What does "QNH" refer to in aviation?

"QNH" refers to the atmospheric pressure adjusted to sea level for altimeter settings. This setting is crucial for pilots as it allows them to calibrate their aircraft's altimeter, ensuring that the altitude displayed corresponds to the actual altitude above sea level. Accurate altitude measurement is vital for maintaining safe separation between aircraft and avoiding terrain or obstacles.

When QNH is set correctly, it ensures that the altimeter reads zero when the aircraft is on the ground at the airport where the QNH measurement was taken. This adjustment compensates for changes in atmospheric pressure due to weather variations, which can influence flight safety and efficiency. Understanding QNH and its implications plays a key role in effective flight operations and navigation. Thus, option C accurately captures the significance of QNH in aviation, making it the correct choice.