Much like temperature, pressure changes as we fly from A to B. Typically we use the QNH from the nearest airfield and continually update our altimeter subscale setting as we fly cross country, however this is not always possible. In more remote areas there may be 50 miles or more between airfields and we may end up using a somewhat inaccurate pressure setting for a period of time.
The same rule of thumb applies: High to Low – Look Out Below! As we fly from higher to lower pressure, our true altitude will decrease if we maintain a constant indicated altitude on our altimeter. Flying from lower pressure to higher pressure, our true altitude will increase.
The rule of thumb is 30ft per hectopascal. So if the QNH decreases by 15hPa and you don’t update the altimeter subscale, you will be 450ft lower than indicated (30ft x 15hPa = 450ft).