Every pilot relies on the altimeter to know how high the aircraft is flying, yet the instrument cannot directly measure height. Instead, it senses atmospheric pressure and converts that pressure reading into an altitude indication using a fixed mathematical model — the standard atmosphere. Because the real atmosphere almost never matches that model exactly, the altitude displayed on the altimeter is almost never your actual height above the ground. Understanding exactly why, by how much, and what to do about it is essential for safe flight, particularly during instrument approaches and operations over mountainous terrain.
The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 8, provides the foundational framework for this topic. This article expands on that source to give pilots a thorough, practical understanding of altimeter errors caused by nonstandard temperature and pressure conditions.
How the Altimeter Works
The altimeter is essentially an aneroid barometer — a sealed, flexible capsule that expands and contracts as the surrounding air pressure changes. The key difference from a plain barometer is that the scale is graduated in feet (or meters) of altitude rather than units of pressure. That graduation is derived from the standard atmosphere, which defines a specific pressure at every altitude. At sea level, standard pressure is 29.92 inches of mercury (inHg), or 1,013.2 millibars. Pressure decreases predictably with altitude in the standard model.
Because pressure changes are the only input the aneroid capsule receives, the altimeter cannot distinguish between a pressure change caused by a change in actual height and a pressure change caused by a shift in the surface weather system below. A movable Kollsman window (the altimeter setting knob) allows pilots to enter the local altimeter setting — the sea-level equivalent pressure reported by a nearby weather station — so that the instrument reads true altitude at field elevation. This is the altimeter setting: the value set on the pressure altimeter so it indicates true altitude at field elevation. Critically, there is no knob or correction for temperature; the altimeter is permanently calibrated against the standard atmosphere lapse rate and cannot account for a warmer or colder-than-standard air column.
Types of Altitude: A Critical Distinction
True Altitude
True altitude is your actual vertical distance above mean sea level (MSL). It is what you would measure with a perfect ruler. Because real atmospheric conditions rarely match the standard atmosphere, your altimeter almost never shows true altitude — it shows indicated altitude.
Indicated Altitude
Indicated altitude is what the altimeter displays when set to the local altimeter setting. It is the standard reference used for traffic separation, minimum en route altitudes, and obstacle clearance on charts. All aircraft in the same area use the same altimeter setting, so their indicated altitudes are mutually consistent — but none of them may match true altitude when temperature is nonstandard.
Corrected (Approximately True) Altitude
If a pilot applies a temperature correction using outside air temperature (OAT) and a flight computer or E6B, the result is corrected altitude — a close approximation to true altitude. The correction is based on the estimated deviation of the existing temperature from standard atmosphere temperature at that level. This value is accurate enough for terrain clearance purposes, provided the altimeter is set to a current, nearby reporting station.
Pressure Altitude
Pressure altitude is the indicated altitude when the altimeter is set to 29.92 inHg. It represents the altitude associated with a specific pressure in the standard atmosphere. Pressure altitude is used for performance calculations (takeoff, climb, engine power) and is the basis for flight levels at or above FL180 in the United States.
Density Altitude
Density altitude is pressure altitude corrected for temperature deviations from standard. It is the altitude in the standard atmosphere at which the current air density would exist. Density altitude is not directly readable on the altimeter; it is computed and serves as an index of aircraft performance. Hot, high, or humid conditions raise density altitude and degrade performance. Cold, low-elevation airports have low density altitude and excellent performance.
How Pressure Errors Affect the Altimeter
When a pilot flies from a high-pressure area into a low-pressure area without updating the altimeter setting, the instrument reads higher than the aircraft's actual altitude. The classic memory phrase for this is: from high to low, look out below. The aircraft is physically lower than the altimeter indicates, because the surface pressure ahead is lower than what the Kollsman window was set for. The fix is simple: update the altimeter setting frequently by requesting the current altimeter from nearby ATIS, AWOS, ASOS, or ATC. The FAA recommends updating the setting as the aircraft passes within range of each new reporting station.
Flying from a low-pressure region to a high-pressure region without correcting causes the opposite: the altimeter reads lower than actual altitude. This is less immediately dangerous for terrain clearance but causes the pilot to climb unnecessarily to reach a desired indicated altitude, wasting fuel and potentially conflicting with traffic.
How Temperature Errors Affect the Altimeter
Temperature errors are more insidious than pressure errors because they cannot be corrected by adjusting the Kollsman window. The altimeter is calibrated on the assumption that air cools at the standard lapse rate of approximately 2°C per 1,000 feet. When the real atmosphere is colder than standard, the air is denser, pressure levels are compressed closer to the surface, and the pressure the altimeter senses corresponds to a lower altitude in the standard atmosphere than where the aircraft actually is. The result: the altimeter reads higher than the aircraft's true altitude. The aircraft is lower than it thinks it is.
Conversely, in warmer-than-standard air, the atmosphere expands, pressure levels are lifted higher, and the altimeter reads lower than the aircraft's true altitude. The aircraft is actually higher. While this is generally safe for terrain clearance, it means the aircraft is above its charted altitude, which can cause conflicts on instrument approaches.
