Hi Piper.
The altimeter just compares the pressure you set on the subscale with the actual pressure it is sensing. If they are the same, the altimeter will read zero.
If the actual pressure is
less than the one you have set, the altimeter will show a positive reading.
If the actual pressure is
more than the one you have set, the altimeter will show a negative reading.
So, if you decrease the pressure you have set, whilst the actual pressure remains the same, the altimeter reading will decrease. (By approx. 30 feet per hectopascal.)
Hope that helps
MJ
