What does QNH indicate?

QNH is a term used in aviation meteorology that indicates the altimeter setting used to calibrate an aircraft’s altimeter for flight at or near sea level. It represents the barometric pressure at sea level, allowing pilots to measure the aircraft's altitude in relation to Mean Sea Level (MSL). When a pilot sets the altimeter to the QNH value provided by air traffic control, it adjusts the instrument to read altitude in MSL, ensuring that the altitude readout is accurate and consistent with the ground level regardless of variations in atmospheric pressure.

This value is critical for safe flight operations as it helps in maintaining proper altitude separation from terrain and other aircraft. Understanding QNH and its usage is essential for pilots to ensure accurate altitude readings throughout their flight.