A barometric altimeter measures altitude by sensing atmospheric pressure. This device does not directly measure the aircraft’s height above the ground or sea level. Instead, it operates on the principle that air pressure decreases predictably as altitude increases. Since the instrument translates measured pressure into a corresponding altitude reading, any factor causing the ambient air pressure to deviate from the expected value will introduce an error. Atmospheric temperature is the largest variable causing the instrument’s reading to diverge from the aircraft’s true vertical position.
The Standard Atmosphere Baseline
To function as a reliable reference, every altimeter is calibrated against the International Standard Atmosphere (ISA). This theoretical model establishes a fixed set of atmospheric conditions. The ISA begins at mean sea level with a standard temperature of $15^\circ\text{C}$ ($59^\circ\text{F}$) and a standard pressure of 1013.25 hectopascals (or 29.92 inches of mercury).
The model dictates how these values change with height using a fixed temperature lapse rate. Within the troposphere, the ISA assumes temperature decreases at a constant rate of $6.5^\circ\text{C}$ per 1,000 meters (approximately $2^\circ\text{C}$ per 1,000 feet). This fixed temperature gradient determines the rate at which pressure decreases with altitude, providing the altimeter with its predictable pressure-to-altitude correlation.
Temperature’s Effect on Air Density
Temperature influences an altimeter’s reading through the fundamental physics of air density. Air obeys the ideal gas law, linking its pressure, temperature, and density. Warmer air expands, causing molecules to spread farther apart, resulting in lower density. Conversely, colder air contracts, packing molecules closer together, resulting in higher density.
This change in density directly affects the pressure gradient—the rate at which atmospheric pressure drops with altitude. In warm, less dense air, the column is vertically stretched; pressure decreases more slowly with height. In cold, denser air, the column is vertically compressed, and pressure drops more rapidly. The altimeter, calibrated against the ISA standard, is misled by this vertical compression or expansion of the atmosphere.
Translating Density Changes into Altimeter Error
When flying in cold air, the atmosphere is denser and vertically compressed, causing pressure layers to be closer together. The altimeter senses this steeper pressure gradient and interprets the rapid pressure drop as a greater climb than what occurred. This causes the altimeter to over-read the altitude; the indicated altitude is higher than the aircraft’s true altitude above sea level. This is a dangerous scenario, particularly near terrain, summarized by the mnemonic: “High to low, look out below.”
The opposite effect occurs in warmer-than-standard air, where the atmosphere is less dense and vertically expanded. Pressure layers are farther apart, and the altimeter senses a shallower pressure gradient. The instrument interprets the slow pressure drop as a smaller climb than the aircraft actually made. This causes the altimeter to under-read the altitude, meaning the aircraft is actually higher than indicated.
Operational Methods for Temperature Correction
Temperature-induced altimeter error requires specific correction methods, especially during instrument approaches near mountainous terrain. The local altimeter setting (QNH) corrects for non-standard sea-level pressure variations but does not account for temperature-related density changes in the air column. The error is proportional to the difference between the actual temperature and the ISA temperature, and it increases with height above the altimeter setting source.
Manual Correction
In extremely cold conditions, pilots must manually apply a temperature correction to published altitude values on approach charts. A common method uses the ICAO Cold Temperature Error Table to calculate the required adjustment, based on the reported airport temperature and the aircraft’s height above the airport. A simplified approximation suggests adding about four percent to the height above the airport for every $10^\circ\text{C}$ the ambient temperature is below the ISA standard.
Automated Correction
Modern aircraft equipped with advanced Flight Management Systems (FMS) can automatically calculate and apply this correction. They integrate real-time air temperature data from external probes into their altitude computations, ensuring the displayed altitude more closely reflects the aircraft’s true vertical position.