Altimeter Errors

[lde1988]

I have just read the following -

"Temperature/Density Error - altitude is calibrated with respect to pressure lapse rate at ISA value. If temperature is lower, density will be higher and altitude will over read"

I may be being stupid, but my thinking is that if density is higher, then the altimeter should under read. Any ideas???

[ft]

The air pressure is really a measure of how much the air above you weighs. As you ascend, you remove the air you climb through from the weight felt by the altimeter.

If the air you climb through is cold, and hence has higher density, you remove weight at a faster rate and the altimeter will indicate a faster climb for the same actual rate of climb.

Think of it as the atmosphere having a constant weight, meaning the more the air below you at a given altitude weighs, the less the air above you will weigh. Denser (colder) air below you -> higher weight of the air below you -> lower weight of the air above you -> lower pressure -> altimeter overreads -> look out below!

Cheers,
/Fred

[Deeday]

Put in another way: take a volume of air, e.g. a cylinder, that starts on the ground and goes all the way up to the end of the atmosphere, at ISA conditions.
Inside it, at an altitude of say 1000 ft, an altimeter will read exactly that value.
Now cool down the whole cylinder. The pressure at ground level will not change, because, as user ft points out, it depends exclusively on the total mass of air contained by the cylinder. The density, instead, can only increase, because P = ρ R T (if T goes down, ρ must go up, for P to stay the same; R is constant).
A higher density means that a bigger mass of air settles down below our 1000 ft level, compared to ISA conditions, therefore less air will sit on top of it, and the pressure at 1000 ft will be lower than ISA, i.e. the altimeter will over-read.
I hope it makes sense.

Deeday

[Robin400]

As a pilot just remember......from high to low temperature and pressure LOOK BELOW.

Be aware of thine altitude, or the earth shall arise and smite thee.

I say this to myself before descending in IMC