The Altimeter: Types of Altitude, Cold-Weather Errors, the Kollsman Window, and Pressure Altitude
Updated: Sep 26
The altimeter is one of the most critical instruments in an airplane cockpit. It tells pilots how high the aircraft is above mean sea level — information essential for terrain clearance, obstacle avoidance, airspace compliance, and safe separation from other aircraft. Unlike some modern navigation tools, the altimeter relies on a simple but precise mechanical principle tied to the pitot-static system. But there's real depth beyond "it shows your altitude": there are actually FIVE distinct types of altitude, the altimeter can dangerously overread in cold weather (a real CFIT hazard requiring a specific correction), and setting it correctly involves understanding pressure altitude and the "1 inch = 1,000 feet" relationship. Understanding how the altimeter works, what kind of altitude it's showing, and how it can mislead helps pilots interpret it correctly and recognize dangerous errors.
This post covers the altimeter in practical depth: how it works internally, the Kollsman window and altimeter setting, the five types of altitude, cold-temperature errors and the correction table, pressure altitude and 29.92, the pressure-change rule of thumb, transition altitude and flight levels, and blockage failures.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
How the Altimeter Works
The altimeter measures static pressure and translates it into altitude using sealed aneroid wafers.
The aneroid wafers:
Thin, sealed metal capsules (evacuated or sealed at a reference pressure)
Inside the altimeter case
Expand and contract with static pressure changes
The sensing element
The operation:
Static pressure from the static port enters the altimeter CASE (around the wafers)
The wafers are sealed at a reference pressure
Climbing (static pressure decreases): the wafers EXPAND
Descending (static pressure increases): the wafers CONTRACT
Mechanical linkage translates the movement to the needle(s)/display
Continuous altitude indication
Why it uses static only:
The altimeter measures ambient (static) pressure
Static pressure decreases predictably with altitude
No pitot (dynamic) pressure involved
Static-only instrument
The pressure-altitude relationship:
The altimeter converts static pressure to altitude
Using the standard atmosphere model
Lower pressure = higher altitude
The core conversion
(For the pitot-static plumbing and the static-system failures, see our pitot-static system post.)
The Standard Atmosphere and Calibration
The reference the altimeter is built on.
The International Standard Atmosphere (ISA):
Altimeters are calibrated to the ISA
Standard sea-level pressure: 29.92 inHg (1013.25 hPa)
Standard sea-level temperature: 15°C
Standard lapse rate: about 2°C per 1,000 feet (temperature) and ~1 inHg per 1,000 feet (pressure, near sea level)
The model
Why calibration matters:
The altimeter assumes standard conditions
Real conditions rarely match the standard exactly
So corrections (the altimeter setting, and awareness of temperature) are needed
The assumption vs. reality
The real-world deviation:
Actual pressure and temperature differ from standard
The altimeter setting corrects for non-standard PRESSURE
Temperature errors require awareness (and correction in extreme cold)
Adjusting for reality
The Kollsman Window and Altimeter Setting
The adjustment, expanded with the rule of thumb.
The Kollsman window:
The small window on the altimeter showing the current pressure setting
Adjustable via a knob
Set to the local altimeter setting
The pressure reference adjustment
Why it matters:
If set incorrectly, the indicated altitude is wrong
Terrain clearance and separation can be compromised
Set it correctly and update it
Accuracy
The "1 inch = 1,000 feet" rule of thumb:
Changing the altimeter setting by 1 inHg changes the indicated altitude by about 1,000 feet
Set it 1 inch too high → the altimeter reads ~1,000 feet too high
Set it 1 inch too low → reads ~1,000 feet too low
A useful relationship
The setting-error direction:
Altimeter setting too HIGH → indicated altitude too HIGH → you're actually LOWER than indicated
Altimeter setting too LOW → indicated altitude too LOW → you're actually HIGHER than indicated
The direction matters (safety)
Know which way
"High to low, look out below":
Flying from HIGH pressure to LOW pressure (without updating the setting)
The altimeter reads HIGHER than actual (you're LOWER than indicated)
"Look out below" (you're closer to the ground than the altimeter shows)
The classic warning
The same for temperature:
Flying from warm to cold (without correction) also makes you lower than indicated
"High to low" applies to both pressure AND temperature
Cold = low (like low pressure)
The extended rule
Getting the setting:
From ATIS, AWOS, ASOS, ATC, or Flight Service
Update it as you fly (especially on long flights)
Within 100 NM (regulatory, for the nearest station)
Keep it current
The Five Types of Altitude
Here's the framework the basic treatment doesn't lay out but that's fundamental and heavily tested: altitude comes in five forms.
