Define the term "QNH" in aviation weather reporting.

The term "QNH" in aviation weather reporting specifically refers to the altimeter setting that provides the sea level pressure. When pilots adjust their altimeters to the QNH setting, they are calibrating their instruments to display their altitude above sea level, rather than above the local ground level. This ensures that pilots maintain accurate altitude readings and can manage safe vertical separation from terrain and other aircraft.

Setting the altimeter to QNH allows pilots to understand the atmospheric pressure at sea level and provides a standardized reference for altitude, which enhances safety during flight operations. It is critical for pilots, especially when flying at lower altitudes, to have their altimeters set correctly so they can maintain proper altitude and avoid obstacles.

The other options do not accurately capture the definition of QNH. For instance, QNH does not relate to temperature or pressure measurements at specific altitudes like 10,000 feet, nor does it pertain to the reliability of navigational aids. Thus, the choice referencing QNH as the altimeter setting that provides sea level pressure is correct.