrote in message <2750ac20...@usw-ex0101-006.remarq.com>...
>So I'm understanding that altimeter errors due to nonstandard
>temperature are (drumroll) the result of the _nonlinear_ function
>between pressure and altitude. Cause if it WAS linear, then the
>altimeter setting would be able to compensate for nonstandard temps, no
>matter whether you were at airport elevation or 10,000 ft above.
>
>So to bring lapse rate into the equation, it would seem that a higher
>than standard temperature lapse would tend to EXACERBATE altimeter
>error, and lower than standard would tend to MITIGATE it. Thus, to
>complete the spiel, we could say: hot to cold and well above the
>reporting station with a higher than standard lapse rate = look out
>below.
>
>I can visualize the iso-pressure lines stacking up in the atmosphere
>(at cold temperature) and causing the altimeter to "think" it is higher
>than it is, but it still seems weird that density and pressure move in
>inverse directions. Ah well. I'm a pilot, not a physicist.
>
>
Mike Rapoport
Mike
MU-2
jcrogin wrote in message <1415c574...@usw-ex0101-006.remarq.com>...
>I was thinking in terms of temperature. Presumably it's not cold
>enough to snow inside his office, so wouldn't the 'meter read
>differently than if it were outside?
>
>
>> It doesn't matter since the pressure will equalize.
>> Mike
>> MU-2
>
>> jcrogin wrote in message:
>> >Randy, is your barometer inside or outside?
>> >
>> >
>> >> It snowed here in Toronto today <brrrrrr>. My barometer is 500
>> >> feet above
>> >> the reference source at the nearby airport. Mine reading wrong.
>> >> Instead of
>> >> being on the 11th floor, I think I'm on the 13th floor. It's a
>> >> good thing,
>> >> because rent is higher on higher floors, so I feel like I'm
>> >> getting a deal.
>> >> Really, I'd get a weather balloon and do a skew-t from the
>> ground to get
>> >> lapse rates up to my balcony, if you want, but I'm sure also,
>> that it's temperature that's solely at issue here.
>
>
>
Mark Mallory
> So I'm understanding that altimeter errors due to nonstandard
> temperature are (drumroll) the result of the _nonlinear_ function
> between pressure and altitude.
No, I don't believe anyone said this.
> Cause if it WAS linear, then the
> altimeter setting would be able to compensate for nonstandard temps, no
> matter whether you were at airport elevation or 10,000 ft above.
No, doesn't matter if it's linear or not. The only way you could
compensate for nonstandard temps would be for the altimeter to sense
temperature (at all points from the ground up to the altitude of the
altimeter). You couldn't do it just with the altimeter setting.
> So to bring lapse rate into the equation, it would seem that a higher
> than standard temperature lapse would tend to EXACERBATE altimeter
> error, and lower than standard would tend to MITIGATE it. Thus, to
> complete the spiel, we could say: hot to cold and well above the
> reporting station with a higher than standard lapse rate = look out
> below.
Actually, a lapse rate GREATER than standard would tend to MITIGATE the
error on a HOT day but EXACERBATE it on a COLD day; and a lapse rate
LESS than standard would tend to MITIGATE the error on a COLD day but
EXACERBATE it on a hot day. For 10 points: why?
> I can visualize the iso-pressure lines stacking up in the atmosphere
> (at cold temperature) and causing the altimeter to "think" it is higher
> than it is,
A good way to look at it.
> but it still seems weird that density and pressure move in
> inverse directions. Ah well. I'm a pilot, not a physicist.
Nobody said that density and pressure move in inverse directions.
rober...@my-deja.com
of taskemcorp (they make altimeters).
My question to him was "Do altimeters skew their display
to account for standard temperature changes expected when
altitude changes".
Here is his response....
"Dear Robert,
I don't know if I can settle your argument, but I can tell you how
our
altimeter responds to temperature changes. In fact, this is how ALL
mechanical altimeters (TSO'd clockworks devices) respond too.
There is no temperature correction that takes place within the
altimeter. The altimeter assumes a "Standard Atmosphere" curve and that
curve is adjusted for the current barometric pressure with the BARO
setting.
Any deviation from standard temperature at altitude causes an error in
the
assumed air density and that results in a small error in displayed
altitude.
