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.