QNH, an acronym standing for "Quasi Non Hydrostatic," is a meteorological term with significant implications in aviation. QNH refers to the atmospheric pressure at mean sea level (MSL) adjusted to the local area or airport elevation. It provides a reference point for altimeter settings and is crucial for ensuring precise altitude measurements during flight.
In aviation, QNH is important because altimeters measure the aircraft's altitude relative to the pressure of the surrounding atmosphere. By setting the altimeter to the QNH value of the airport or area, pilots can accurately determine their height above mean sea level, making it an essential component for safe and accurate navigation.
The QNH value is determined by considering the prevailing atmospheric pressure at sea level and adjusting it for elevation. It accounts for variations in local air pressure due to weather, terrain, and altitude. Aviation authorities and meteorological agencies regularly update QNH values to ensure accurate altitude readings, particularly during takeoff, landing, and en-route phases of flight.
Pilots receive QNH information from air traffic control or through automatic weather stations at airports. The QNH is usually reported in hectopascals (hPa) or inches of mercury (InHg) and is always adjusted relative to mean sea level.
Incorrectly set QNH values can lead to altitude deviations, posing significant risks during flight operations. Therefore, pilots must diligently set the accurate QNH reading provided to them to guarantee safety and precision in altitude measurement while navigating through the skies.