BUT, this same error is experienced by ALL aircraft in the area, so it's
of
no great consequence. If ATC directs you to climb to 8000 and another
craft
to 10000, there will still be almost exactly 2000 separation even though
your true altitudes may be off.
I understand what you mean when you say that ATIS "accounts for
temperature" on the ground. They report the local atmospheric pressure
"corrected to sea level", and this requires a temperature correction if
the
local temperature is far from the norm. Otherwise, their own altitude
would
not come out correct. However, they have no way of knowing the
temperatures
at altitude (on a minute-to-minute basis), so they simply depend on the
standard lapse rate from there.
I hope this is of some help. I make no pretense to be an expert,
but
I've been in this business for quite some time and believe that this is
correct."
Mike Rapoport
Mike
MU-2
rober...@my-deja.com wrote in message <81bte0$7h4$1...@nnrp1.deja.com>...
Mark Mallory
rober...@my-deja.com wrote:
> Here we go. I just got a response back from Tom Riley
> of taskemcorp (they make altimeters).
> My question to him was "Do altimeters skew their display
> to account for standard temperature changes expected when
> altitude changes".
> Here is his response....
>
> "Dear Robert,
> There is no temperature correction that takes place within the
> altimeter. The altimeter assumes a "Standard Atmosphere" curve and that
> curve is adjusted for the current barometric pressure with the BARO setting.
> Any deviation from standard temperature at altitude causes an error in the
> assumed air density and that results in a small error in displayed altitude.
> BUT, this same error is experienced by ALL aircraft in the area, so it's of no great consequence. > If ATC directs you to climb to 8000 and another craft to 10000, there will still be almost exactly > 2000 separation even though your true altitudes may be off.
>
> I understand what you mean when you say that ATIS "accounts for
> temperature" on the ground. They report the local atmospheric pressure
> "corrected to sea level", and this requires a temperature correction if
> the local temperature is far from the norm. Otherwise, their own altitude
> would not come out correct.
>
Incorrect. Your respondent appears to be making the same error here
that you have made. The altimeter setting does NOT include or require
any "temperature correction". The altimeter setting is simply the
number that you dial into the Kollsman window to make the altimeter
(calibrated
to Standard Atmosphere pressures) read field elevation when on the
ground. Because the altimeter responds ONLY to pressure, the Altimeter
Setting is a function of ambient station pressure ONLY; temperature has
no effect on the setting.
The standard atmosphere table states that at an altitude of 3000 ft, the
ambient pressure is 26.81 inHg, and the temperature is 48.3 degF.
Therefore, if you were located on the ground at an airport where the
elevation was 3000 MSL, and using your trusty Barometer you measured an
ambient pressure of 26.81 (the standard value), the altimeter setting
for that airport would be 29.92 (standard sea-level pressure). It
wouldn't make any difference if the outside temperature was 105 degF or
-10 degF; an altimeter on the ground set to 29.92 would indicate the
field elevation correctly in both cases.
However, (assuming standard lapse rates) an altimeter in an airplane at
1000 AGL would indicate LOWER than 1000 AGL on the 105 degree day, and
HIGHER than 1000 AGL on the -10 degree day. Only when the surface
temperature was STANDARD (ie, 48.3 degF) would the altimeter correctly
indicate 1000 AGL.
> However, they have no way of knowing the
> temperatures at altitude (on a minute-to-minute basis), so they simply depend on the
> standard lapse rate from there.
>
Replace the words "standard lapse rate" with "standard atmosphere
temperatures" and this last sentence would be correct.
Charlie Wendell
Mark Mallory (mar...@earthlink.net) writes:
> rober...@my-deja.com wrote:
>> Here we go. I just got a response back from Tom Riley
>> of taskemcorp (they make altimeters).
>> My question to him was "Do altimeters skew their display
>> to account for standard temperature changes expected when
>> altitude changes".
>> Here is his response....
>>
>> "Dear Robert,
> <SNIP>
however .. I'm not so sure about the following paragraph
>
> The standard atmosphere table states that at an altitude of 3000 ft, the
> ambient pressure is 26.81 inHg, and the temperature is 48.3 degF.
