Barometric Altimeters are calibrated for ISA, therefore when it's not ISA they will not read accurately.
When it's ISA + they underead which is safe so ignored.
When it's ISA - they overread which is potentially deadly and must be allowed for.
Most carriers/operators who operate in VERY cold climates will have a tabulated correction card to tell the pilot how much the altimeter will be overreading and therefore how much altitude to add to LSALTs/MSAs/MORAs/Minimas etc.
The correction must be added to all heights & altitudes except assigned altitudes (as ATC will have figured in the correction).
A usefull little formula which works to within a few feet until temperatures get bizarre cold and heights above the temperature datum (airfield) get really high is;
+/- 4' per degree of isa devn x (alt/1000)
Note in this formula that alt = height above the datum. The datum is the temperature reference point i.e the airfield.
So you are flying an ILS to a msl desto @ ISA + 20.
ILS published crossing height is 1800'
-( 4 x 20)x (1.8)= 144'
So you would expect the altimeter to read 1656' as you crossed the fix. When it does you can confidantly say "Checked".
At ISA - 20 the altimeter would overead by 144'. But don't just add this to the minima...as some will tell you is mandatory.
If the minima is 250' the math would be + 80 x .250 = +20'. If you then adjusted your minima to 270' when you reached it you would be at the correct DH...and the radalt would confirm that.
As you can see it's not really an issue on precision approaches...which is probably why it is so badly understood...but inbound to a very cold place over high terrain it can get far more interesting.
MSL airport with MSA of 10000', ISA - 20.
+(4x20)x (10) = + 800'.
Obstacle clearance looking a tad sad now isn't it?
Just one good reason to set 250' on the radalt in cruise...and follow ground prox warnings in IMC/night...no matter what you think the altimeter is telling you!!!
Chuck.