If a flight is made from an area of high pressure into an area of lower pressure without the altimeter setting being adjusted, the altimeter will indicate which of the following?

• A. Higher than the actual altitude above sea level
• B. Lower than the actual altitude above sea level
• C. The same as the actual altitude above sea level
• D. No change in indicated altitude

Answer: (A)

The altimeter reads altitude by comparing the ambient air pressure to a reference pressure you set for sea level. If you move from a high-pressure region to a lower-pressure region and don’t adjust the altimeter, the instrument keeps using the higher reference. The actual ambient pressure in the new area corresponds to a lower altitude than the high-pressure setting would imply, so the altimeter interprets it as being higher above sea level than you really are. In practice, this means your indicated altitude is higher than your true altitude (look out below).