What is QNH and why is it significant?

QNH refers to the altimeter setting that indicates the atmospheric pressure at sea level. It is significant because it allows pilots to calibrate their altimeters to accurately measure their altitude above sea level. This is crucial for maintaining safe separation between aircraft, especially during takeoff and landing when precise altitude readings are essential for avoiding terrain and other obstacles.

When pilots set their altimeters to the QNH value provided by air traffic control or weather reports, they ensure that all aircraft in the vicinity are operating with a standard reference point for altitude. This synchronization is vital for situational awareness among pilots and air traffic controllers, preventing altitude deviations that could lead to conflicts in busy airspace.

Furthermore, knowing the QNH helps pilots adjust for changes in atmospheric pressure caused by weather systems, ensuring their altitude readings remain accurate despite variations in pressure that may occur as they ascend or descend. This level of precision contributes significantly to overall flight safety and operational efficiency in aviation.