What is indicated if a pilot notices that their altimeter is reading significantly higher than the actual altitude?

When a pilot observes that the altimeter is reading significantly higher than the actual altitude, it indicates that the barometric pressure at the aircraft's location is higher than the standard atmospheric pressure set for the altimeter. The altimeter is essentially calibrated to a standard pressure of 29.92 inches of mercury (inHg) at sea level. If the local pressure increases, the altimeter will show a higher altitude than the actual altitude because it assumes the standard pressure for its calculations.

Consequently, if the atmospheric pressure is indeed higher than the standard value, the altimeter reading will not correspond accurately to how high the airplane truly is above sea level. Pilots must then verify current barometric conditions and adjust their altimeter settings accordingly to ensure they have an accurate reading of their altitude, which is crucial for maintaining safe operations, particularly during approaches and landings.