1. Indicated Altitude:
What the altimeter READS (with the current setting)
The raw indication
What you fly by (assigned altitudes, etc.)
The displayed altitude
2. Pressure Altitude:
The altitude when the altimeter is set to 29.92 inHg
Height above the standard datum plane
Used for flight levels (above the transition altitude) and performance calculations
The 29.92 altitude
3. Density Altitude:
Pressure altitude CORRECTED for non-standard TEMPERATURE
The altitude the aircraft "feels" (performance-wise)
High density altitude = poor performance (hot, high, humid)
The performance altitude
4. True Altitude:
The ACTUAL height above mean sea level (MSL)
What the altimeter approximates (with correct setting, standard temperature)
Charted elevations (airports, obstacles) are true altitude
The real MSL height
5. Absolute Altitude:
Height above the GROUND (AGL — above ground level)
Measured by a radar altimeter (or indicated altitude minus terrain elevation)
"Absolute" = above the surface
The AGL height
The relationships:
Indicated: what you read (with the setting)
Pressure: set 29.92
Density: pressure altitude + temperature correction (performance)
True: actual MSL height
Absolute: height above the ground (AGL)
Why the types matter:
Indicated: flying assigned altitudes
Pressure: flight levels, performance charts
Density: aircraft performance (takeoff, climb)
True: terrain/obstacle clearance (charted elevations)
Absolute: height above terrain (radar altimeter)
Different uses
The memory aid (ordering):
Some remember them by how they're derived
Indicated → (set 29.92) → Pressure → (temp correction) → Density
True is the real MSL; Absolute is AGL
The framework
Cold-Temperature Errors: The CFIT Hazard
Here's the operational depth the basic treatment gestures at but doesn't fully develop: the altimeter dangerously overreads in cold weather.
Why cold air causes overreading:
The altimeter assumes standard temperature
Cold air is DENSER than standard (pressure levels are compressed closer to the ground)
So a given pressure occurs at a LOWER true altitude than standard
The altimeter (assuming standard) reads HIGHER than your true altitude
You're LOWER than indicated
The danger:
In cold weather, your TRUE altitude is LOWER than the altimeter shows
You're closer to terrain/obstacles than indicated
A CFIT (controlled flight into terrain) hazard
Especially on approaches in cold weather
When it's critical:
Winter operations (cold temperatures)
Mountain flying (terrain close)
Instrument approaches (descending toward terrain in IMC)
The combination (cold + terrain + IMC) is most dangerous
The magnitude:
The error grows with how cold it is AND how high above the altimeter source you are
At very cold temperatures and significant heights, the error can be hundreds of feet
Enough to matter on an approach
Significant
The cold-temperature correction:
The AIM (and ICAO) provides a cold-temperature altitude correction TABLE
Based on the temperature and the height above the airport (or reporting station)
Add the correction to the published altitude (to fly a higher true altitude)
Corrects for the error
Using the correction table:
Enter the table with the reported temperature and the height above the airport
Find the correction (in feet)
ADD it to the altitude you fly
This keeps your TRUE altitude safe
Cold-Temperature Restricted Airports:
The FAA designates certain airports as "cold temperature restricted airports" (with a snowflake icon)
At these airports, below a specified temperature, the correction MUST be applied on the specified approach segments
Mandatory correction
Know if your destination is one
The practical takeaway:
In cold weather, remember: the altimeter overreads (you're lower than indicated)
Apply the correction on approaches when cold (especially at restricted airports)
"Cold = low" (like high-to-low)
A real safety item
Pressure Altitude and 29.92
The 29.92 setting, expanded.
When you set 29.92:
At or above the TRANSITION ALTITUDE (in the US, 18,000 feet MSL)
Above 18,000, everyone sets 29.92 (standard)
Below 18,000, you use the local altimeter setting
The transition
Why everyone uses 29.92 up high:
Above the transition altitude, all aircraft use the same reference (29.92)
So everyone's altimeter agrees (same reference)
Ensures separation (everyone measuring from the same datum)
Standardization at altitude
Flight levels:
Above 18,000 (with 29.92 set), altitudes are called FLIGHT LEVELS
FL180 = 18,000 feet (pressure altitude), FL350 = 35,000 feet, etc.
