From: elmore@virga.ucar.edu (Kim Elmore)
Subject: Re: Temperature Related Altimeter Errors
Organization: Research Applications Program/NCAR, Boulder, CO
Date: Mon, 27 Jul 1992 20:07:19 GMT

In article <vdkmw!c.rkaplan@netcom.com> rkaplan@netcom.com (Richard Kaplan) writes:
>The current issue of IFR has an interesting article on 
>temperature related altimeter errors, including some frightening
>examples of potential impact into terrain when flying 
>an instrument approach on a very cold day.
>
>This makes me wonder.... why do we not adjust our altimeter
>setting on the ground to correct for this?  What if the official
>altimeter setting were established not by a barometric
>reading but by the weather man fiddling with his altimeter
>until it read exactly field elevation?

	Ah!  Because of the Gas Law.  Lemme explain it...

	Meterologists keep track of the height of constant pressure
surfaces because it is a direct measurable; radiosondes directly
measure temeperature, pressure and humidity.  Because the balloons are
filled to lift a specific weight, the rise rate is known very
precisely and so we can accurately measure the height by knowing how
long the balloon has been in flight.  All the equations of motion for
the atmosphere are then expressed in units of geopotential height.
Were it not for this, we would have to derive the density at every
level, which can introduce significant errors.

	"So get to the point already!"

	OK, OK... Altimeters are analogue computers that yield
altitude given pressure.  Thus when you're flying at a constant
altitude indication, you're *really* flying at a constant pressure.
The pressure-to-altitude conversion *only* works if the density
profile of the atmosphere is known.  For altimeters, the density
profile is assumed to follow that of the Standard Atmosphere, wherein
the atmosphere is assumed to be dry (without any water vapor) and the
lapse rate is given.  BUT, if the atmosphere is much *colder* that the
standard atmosphere, the constant pressure surfaces are closer
together and a large change in pressure occurs over only a small
height.  If you really want to know your altitude, you have to also
have a local sounding handy.

	So, no surface correction can account for the
different-from-standard spacings of constant pressure surfaces
throughout the depth of the atmosphere (or eve the troposphere, for
that matter).  A surface altimeter setting can yield a correct
pressure-to-altitude conversion at only one height if the atmospheric
lapse rate is not "standard".  When the atmosphere is very cold,
altimeters indicate an altitude higher that actual and when it is very
warm, lower than actual.


					Kim Elmore
					N5OP, PP ASEL/Glider 2232456
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