Altimeter error

[MichaelP]

Who has a simple explaination for why an altimeter over-reads at very low temperatures?

[Grey_Wolf]

Recall how an altimeter works .... decrease in pressure = increase in altitude. We know from met, increase in alt = decrease in temp.

Hence going from High Pressure/Warm Air to Low Pressure/Cold Air, if not corrected by a proper altimeter setting, the altimeter will read high.

[Hedley]

The way I explain it ...

Remember, George from Seinfeld that suffered from
(ahem) "shrinkage" when it got cold?

Well, the atmosphere does the same thing. It vertically
shrinks when it gets cold.

Now, what your altimeter does is measure pressure,
which is really vertical position in the atmosphere.

So, as it gets colder, an altimeter reading the same
pressure, will go down as it gets colder. This means
that when it is cold, the altimeter reads too high,
and you will hit things.

Worth what you paid me for it!

[Single-Engine IFR]

Recall how an altimeter works .... decrease in pressure = increase in altitude. We know from met, increase in alt = decrease in temp.

Hence going from High Pressure/Warm Air to Low Pressure/Cold Air, if not corrected by a proper altimeter setting, the altimeter will read high.

If that were the case, the problem could be solved simply by adjusting for the new pressure in the kolsman (sp?) window. The fact is that warm and high and cold and low are not mutually exclusive. You can have very cold weather with a high pressure and vice versa.

At standard temperature the pressure levels that indicate 1,000 ft increments are, in fact 1,000 ft apart. ( 1 in. HG. per 1,000 ft).

Hedley said it best when he referred to shrinkage. When the air is warmer that normer, it expands and the pressure levels are further apart and you you would actually be higher than indicated, but that isn't a problem so we don't bother to correct for it.

When the air is sub-zero, the air contracts and the pressure levels are closer together so you would indicate higher. (Don't get pressure confused with density)

Picture a multi layer mass of air, spanning across an area from standard temperature through sub-zero and back up to standard. You would see that in the cold area the layers would be squeezed closer together and be closer to the ground. So a person who is flying in the cold area and indicating 10,000 ft could actually only be 9,200 for example.

Hope this helps.


Regards,

SEI

[trey kule]

When I read that post, I assumed it was inted to mean either, not high/cold-low/warm, but from high to low or warm to cold.
The problem is you dont have to transition from warm to cold for the error to occur. Severe cold in itself will cause an overreading, sometimes by thousands of feet. It is why over the no. 1 mountain region (if thats still what they refer to it as), they suggest increasing the minimum enroute altitude during severe cold, and also why the CAP has temp correction tables.

Hope this helps, unless you simply wanted the mechanical cause of the overreading, in which case i think some of the others already explained it.

[Grey_Wolf]

Don't get pressure confused with density

"The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics. As does air pressure, air density decreases with increasing altitude and temperature"
http://en.wikipedia.org/wiki/Density_of_air


"Atmospheric pressure is the pressure at any point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air 1 square inch in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 lbf. A 1 m² (11 sq ft) column of air would weigh about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface)."
http://en.wikipedia.org/wiki/Atmospheric_pressure



So what's the difference ?

[MichaelP]

Thank you, I like Hedley's explanation best.

[THEICEMAN]

The way I explain it ...

Remember, George from Seinfeld that suffered from
(ahem) "shrinkage" when it got cold?

Well, the atmosphere does the same thing. It vertically
shrinks when it gets cold.

Now, what your altimeter does is measure pressure,
which is really vertical position in the atmosphere.

So, as it gets colder, an altimeter reading the same
pressure, will go down as it gets colder. This means
that when it is cold, the altimeter reads too high,
and you will hit things.

Worth what you paid me for it!

Hedley you should re-write books like from the ground up, c.u.l.h.a.ne, aerocourse, etc...

Your methods will revolutionize flight training in Canada!

[Single-Engine IFR]

Don't get pressure confused with density

"The density of air, ρ (Greek: rho) (air density), is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics. As does air pressure, air density decreases with increasing altitude and temperature"
http://en.wikipedia.org/wiki/Density_of_air



So what's the difference ?

I was referring to Grey_Wolf's alluding to flying to a low pressure/temperture area and assuming that one could solve the cold temperature correction by setting the current altimeter setting on the altimeter. I assert again that cold air is not necessarily low pressure.

On the subject of density versus pressure, do the following experiment:

Inflate a regular party balloon with room air that is the same temperature as the surrounding air.
Allow the balloon to warm up over a heat radiator.
The balloon will expand, indicating a greater pressure as evidenced by the further stretching of the plastic.
The air in the balloon will be of a lesser density as evidenced by the fact that the balloon will start to float up to the ceiling.
Cold air is often denser that warm air and will often flow off a glacier down a mountain valley.