"Flight level three five zero"
Pressure-altitude-based
Reading pressure altitude:
Set the altimeter to 29.92
Read the altitude — that's pressure altitude
Used for performance charts and flight levels
The 29.92 reading
The transition altitude/level:
Transition altitude (climbing): 18,000 in the US (set 29.92 climbing through)
Transition level (descending): set the local altimeter setting descending through
The boundary between local settings and 29.92
Where the reference changes
Temperature and Pressure Error Summary
The two main errors, consolidated.
Pressure errors:
Flying to LOWER pressure without updating → altimeter reads HIGH → you're LOWER than indicated ("high to low, look out below")
Flying to HIGHER pressure without updating → altimeter reads LOW → you're HIGHER than indicated
Update the setting to avoid
The "1 inch = 1,000 ft" relationship
Temperature errors:
COLD (below standard) → altimeter reads HIGH → you're LOWER than indicated ("cold = low")
WARM (above standard) → altimeter reads LOW → you're HIGHER than indicated (less of a safety concern)
Cold is the dangerous one (you're lower than indicated)
Apply the correction when cold
The combined "look out below":
HIGH to LOW (pressure) → lower than indicated
WARM to COLD (temperature) → lower than indicated
Both make you lower than indicated (the dangerous direction)
"High to low OR hot to cold, look out below"
Position errors:
Static port airflow disturbance (attitude, configuration, sideslip)
Usually small
The pitot-static system's inherent error
Minor
Reading the Altimeter (and the Classic Misread)
A practical point the basic treatment doesn't cover.
The three-pointer altimeter:
The traditional altimeter has three pointers (like a clock)
Long pointer: hundreds of feet
Short pointer: thousands of feet
Smallest pointer: ten-thousands of feet
Three hands
The classic misread:
The three-pointer altimeter is easy to misread (mistaking the thousands)
A famous cause of altitude errors (and accidents)
Reading it wrong by 10,000 feet is a real risk
The three-pointer problem
The drum-pointer altimeter:
Modern altimeters use a drum (digital numbers) plus a single pointer (hundreds)
Easier to read (the drum shows thousands/ten-thousands numerically)
Reduces the misread risk
The improvement
The glass-cockpit tape:
Glass cockpits show altitude as a vertical tape with a digital readout
Even easier to read
Trend indicators
Modern presentation
The reading habit:
Read the altimeter carefully (especially three-pointer types)
Cross-check with other cues
Avoid the misread
Careful interpretation
Altimeter Failures Related to the Pitot-Static System
The blockage failures, brief (the pitot-static post covers them fully).
Blocked static port:
The altimeter FREEZES at the blockage altitude (trapped static)
Reads the same climbing or descending
(The VSI reads zero and the airspeed becomes unreliable too)
Stuck reading
The alternate static source:
Opens the static system to cabin air (backup)
Cabin static is lower → the altimeter reads HIGHER than actual
Account for the error
The backup (reads high)
The clue:
A frozen altimeter (plus zero VSI) suggests a static blockage
Recognize the pattern
Use the alternate static source
Diagnostic
(For the full static-blockage behavior and the alternate static source, see our pitot-static system post.)
Altimeter Use in Flight Operations
The operational use, expanded.
Pilots rely on the altimeter for:
Maintaining assigned altitudes (ATC)
Terrain and obstacle clearance
Airspace compliance (altitude limits)
Instrument approach minimums (DA/MDA)
The approach-minimums criticality:
On an instrument approach, the altimeter determines when you reach minimums (DA/MDA)
Accuracy is critical (a wrong setting or cold error = wrong minimums)
Set correctly, apply cold correction
Life-and-death precision
The verification habits:
Cross-check altitude with other instruments
Verify the altimeter setting frequently (update it)
Monitor trends (not just isolated readings)
Set the current setting before an approach
The pre-approach setting:
Get the current altimeter setting before the approach (from ATIS/ATC)
Set it (the approach minimums depend on it)
A critical step
Accuracy for minimums
Altimeters in Modern Glass Cockpits
The glass reality, brief.
In glass cockpits:
Static pressure is sensed electronically (Air Data Computer)
The ADC computes altitude
Displayed as a digital tape (with a readout)
Same static-pressure dependence
The advantages:
Easier to read (digital)
May include alerts (altitude deviation)
Trend indicators
Reduced misread risk
The failures still apply:
A blocked static still affects the altitude (the ADC gets bad data)
The fundamentals are unchanged
Understand them
Same principles
Why Understanding the Altimeter Matters
The stakes, expanded.
