Altimeter setting != Sea Level Pressure - Why?

[JT Wright]

I have a question regarding the relationship between an altimeter
setting and sea level pressure.

Here is the current METAR for Abilene Regional Airport, Texas:

KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098
T01940161

As you can see, the altimeter setting is 29.87" and the Sea Level
Pressure is 1009.8 millibars (hectoPascals)

But the altimeter setting 29.87" is 1011.6 milibars, or 1.8
millibars higher than the sea level pressure.

Why is there a difference between these two values? What
accounts for the 1.8 millibars?

I've looked at similar METARs, and some with a temperature
greater than standard temp have a lower difference between alt
settings and SLP, so this apparently does not account for the
difference.

[PS2727]

Altimeter settings are corrected for elevation of the station. Imagine an
airport which is at 1000 ft. in elevation. On a standard day the altimeter
setting on ATIS will say 29.92, but the actual air pressure is closer to 28.92
(using 1in/hg per 1000 ft). Yet when set to 29.92 your altimeter will read
field elevation of 1000 ft when you land.

[JT Wright]

PS2727 wrote:

> Altimeter settings are corrected for elevation of the station. Imagine an
> airport which is at 1000 ft. in elevation. On a standard day the altimeter
> setting on ATIS will say 29.92, but the actual air pressure is closer to 28.92
> (using 1in/hg per 1000 ft). Yet when set to 29.92 your altimeter will read
> field elevation of 1000 ft when you land.

That's certainly true, but it's not related to my question. Both the altimeter
setting and the SLP are corrected for the elevation of the station. My question
relates to why sometimes (but not always) there is a difference between these
figures.

For example, the latest METAR from KABI shows that the difference between the
altimter setting and the sea level pressure (SLP) has increased by 0.1 mb in the
past few hours.


KABI 280452Z 17017KT 10SM FEW032 18/16 A2989 RMK AO2 SLP104 T01830156
Altimeter 29.89 inches Hg or1012.3 mb, Sea-level pressure: 1010.4 mb:
Difference of 1.9 mb


earlier METAR in previous post:
KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098 T01940161
29.87" (1011.6 milibars) and the Sea Level Pressure was 1009.8
Difference of 1.8 mb

[JT Wright]

JT Wright wrote:

> For example, the latest METAR from KABI shows that the difference between the
> altimter setting and the sea level pressure (SLP) has increased by 0.1 mb in the
> past few hours.
>
> KABI 280452Z 17017KT 10SM FEW032 18/16 A2989 RMK AO2 SLP104 T01830156
> Altimeter 29.89 inches Hg or1012.3 mb, Sea-level pressure: 1010.4 mb:
> Difference of 1.9 mb
>
> earlier METAR in previous post:
> KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098 T01940161
> 29.87" (1011.6 milibars) and the Sea Level Pressure was 1009.8
> Difference of 1.8 mb

I think I have found the answer:
The altimeter setting value does not take into account the temperature. The
temperature profile of a "standard" atmosphere is assumed. The SLP takes into
affect the actual temperature reading (12 hour mean temperature average). So it
looks like the difference between these two readings can be greater at higher
station elevations. Still, I'm curious about the details of how this is calculated.

http://www.crh.noaa.gov/den/awebshtml/definitions_pressure.shtml
http://www.faa.gov/atpubs/SWO/chapter_14Sect1.htm#14-2.%20%20%20ALTIMETER%20SETTING%20(ALTSG)

[Julian Scarfe]

"JT Wright" > wrote in message
...

> The altimeter setting value does not take into account the temperature.
The
> temperature profile of a "standard" atmosphere is assumed. The SLP takes
into
> affect the actual temperature reading (12 hour mean temperature average).
So it
> looks like the difference between these two readings can be greater at
higher
> station elevations.

That's right. Bear in mind that SLP is somewhat theoretical, in that you
don't dig a mineshaft down to sea level to check it. :-) In some senses so
is altimeter setting.

I think the reason for having SLP numbers is so that you can plot higher and
lower elevation stations on the same chart without getting anomalies. If
you used altimeter settings, then on hot or cold days the high elevation
stations would stand out, because their altimeter settings would be much
higher or lower than low level stations nearby.

Julian Scarfe

[Martin Hellman]

To add to the other replies so far, if you have a GPS with you and
especially if you fly high, you'll notice that the GPS altitude and
altimeter altitude can differ significantly for the same reason as
mentioned earlier: non-standard temperature. On a hot day, the air
"column" above the airport expands and becomes higher. So the point at
which the air pressure is half of sea level pressure is higher. On a
cold day, the reverse happens. Net result? On a hot day your altimeter
will read low compared to actual altitude, and on a cold day it will
read high.

ATC goes by the altimeter altitude, so on a hot day it is legal to fly
VFR at an altitude of 17,900' on your altimeter (assuming it is set to
the local altimeter setting) even though your GPS may read 19,000'.
[Note: This is USA-centric where class A airspace starts at 18,000.]

I've also heard that, on hot days, ATC will not assign an IFR altitude
of FL185 or even FL190 due to this effect. They want to make sure the
IFR traffic is well separated from the VFR guys.

Hope this helps.

Martin

JT Wright > wrote in message >...
> I have a question regarding the relationship between an altimeter
> setting and sea level pressure.
>
> Here is the current METAR for Abilene Regional Airport, Texas:
>
> KABI 280152Z 17014KT 10SM CLR 19/16 A2987 RMK AO2 SLP098
> T01940161
>
> As you can see, the altimeter setting is 29.87" and the Sea Level
> Pressure is 1009.8 millibars (hectoPascals)
>
> But the altimeter setting 29.87" is 1011.6 milibars, or 1.8
> millibars higher than the sea level pressure.
>
> Why is there a difference between these two values? What
> accounts for the 1.8 millibars?
>
> I've looked at similar METARs, and some with a temperature
> greater than standard temp have a lower difference between alt
> settings and SLP, so this apparently does not account for the
> difference.