> Therefore, if you were located on the ground at an airport where the
> elevation was 3000 MSL, and using your trusty Barometer you measured an
> ambient pressure of 26.81 (the standard value), the altimeter setting
> for that airport would be 29.92 (standard sea-level pressure). It
> wouldn't make any difference if the outside temperature was 105 degF or
> -10 degF; an altimeter on the ground set to 29.92 would indicate the
> field elevation correctly in both cases.
I dont think you can say that, when the pressure at a given altitude
conforms to ISA (Stanadard) that the pressure at Sea Level will also be at
ISA (29.92). That .. IMHO .. would be the case when the temperatures at
that given altitude ( in this case 3000 ft) and from that altitude are the
same as those found in the (ISA) or Standard Atmosphere.
My 2 cents worth .. please flame me good if I'm not correct on this
particular point. .. Because I do believe it is worth clearing up for all
who are involved in this thread .. as participants and lurkers.
The rest of the post ..I agree with ..>
> However, (assuming standard lapse rates) an altimeter in an airplane at
> 1000 AGL would indicate LOWER than 1000 AGL on the 105 degree day, and
> HIGHER than 1000 AGL on the -10 degree day. Only when the surface
> temperature was STANDARD (ie, 48.3 degF) would the altimeter correctly
> indicate 1000 AGL.
>
>> However, they have no way of knowing the
>> temperatures at altitude (on a minute-to-minute basis), so they simply depend on the
>> standard lapse rate from there.
>>
> Replace the words "standard lapse rate" with "standard atmosphere
> temperatures" and this last sentence would be correct.
>
--
Cheers, Charlie
Mark Mallory
Charlie Wendell wrote:
>
> Mark Mallory (mar...@earthlink.net) writes:
> > The standard atmosphere table states that at an altitude of 3000 ft, the
> > ambient pressure is 26.81 inHg, and the temperature is 48.3 degF.
> > Therefore, if you were located on the ground at an airport where the
> > elevation was 3000 MSL, and using your trusty Barometer you measured an
> > ambient pressure of 26.81 (the standard value), the altimeter setting
> > for that airport would be 29.92 (standard sea-level pressure). It
> > wouldn't make any difference if the outside temperature was 105 degF or
> > -10 degF; an altimeter on the ground set to 29.92 would indicate the
> > field elevation correctly in both cases.
>
> I dont think you can say that, when the pressure at a given altitude
> conforms to ISA (Stanadard) that the pressure at Sea Level will also be at
> ISA (29.92).
No, you cannot say that, and I never did. Please note I did not say the
actual SEA LEVEL PRESSURE was 29.92, but rather that ALTIMETER SETTING
was 29.92. The two are NOT necessarily the same.
If one were to dig a 3000 foot deep hole down from the surface of a 3000
MSL airport and lower an altimeter (set to the surface Altimeter
Setting) down the hole, it would not necessarily read Sea Level at the
bottom. Just as in the case of an altimeter that climbs to a given
height ABOVE the reporting station, an altimeter descending BELOW the
reporting station will also show a greater change in altitude when the
air is COLD than is actually the case. Thus, if the air in the hole
were COLDER than Standard, the altimeter would read BELOW sea level at
the bottom, and vice-versa.
Charlie Wendell
Mark Mallory (mar...@earthlink.net) writes:
> Charlie Wendell wrote:
>>
>> Mark Mallory (mar...@earthlink.net) writes:
>> > The standard atmosphere table states that at an altitude of 3000 ft, the
>> > ambient pressure is 26.81 inHg, and the temperature is 48.3 degF.
>> > Therefore, if you were located on the ground at an airport where the
>> > elevation was 3000 MSL, and using your trusty Barometer you measured an
>> > ambient pressure of 26.81 (the standard value), the altimeter setting
>> > for that airport would be 29.92 (standard sea-level pressure). It
>> > wouldn't make any difference if the outside temperature was 105 degF or
>> > -10 degF; an altimeter on the ground set to 29.92 would indicate the
>> > field elevation correctly in both cases.
>>
>> I dont think you can say that, when the pressure at a given altitude
>> conforms to ISA (Stanadard) that the pressure at Sea Level will also be at
>> ISA (29.92).
