The altimeter setting, known in aviation as QNH, is the pressure value given to pilots to ensure their altimeter displays an accurate altitude above mean sea level (AMSL). Aircraft altimeters operate by measuring atmospheric pressure, which decreases as altitude increases. Since atmospheric pressure fluctuates with weather and location, a standard baseline is required for safe vertical separation and navigation. QNH represents the current atmospheric pressure at a specific point, corrected to what it would be at sea level under the conditions of the International Standard Atmosphere (ISA). Because weather systems are constantly moving, the altimeter setting is rarely the same across distant reporting points. The variations observed in these settings are a direct result of three primary physical phenomena: the movement of large air masses, the influence of non-standard air temperature on density, and highly localized surface effects.
Large-Scale Pressure Systems
The presence of synoptic-scale pressure systems, which involve the movement of air masses, is a major cause of variation in altimeter settings over large distances. Atmospheric pressure is the weight of the air column pressing down on a given point on the Earth’s surface. This column of air can weigh more or less depending on the current weather system overhead. These large-scale systems are categorized as high-pressure and low-pressure areas, and they dictate the overall pressure gradient across entire regions.
High-pressure systems, or anticyclones, are characterized by descending air that is relatively cold and dry. This sinking air compresses the layers below it, increasing the total weight of the air column above the ground reporting station. Consequently, a reporting point located within a high-pressure system will report a higher QNH value, which can often exceed the standard pressure of 1013.25 hectopascals (hPa). The higher QNH reflects a greater overall atmospheric weight, meaning the air is more dense near the surface.
Conversely, low-pressure systems, or cyclones, are associated with rising air and stormy weather. As air rises, it expands and cools, which reduces the total weight of the air column pressing down on the surface. Reporting stations inside a low-pressure center will therefore measure a lower surface pressure, resulting in a lower QNH value. This difference in atmospheric weight creates a horizontal pressure gradient, where the altimeter setting smoothly transitions from the high values in one system to the low values in another over hundreds of miles. The movement of these systems causes the altimeter setting at any fixed location to change over time, which is why pilots must constantly update their setting during flight.
The Influence of Temperature and Density
While large-scale systems determine the pressure variation, the actual temperature of the air column introduces another source of QNH variation by altering air density and the standard calculation model. The process of calculating QNH involves extrapolating the measured surface pressure down to sea level, assuming an idealized temperature-lapse rate defined by the International Standard Atmosphere (ISA). This ISA model assumes a standard temperature of 15°C at sea level and a decrease of 2°C for every 1,000 feet of altitude gain.
When the actual temperature of the air column deviates from the ISA model, the density of the air changes, altering the vertical spacing between the constant pressure surfaces. Cold air is denser than standard air, meaning the pressure drops more rapidly with altitude. When a station calculates its QNH in extremely cold conditions, the calculation, based on the ISA model, can yield a setting that results in the altimeter indicating an altitude higher than the aircraft’s true altitude. This is due to the denser, colder air column being vertically compressed, placing the actual pressure level closer to the ground than the altimeter assumes.
The opposite occurs in air that is warmer than the ISA standard. Warm air is less dense, causing the pressure to drop more slowly with an increase in altitude. In this scenario, the altimeter will indicate an altitude lower than the aircraft’s true altitude because the pressure levels are vertically expanded. These temperature-induced variations mean that two reporting points within the same overall high-pressure system can still report slightly different QNH values if one is experiencing colder or warmer air than the ISA model assumes for its elevation.
Localized Topographic and Thermal Effects
Micro-scale variations in the altimeter setting occur over short distances due to localized influences from the surrounding terrain and surface heating. Physical features, such as mountain ranges, affect the airflow, creating areas of localized compression and expansion that cause pressure fluctuations. Air flowing over a mountain, for instance, can temporarily increase pressure on the windward side and decrease it on the leeward side.
Thermal effects from the surface also contribute to temporary pressure changes that affect the QNH at nearby stations. Large bodies of water and landmasses heat and cool at different rates, generating local wind patterns like sea breezes and land breezes. These localized air movements are driven by pressure differences that are not captured by the large-scale weather models, resulting in measurable QNH differences between a coastal airport and one just a few miles inland. Urban heat islands, where cities retain more heat than the surrounding rural areas, can cause localized pockets of less dense, warmer air, leading to slight pressure variations. These topographic and thermal influences are usually short-lived and specific to the immediate geographical area.