My main reason for posting this is to present the fact that setting the altimeter will not correct for the error at all, and you must use the tables in the CAP Gen.

Rgards

SEI

[wtf_over]

I wouldn't put to much stalk in what Greywolf says He's an idiot.

I'd be willing to bet he's been about 150miles outside the circuit in his career and has never seen the cold weather correction charts in the capgen.

[Grey_Wolf]

Fun .... Retaliation

Don't you mean "stock" ?




Edit: How far is Yellowknife to Ottawa ?

[MichaelP]

Sometimes these end up with more than a five minute argument... At least it's not costing me five quid!

But this is the reply I got from someone:

"Im not buying it. That shrinking air is denser, and so, even though it is a shorter stack of air above you, it is a denser shorter stack, AND, what about cold clear days with HIGH pressure. There must be a good answer somewhere..."

Comments please...

[scm]

Im not buying it. That shrinking air is denser, and so, even though it is a shorter stack of air above you, it is a denser shorter stack,

The altimeter doesn't measure density, it measures pressure. The weight of the column of air above it. Even though the columns of air above are of different length and density, the force they exert downwards is exactly the same.

what about cold clear days with HIGH pressure.

On a colder than standard temperature day, the altimeter will read too high.
On an above standard pressure day, the altimeter will read too low.
If we don't correct the altimeter subscale, and the numbers are just right, the two errors may cancel themselves.

As said before the altimeter reads pressure, and is calibrated for standard. Pressure is weight over an area. Lets assume you're on mountain and the pressure is 13 psi. The next day the pressure is 17 psi. If you were an altimeter, the higher pressure you feel today would make you believe that you were lower down the mountain.

The only time there is no altimeter error is when the air is ICAO standard.

Something else to consider. For altimeter error, were considering high pressure anything above 29.92 ( ). In meteorology, a pressure of 29.72 may be 'high' relative to the areas around it.

[767]

*Someone confirm this before i get flamed!! Im sure this is how it works, but please confirm with me in case im wrong.
---------------------------------------------------------------------------------------------------------------------

1. Standard temperature is 15 deg. Standard pressure is 29.92.

2. Temperature above standard = Pressure increases "above standard (29.93 and higer)"
(higer pressure "above standard" causes altimeter to under read)

3. Temperature below standard= Pressure decreases "below standard (29.91 and lower)"
(lower pressure below standard causes the altimeter to over read)


Q: Why does pressure difference change the altimeter reading?

A: Well, when you take off and gain altitude, you are flying from an area of higer pressure (surface), to an area of lower pressure (altitude). Becuase the pressure decreases (gets lower) with altitude, your altimeter will over read.


Hope that helps!

[skye]

I explain it this way:

1. As the atmospheric temperature decreases (e.g. in winter, or flying into colder regions), the density increases (air molecules more closely packed). As a result, the air gets heavier and is pressed down because of its own weight - on average, more of the air particles are closer to the ground. That is, the height of the atmoshere decreases in colder areas.

2. The altimeter is a pressure measurement device...meaning it measures the weight of the air above it. Analogy is scuba diving where the pressure increases the deeper you dive, because there is a greater weight of water above applying hydrostatic pressure.

3. When the altimeter is used to fly a constant altitude (level flight), the plane will naturally "follow" an altitude of constant pressure above the aeroplane. As noted in (1), with a "lower atmosphere" in a colder area (and without adjusting for the local altimeter setting), the aeroplane will fly lower to the ground to follow the same pressure (now closer to the ground) ... "watch out below"!

4. However...the correct (local) altimeter setting (QNH) should correct for all or most of this temperature "error". Most altimeters have built-in temperature-compensation (bi-metallic) strips which provide a correction for temperature effects.

CF

Hope this helps.

[scm]

*Someone confirm this before i get flamed!! Im sure this is how it works, but please confirm with me in case im wrong.
---------------------------------------------------------------------------------------------------------------------

1. Standard temperature is 15 deg. Standard pressure is 29.92.

2. Temperature above standard = Pressure increases "above standard (29.93 and higer)"
(higer pressure "above standard" causes altimeter to under read)

3. Temperature below standard= Pressure decreases "below standard (29.91 and lower)"
(lower pressure below standard causes the altimeter to over read)


Q: Why does pressure difference change the altimeter reading?

A: Well, when you take off and gain altitude, you are flying from an area of higer pressure (surface), to an area of lower pressure (altitude). Becuase the pressure decreases (gets lower) with altitude, your altimeter will over read.


Hope that helps!