>
> No, you cannot say that, and I never did. Please note I did not say the
> actual SEA LEVEL PRESSURE was 29.92, but rather that ALTIMETER SETTING
> was 29.92. The two are NOT necessarily the same.
Mark , I will agree that you did not say that, however, with all due
respect, if I may, I would expect that, if the Sea Level Pressure is
fairly close to 29.92. I still maintain my stand stated in my repy.
>
> If one were to dig a 3000 foot deep hole down from the surface of a 3000
> MSL airport and lower an altimeter (set to the surface Altimeter
> Setting) down the hole, it would not necessarily read Sea Level at the
> bottom.
> Just as in the case of an altimeter that climbs to a given
> height ABOVE the reporting station, an altimeter descending BELOW the
> reporting station will also show a greater change in altitude when the
> air is COLD than is actually the case. Thus, if the air in the hole
> were COLDER than Standard, the altimeter would read BELOW sea level at
> the bottom, and vice-versa.
>
The above para .. I got to think about ... I'm trying to rationalize it
a different way, like .. suppose that airfield at 3000 ft was on a plateau
( at 3000 feet of course) and just a short distance away we find a cliff
overlooking a sea plane base at Sea Level. And suppose the sea level
pressures were the same at both. If the temperatures were different from
Standard .. then the QNH's would have to be different .. (QNH = Altimeter
setting) .. and may be quite different ... hmmmmmmm gonna have to look at
this some more ..
--
Cheers, Charlie
Mark Mallory
Charlie Wendell wrote:
> .. suppose that airfield at 3000 ft was on a plateau
> ( at 3000 feet of course) and just a short distance away we find a cliff
> overlooking a sea plane base at Sea Level. And suppose the sea level
> pressures were the same at both. If the temperatures were different from
> Standard .. then the QNH's would have to be different .. (QNH = Altimeter
> setting) .. and may be quite different
Precisely as I said. If on a COLD day, an altimeter at the 3000 MSL
airfield is set to the current Altimeter Setting for the field (say
29.92), and the altimeter is taken from 3000 down to Sea Level, it will
read BELOW sea level when it gets there. In order for the altimeter to
properly indicate at sea level, it must be RESET to a different (higher)
setting (say 30.10).
> ... hmmmmmmm gonna have to look at this some more ..
Good idea. Do try and THINK before your next post.
Butch Grafton
>
> Good idea. Do try and THINK before your next post.
If you want to read a very good article on how cold affects altimeters then
check this link out.....
http://www-afsc.saia.af.mil/magazine/htdocs/octmag98/itscold.htm
Russell Fairchild
not the absolute temperature at the bottom (within reason). i.e. if you
know it is 3,000 feet and you know the pressure at that point, as long
as the lapse rate is standard between the bottom of the hole and the
surface, the altimeter should read sea level at the bottom of the hole.
Mark Mallory wrote:
>
> Charlie Wendell wrote:
> >
> > Mark Mallory (mar...@earthlink.net) writes:
> > > The standard atmosphere table states that at an altitude of 3000 ft, the
> > > ambient pressure is 26.81 inHg, and the temperature is 48.3 degF.
> > > Therefore, if you were located on the ground at an airport where the
> > > elevation was 3000 MSL, and using your trusty Barometer you measured an
> > > ambient pressure of 26.81 (the standard value), the altimeter setting
> > > for that airport would be 29.92 (standard sea-level pressure). It
> > > wouldn't make any difference if the outside temperature was 105 degF or
> > > -10 degF; an altimeter on the ground set to 29.92 would indicate the
> > > field elevation correctly in both cases.
> >
> > I dont think you can say that, when the pressure at a given altitude
> > conforms to ISA (Stanadard) that the pressure at Sea Level will also be at
> > ISA (29.92).
>
> No, you cannot say that, and I never did. Please note I did not say the
> actual SEA LEVEL PRESSURE was 29.92, but rather that ALTIMETER SETTING
> was 29.92. The two are NOT necessarily the same.
