An altimeter is an instrument designed to measure an object’s altitude, or height, above a fixed reference level, commonly sea level. In aviation, this device is indispensable, providing pilots with important information about their vertical position. While altimeters primarily function by sensing atmospheric pressure, their accuracy is susceptible to variations in temperature. Understanding this relationship is important for safe and precise navigation, affecting aircraft performance and flight safety.
How Pressure Altimeters Work
A pressure altimeter operates on the principle that atmospheric pressure decreases predictably with increasing altitude. It is essentially a barometer, calibrated to translate ambient air pressure into an altitude reading. Within the altimeter, sealed aneroid capsules expand or contract in response to changes in external static pressure. This mechanical movement is then displayed as altitude.
This instrument relies on a standardized atmospheric model, known as the International Standard Atmosphere (ISA), for its calibration. The ISA defines average conditions, including a standard sea-level pressure (1013.25 millibars or 29.92 inches of mercury) and a specific temperature lapse rate, with temperature decreasing consistently with altitude. This standard provides a consistent baseline for altimeter readings, allowing for consistent flight operations.
The Influence of Air Density
Temperature directly influences air density, a fundamental factor affecting altimeter accuracy. Warm air is less dense because its molecules are more spread out. Conversely, cold air is denser because its molecules are packed more closely. This difference in density means that the weight of a column of air, and the pressure it exerts, varies with temperature.
Altimeters measure pressure, and atmospheric pressure is a function of the weight of the air column above it. When air density changes due to temperature variations, the weight of this air column also changes. Consequently, changes in air density mean that a specific pressure level will be found at a different true altitude than what the altimeter, calibrated to a standard atmosphere, anticipates.
How Temperature Affects Altimeter Readings
Deviations from standard temperature cause an altimeter to indicate an altitude that differs from the aircraft’s true altitude. In air that is warmer than the standard, a given pressure level is found at a higher true altitude. This occurs because the warmer, less dense air expands, causing pressure surfaces to be farther apart. Since the altimeter interprets pressure as an altitude based on the standard, it will indicate a lower altitude than the aircraft’s actual height above sea level in warmer-than-standard conditions.
Conversely, in air colder than the standard, a given pressure level exists at a lower true altitude. The colder, denser air contracts, bringing pressure surfaces closer together. Because the altimeter is calibrated for standard air, it will consequently indicate a higher altitude than the aircraft’s true position. This means the aircraft is actually lower than the altimeter suggests. For pilots, a common adage, “High to Low, Look Out Below,” serves as a reminder that flying from an area of high pressure or warmer temperatures to an area of low pressure or colder temperatures without adjusting the altimeter setting can lead to the aircraft being at a lower true altitude than indicated, increasing the risk of terrain contact.
Practical Considerations for Flight
Understanding the effect of temperature on altimeter readings is important for aviation safety, particularly for terrain clearance. Discrepancies between indicated and true altitude, particularly in cold or mountainous terrain, pose a significant risk. For example, an altimeter indicating 10,000 feet in very cold air means the aircraft’s true altitude might be considerably lower, reducing the safe margin above obstacles. Pilots apply specific corrections during cold weather operations, especially during instrument approaches, to maintain terrain separation.
Temperature also impacts aircraft performance through a concept known as density altitude. While not a direct altimeter reading, density altitude represents the pressure altitude corrected for non-standard temperature. Higher temperatures result in higher density altitude, meaning the air behaves as if the aircraft is operating at a greater altitude.
This reduces engine power, propeller efficiency, and wing lift, impacting takeoff, climb, and landing performance. Pilots account for these temperature-related effects through thorough pre-flight planning, using current temperature and pressure data from sources like ATIS or AWOS. They also apply cold temperature corrections, particularly for instrument approaches and minimum safe altitudes, to ensure accurate altitude awareness and safe operations.