Remember that standard pressure and temperature is supposed to decrease uniformly with height (realisticly, it doesn't). 1'' of mercury and 1.98 degrees per 1000 feet. That is, for 2000 ASL, standard is 27.92'' and 11.04 degrees. If you climb in standard conditions, even though there is a drop in pressure, there will be no error.

..and i dont see why anyone would flame you or anyone else here, its a flight training forum after all.

Its easier to think of temperature/pressure errors seperately. For example, Regina is currently -13 and 29.39, (below standard pressure/temperature) while Gillam is -32 and 30.08 (below standard temperature/above standard pressure).

Take a look at the GFAs. The entire prairie region is below standard temperature while many places are above standard pressure (1013.25 millibars.)

Im not sure what you mean by #2 and 3.
Anyway im new at this so i could be wrong too.

[767]

Im not sure what you mean by #2 and 3.
Anyway im new at this so i could be wrong too.

The altimter is calibrated to work correctly in standard conditions. The purpose of the knob to change the altimeter setting is to re-calibrate the altimeter to show correct indications according to the current pressure the altimeter is operating in. e.g. if the pressure outside is 28.90, the altimeter will be set to 28.90

What i meant by #2 and #3 is

Lets suppose:

You leave the altimeter set to 29.92 everytime.
Anytime the pressure outside is 29.92, the altimeter will indicate correctly.
Anytime the pressure outside is above 29.92 (altimeter is still set to 29.92), the altimeter will under read.
Anytime the pressure outside is below 29.92 (altimeter is still set to 29.92), the altimeter will over read.

[Single-Engine IFR]

I explain it this way:


4. However...the correct (local) altimeter setting (QNH) should correct for all or most of this temperature "error". Most altimeters have built-in temperature-compensation (bi-metallic) strips which provide a correction for temperature effects.

CF

Hope this helps.

The local altimeter setting will not correct for the cold temperature error. The local altimeter setting is the local pressure corrected down to sea level and has nothing at all to do with the temperature. It is very important that nobody goes away from this with that idea.
The temperature correction and the altimeter pressure correction must be taken as separate actions. You must not think that just because you set the altimeter to the local pressure setting you can get away with not doing the temperature correction. Take my word for it.

Regards,

SEI

[skye]

SEI - Thanks for the additional clarification.

Perhaps my statement in para 4 would be more accurate if I had stated that "...the correct (local) altimeter setting (QNH) should correct for all or most of this temperature "error" when on the ground, with the error increasing as a function of altitude."

Cheers.

[Single-Engine IFR]

SEI - Thanks for the additional clarification.



Cheers.

Your welcome.


Regards,

SEI

[ottawa,kan]

Wouldn't the solution to this be to replace the idiot air pressure altimeter with a similar sized GpS altimeter??? That way youwould never have to correct for anything?? Sort of like foregtting all that VOR angles to/from crap and just followingthe GPS????

[husky]

I see a lot of incorrect information here, and some correct information.

Your altimeter is calibrated to standard temperatures, that means the standard atmospheric temperature profile.

The temperature error in the altimeter comes from NON-STANDARD average temperature of the atmosphere. It is in fact the average temperature of the atmosphere affects the thickness of pressure levels, NOT the local temperature. Furthermore, a change in temperature cannot precipitate a change in pressure at sea level. The change in temperature affects the thickness of pressure levels. For example, at standard sea level pressure and temperature, 101.32 kPa and 15C respectively, you will lose 20kPa of pressure by climbing roughly 6000', so you say the thickness of the 101.32-81.32kPa layer is close to 6000'. If you cool the average temperature of the atmosphere over the depth of the 101.32-81.32kPa layer, the thickness of this layer will decrease.

Let's do a real world example now. You take off from Boundary Bay, pressure and temperature are standard. You climb to 6000'. You cruise for an hour to your destination where the average temperature below you is lower, but the altimeter setting has not changed. The thickness of the air below you has decreased, but you are still flying along the same 81.32kPa pressure level, so you are flying lower.

It's pretty hard to know what the average temperature of the atmosphere is, and certainly no onboard instrument even on the fanciest widebody can tell you this, so forget that the altimeter has the ability to correct for non-standard temperature profiles. Furthermore, the only way to make a true correction is with an accurate temperature profile from a radio-sonde baloon...which is why the corrections in the CAP are relatively conservative.


Reading some of these replies, I think the understanding of the atmosphere is a little bit lacking by Canadian Pilots. Maybe someone needs to start doing weather seminars Then again, as long as you USE THE CORRECTIONS FOR NON STANDARD TEMPERATURES, you will be alright...even if your understanding of why you are using them is all wrong.