Misinterpreting the altimeter can lead to:
Controlled flight into terrain (CFIT) — the deadliest
Altitude deviations (busting assigned altitudes)
Airspace violations
Loss of separation
The CFIT connection:
CFIT is a leading cause of fatal accidents
Altimeter errors (setting, cold weather) contribute
Correct settings and cold corrections prevent it
Critical
A pilot who understands the altimeter can:
Recognize abnormal indications
Adjust for environmental conditions (cold, pressure)
Respond correctly to failures (static blockage)
Fly accurate, safe altitudes
Competence
Conclusion
The altimeter is a deceptively simple instrument built on precise physics. By measuring static pressure and comparing it to a reference (set in the Kollsman window), it provides one of the most critical pieces of flight information: altitude. But understanding it fully means knowing the five types of altitude (indicated, pressure, density, true, absolute), the "1 inch = 1,000 feet" setting relationship, and — critically — that the altimeter OVERREADS in cold weather, putting you LOWER than indicated (a CFIT hazard requiring the cold-temperature correction).
Understanding how the altimeter works, what altitude it's showing, and how it can mislead — from a wrong setting to cold-weather error to a static blockage — allows pilots to fly accurately, comply with regulations, and maintain safe margins from terrain. In aviation, altitude awareness is non-negotiable, and the altimeter makes that awareness possible.
On the Written Test and Checkride
The altimeter appears on tests and checkride orals. The most commonly tested topics:
How the altimeter works (aneroid wafers, static pressure)
The Kollsman window and altimeter setting ("high to low, look out below")
The five types of altitude (indicated, pressure, density, true, absolute)
Cold-temperature error (overreads, you're lower than indicated)
Pressure altitude and 29.92 (flight levels)
Blocked static behavior
Quick Reference
How It Works:
Aneroid wafers respond to static pressure
Climb (pressure drops): wafers expand → higher reading
Static pressure only (no pitot)
Kollsman window sets the reference
The Standard Atmosphere (ISA):
Sea-level pressure: 29.92 inHg
Sea-level temperature: 15°C
Lapse rate: ~2°C and ~1 inHg per 1,000 ft
Altimeter Setting:
Set the local setting (from ATIS/AWOS/ATC)
"1 inch = 1,000 feet" (setting error = altitude error)
Setting too HIGH → reads HIGH → you're LOWER
"High to low, look out below"
The Five Types of Altitude:
Type | Definition |
Indicated | What the altimeter reads (with the setting) |
Pressure | Altitude with 29.92 set (standard datum) |
Density | Pressure altitude + temperature correction (performance) |
True | Actual height above MSL |
Absolute | Height above the ground (AGL) |
Cold-Temperature Error (CFIT hazard):
Cold air is denser → altimeter reads HIGH → you're LOWER than indicated
Dangerous on approaches in cold weather (terrain close)
Apply the AIM cold-temperature correction (add to the altitude)
Mandatory at Cold Temperature Restricted Airports (snowflake icon)
"Cold = low"
Pressure Errors:
High to low pressure (no update) → lower than indicated
Low to high pressure (no update) → higher than indicated
Temperature Errors:
Warm to cold (no correction) → lower than indicated (dangerous)
Cold to warm → higher than indicated
"High to low OR hot to cold, look out below"
Pressure Altitude / 29.92:
Set 29.92 at/above the transition altitude (18,000 ft in the US)
Above 18,000: flight levels (FL180, FL350) with 29.92
Below: local altimeter setting
Everyone uses 29.92 up high (same reference)
Reading:
Three-pointer altimeter: easy to misread (thousands)
Drum-pointer: easier (digital drum)
Glass tape: easiest
Read carefully
Blocked Static:
Altimeter FREEZES (trapped static)
Alternate static source → reads HIGHER (cabin static lower)
Key Principle:
The altimeter measures static pressure via aneroid wafers, showing indicated altitude with the Kollsman-window setting ("1 inch = 1,000 feet"). Know the five types of altitude (indicated, pressure, density, true, absolute). Critically, the altimeter OVERREADS in cold weather — you're LOWER than indicated — a CFIT hazard requiring the cold-temperature correction, especially on approaches. "High to low, or hot to cold, look out below."
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.