>
Mark Mallory
Russell Fairchild wrote:
>
> doesn't it really depend on the lapse rate between the two points and
> not the absolute temperature at the bottom (within reason). i.e. if you
> know it is 3,000 feet and you know the pressure at that point, as long
> as the lapse rate is standard between the bottom of the hole and the
> surface, the altimeter should read sea level at the bottom of the hole.
No, it depends on TEMPERATURE, not Lapse Rate. In the winter, it may be
-6 degC at 3000 ft and 0 degC at sea level, while in the summer it may
be 30 degC at 3000 ft and 36 degC at sea level. The Lapse Rate is the
SAME (standard) in both these cases, but the air is colder and therefore
DENSER in the winter; this means that in Winter, the altimeter
*indicates* a LARGER altitude change for a given ACTUAL altitude change
than it does in Summer.
And again, the Altimeter Setting has nothing to do with temperature.
It's just as possible that the Sea Level altimeter setting is 29.92 on a
100 degF day as it is on a 0 degF day, even with standard lapse rates.
Russell Fairchild
altimeter reading is obtained. Since I rarely see a difference between
summer and winter in the altitude of the airplane at the ramp when i put
in the local altimeter setting, I am guessing that the local altimeter
setting is
obtained in a special way. Does anyone know how? My simplistic guess
is
that the weather observers have an altimeter like the one I have. To
obtain
the local altimeter setting they put in the altitude in the altimeter
and
read the altimeter setting in the Kollsman Window. This way the density
error is corrected for. Or perhaps they actually have a barometer and
then
go through some complicated formula to obtain the local altimeter
setting.
any other guesses?
Mark Mallory
Russell Fairchild wrote:
> My simplistic guess is that the weather observers have an
> altimeter like the one I have. To obtain the local
> altimeter setting they put in the altitude in the
> altimeter and read the altimeter setting in the
> Kollsman Window. This way the density error is corrected
> for. Or perhaps they actually have a barometer and then
> go through some complicated formula to obtain the local
> altimeter setting.
It can be done either way. Automated observations derive the altimeter
setting directly based on the surface barometric presssure and the field
elevation, using a formula. Manual observations often use an accurate
altimeter set to the field elevation; the altimeter setting is read from
the Kollsman Window.
randomlya...@gmail.com
When flying from a warmer area to a colder area, the altimeter's altitude reading would be higher than the actual altitude the plane is flying at. Since the altitude reading is based on the difference between set pressure in the Kollsman window and the air pressure outside the aircraft, this implies that the the difference read by the altimeter is too high.
So let's say that you have your Kollsman window pressure properly set to 30 Hg for a warm area - this setting would give you the correct altitude if you stayed in the warm area. Let's say that the air pressure outside the aircraft when it's in the colder area is 18 Hg. We know that air pressure decreases as altitude increases. The difference between the now incorrectly-set pressure (30 Hg) and the outside air pressure (18 Hg) is 12 Hg. But since the video said that this difference is larger than it's supposed to be, the actual Kollsman pressure setting should be less than 30 Hg to reduce the difference of 12 Hg (you can't reduce this difference by increasing the outside air pressure). So if the actual Kollsman pressure setting is 26 Hg, the pressure difference would be 26 - 18 = 8. From this, we know that the incorrect altitude reading is 150% (12 / 8 = 1.5) greater than the actual altitude.
jonath...@gmail.com
> jcrogin <jcrogin...@yahoo.com.invalid> wrote [snip]:
>
> > I mean, cold air is more dense than hot air. So if
> >you fly from hot to cold, why doesn't your altimeter think you're
> >flying lower, rather than higher? What am I missing?
>
> I've gotten this one wrong a million times until I read this
> (highly simplified) explanation in a magazine:
>
> Imagine your airplane riding on top of a cylinder of air.The cylinder
> sits on the ground and its sides are rigid, but it's capable of
> expanding vertically.
>
> In general, when air (or any gas) warms, it's volume increases. When
> it cools, its volume decreases. What effect will that have on the
> cylinder your plane is riding on?
>
> Works the same for low pressure -- generally, low pressure means less
> weight -- more expansion -- more volume.
>
> =======================================
> ______|______ Mark Kolber
> \(o)/ Denver, Colorado
> o O o www.midlifeflight.com
> =======================================
> replace "spamaway" with "mlf" for email
Than you for this!!!!!