Units, Measurements & Scientific Instruments20 Concepts & Facts

Altimeter Settings: Kollsman Subscale, QNH and True Elevation

Barometric altimeters determine height above the ground by measuring the weight of the air pressing down from above. As a person climbs higher up a mountain or an aircraft ascends into the sky, fewer air molecules remain overhead. This thinning of the atmosphere causes air pressure to drop in a steady and predictable pattern. Inside a traditional altimeter, a sealed, flexible metal capsule expands slightly as the outside air pressure decreases. A series of tiny gears connects this expanding capsule to a pointer on a circular dial, displaying the current height in feet or meters. However, air pressure is not determined solely by altitude. Daily weather patterns constantly cause air pressure to rise and fall even when an instrument stays in one spot. Because of these natural weather fluctuations, an altimeter must be adjusted regularly so it does not mistake a passing storm for a change in height.

To keep height readings accurate despite shifting weather, aviation pioneer Paul Kollsman invented a specialized adjustment mechanism known as the Kollsman window. This feature consists of a small subscale dial built directly into the face of the altimeter, controlled by an external adjustment knob. Before taking off, a pilot contacts the airport control tower to receive the current local sea-level air pressure, known in aviation terminology as QNH. Turning the adjustment knob aligns the small subscale with this local pressure report. This adjustment physically rotates the entire gear mechanism, ensuring the main needles indicate the true surveyed elevation of the airfield while parked on the runway. Without this calibration, an altimeter simply shows pressure altitude based on an idealized standard atmosphere rather than true physical height above sea level.

Failing to adjust an altimeter before traveling into a different weather region creates serious navigation hazards. When an aircraft or hiker travels toward a low-pressure storm system, the surrounding air pressure drops naturally. An unadjusted altimeter interprets this falling pressure as an increase in height, falsely indicating that the traveler is climbing. In reality, the vehicle or hiker remains much lower than indicated, creating a severe danger of crashing into mountain ridges in foggy weather. Outdoor adventurers remember this hazard through the classic safety warning: from high to low, look out below. Helpful navigation applications such as OneTool: GPS, Compass, Level allow outdoor explorers to recalibrate their digital barometers against known trail benchmarks or regional weather reports. Taking a few seconds to update reference pressure keeps mountain navigation reliable, protecting hikers from confusing cloud cover or sudden weather shifts.

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#1
A barometric altimeter calculates height by measuring ambient air pressure, which naturally decreases as elevation rises.
#2
Sealed aneroid capsules inside mechanical altimeters expand as atmospheric pressure drops, moving a pointer across the display dial.
#3
Natural weather systems cause local air pressure to fluctuate continuously, requiring frequent instrument calibration for accurate elevation readings.
#4
Aviation engineer Paul Kollsman developed the altimeter subscale window in 1928 to let pilots calibrate their instruments before flight.
#5
In aviation and meteorology, QNH represents the measured atmospheric pressure adjusted to mean sea level for a specific geographic region.
#6
When an altimeter is calibrated to local QNH on an airfield runway, the instrument displays the true surveyed elevation of that airport.
#7
Pressure altitude represents the indicated height when an altimeter is set to the standard sea-level reference pressure of 1013.25 hectopascals.
#8
Aircraft flying at high cruising altitudes set their altimeters to standard pressure so all planes share identical vertical separation references.
#9
Flying from a high-pressure weather system into a low-pressure region causes an unadjusted altimeter to read higher than the actual aircraft altitude.
#10
The traditional aviation rule from high to low, look out below warns pilots that uncorrected altimeters overestimate actual clearance above terrain.
#11
Extreme cold weather compresses atmospheric air layers toward the ground, causing barometric altimeters to overstate true physical height above terrain.
#12
Mountain hikers can calibrate their personal altimeters at designated trailhead signs marked with officially surveyed topographic elevation.
#13
Mobile navigation toolkits like OneTool: GPS, Compass, Level allow outdoor adventurers to calibrate barometric readings using known landmarks or local weather updates.
#14
QFE represents airport ground-level pressure, causing an altimeter to read exactly zero while sitting on the runway surface.
#15
Electronic solid-state barometers use microscopic silicon diaphragms that change electrical resistance when atmospheric pressure pushes against them.
#16
Altimeter setting reports are broadcast continuously by airport automated weather stations to assist incoming and departing flights.
#17
A difference of one hectopascal in atmospheric pressure corresponds roughly to about eight meters of elevation change near sea level.
#18
Surveyors historically used sensitive mercurial barometers to calculate mountain summit heights before modern satellite positioning existed.
#19
Satellite positioning provides height above a mathematical reference shape, which often differs from true elevation above mean sea level.
#20
Combining satellite signals with calibrated barometric pressure sensors produces dependable elevation tracking during outdoor mountain journeys.

Subject Specialist Commentary

Analytical perspective & practical exam advice from the Master10 academic board

Educator's Insight

An altimeter is fundamentally a barometer that reads in units of height. Because shifting weather patterns alter air pressure constantly, knowing how to calibrate reference pressure is an essential practical skill. Calibrating an altimeter bridges the gap between changing atmospheric conditions and real physical terrain. This simple adjustment ensures that pilots maintain safe clearances above hills and hikers track their mountain progress accurately, turning fluctuating weather into dependable navigation information.

In competitive civil aviation and geography examinations, students frequently forget how atmospheric pressure changes affect altimeter readings. Always remember that falling weather pressure tricks an unadjusted altimeter into reading too high. To master the essential altimeter calibration concepts, keep in mind the mnemonic KNOB: Kollsman subscale adjustment, Needle points to true elevation, Outdoor weather correction, and Baseline reference pressure alignment.

Altimeter Settings: Kollsman Subscale, QNH and True Elevation

Key Concepts & Self-Assessment20 Key Facts

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