The cold-temperature error is particularly dangerous near high terrain. An aircraft flying an instrument approach in Arctic conditions may be significantly lower than its altimeter suggests. The FAA has addressed this operationally: cold temperature corrections must be applied at certain airports and procedures, and many IFR approach charts now include cold temperature restrictions or correction tables for approach altitudes when the OAT falls below a specified threshold.
The Physical Reason: Density Changes Everything
The underlying physics, as explained in FAA-H-8083-28B Chapter 8, comes down to air density. Pressure and temperature together determine density. Denser air means pressure levels are packed closer together vertically; less dense air means they are spread farther apart. Since the altimeter counts altitude by measuring how much pressure has dropped from the set reference, a denser (colder) column of air between the aircraft and the surface will produce a bigger pressure drop than the standard model expects — the instrument overcounts the altitude. A less dense (warmer) column produces a smaller pressure drop — the instrument undercounts the altitude. The altimeter has no way to know the mean temperature of the air column below; it only senses pressure at its own location.
Key Numbers and Rules
- Standard sea-level pressure: 29.92 inHg (1,013.2 mb). Setting the altimeter here gives pressure altitude.
- Standard lapse rate: approximately 2°C per 1,000 feet (or ~3.5°F per 1,000 ft) — the temperature assumption baked into the altimeter scale.
- Pressure lapse rate (standard atmosphere): approximately 1 inHg per 1,000 feet near sea level; approximately 1 mb per 30 feet.
- Cold temperature error rule of thumb: approximately 4 feet of error per 1,000 feet of height above the airport per 1°C of temperature deviation from standard; this is a commonly taught approximation, and exact values vary by chart or table (such as ICAO cold temperature correction tables). A significant cold day can produce hundreds of feet of error on a high approach.
- High to low, look out below: flying from high pressure to low pressure without updating the altimeter setting causes the aircraft to be lower than indicated.
- Warm to cold, same result: flying from warm air into cold air causes the aircraft to be lower than indicated — without any pressure change at all.
- Flight levels: at or above FL180 in the U.S., all aircraft use 29.92 inHg. Pressure-altitude-based separation applies; temperature errors still affect true altitude but everyone is using the same standard reference.
- Density altitude: equals pressure altitude (and thus field elevation, when pressure altitude equals field elevation) when the outside air temperature equals the standard temperature for that pressure altitude. Hot or high airports can produce density altitudes thousands of feet above field elevation, sharply reducing aircraft performance.
Memory Aid
High to Low, Look Out Below — This mnemonic covers both the pressure and temperature cases: flying from HIGH pressure to LOW pressure (or from WARM air to COLD air), the aircraft is LOWER than the altimeter indicates. The instrument has been fooled in the same direction by two different physical mechanisms.
Why It Matters Operationally
Over flat terrain in VMC, altimeter errors from temperature are largely a shared inconvenience — all aircraft are flying indicated altitudes, not true altitudes, and separation standards account for this. The serious danger emerges in two scenarios. First, instrument approaches in cold weather: minimum descent altitudes (MDAs) and decision altitudes (DAs) on approach plates are expressed as indicated altitudes, but the obstacles and terrain being cleared are at true altitudes. If the indicated altitude overestimates your true altitude, you may be dangerously close to terrain or obstacles while believing you have adequate clearance. Second, en route flight over mountainous terrain in cold air: a minimum en route altitude (MEA) provides obstruction clearance based on standard conditions. In very cold air, the true altitude corresponding to a given indicated altitude can be several hundred feet lower, potentially placing the aircraft in conflict with terrain.
For these reasons, pilots operating IFR in cold temperatures should apply temperature corrections to approach altitudes and minimum safe altitudes whenever required by the procedure or when the temperature is significantly below standard. The AIM (7-2-3) discusses cold temperature altimetry error and directs pilots to designated cold temperature restricted airports and their published correction tables — derived from sources such as ICAO Doc 8168 — which use airport elevation, height above the airport, and OAT to derive the correction to add to indicated altitude to achieve the charted true altitude clearance. Correction is generally required only where charted or specifically directed by the procedure.
Common Test Traps
- Confusing the direction of temperature error: Students often reverse the cold-air rule. Remember: cold air = denser = altimeter reads TOO HIGH = aircraft is LOWER than indicated. Warm air = less dense = altimeter reads too low = aircraft is higher than indicated.
- Assuming the altimeter setting fixes all errors: The Kollsman window only corrects for surface pressure variations. It cannot correct for a nonstandard temperature lapse rate in the column of air below. Temperature error is invisible to the altimeter setting.
- Mixing up pressure altitude and indicated altitude: Pressure altitude requires setting 29.92 inHg; indicated altitude uses the local altimeter setting. They are equal only when local pressure is 29.92 inHg.
- Density altitude as a navigation altitude: Density altitude is a performance index, not something you read off the altimeter or use for separation. Do not confuse it with pressure altitude or true altitude on exam questions about terrain clearance.
- Forgetting water vapor's minor role: The handbook notes that humid air is slightly less dense than dry air (water vapor molecules are lighter than the average dry-air molecule), which technically raises density altitude. However, its contribution to altimetry error is generally negligible compared to temperature and pressure, and exams rarely test it in quantitative terms.