What does the altimeter rely on to accurately measure altitude?

The altimeter measures altitude primarily by relying on barometric pressure. As an aircraft ascends or descends in the atmosphere, the pressure exerted by the air above changes; this change in barometric pressure correlates with changes in altitude. The altimeter is calibrated to recognize how this pressure decreases as altitude increases—roughly, for every 1,000 feet increase in altitude, the pressure drops about 1 inch of mercury (inHg). By measuring the current atmospheric pressure and comparing it to a standard pressure at sea level, the altimeter can determine the aircraft's altitude relative to sea level. This method is crucial for ensuring accurate altitude readings, especially in areas where GPS signals may be weak or unreliable.