When flying in air which is colder than ISA (International Standard Altitude), how will the altimeter read compared to true altitude?
My manual says the altimeter will over read - why?
Air expands as the temperature increases and it compresses when the temperature decreases.
image source: aviationweather.ws
When it is colder than ISA, the air column is compressed and therefore you are flying lower than the altimeter is indicating. In other words: the altimeter will over read in cold air.
DeltaLima has it right, though I still always find it counter-intuitive. I mean, wouldn't denser air mean your altimeter will read lower?
But--the thing to remember is that it's not the weight of the air above you, it's the weight of the air below you. The altimeter is calibrated to the pressure reported by the ground station. Therefore, it IS accurate on the ground. But colder, denser air, as DeltaLima points out, will have a steeper pressure gradient--so as you climb above ground, the altimeter indicates altitude rising faster than the true height.
The moniker is: "High to low, look out below": as temperature falls, you are lower than you think!