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LESSONS · ALTIMETRY AND CRUISING LEVELS · 14 MINUTES · AUSTRALIA AND NZ

Altimeter settings and pressure.

An altimeter is a barometer with a height scale. It shows how far you are above whatever pressure you set in its subscale. Set the wrong pressure, or fly in air colder than standard, and it shows a height you are not at.

The subscale sets the zero

Turning the subscale knob moves the level the altimeter measures from. Set QNH and it reads your height above mean sea level. Set 1013 hPa (1013.2 hPa in the rules) and it reads pressure altitude, your height above the level where the pressure is 1013 hPa.

The altimeter is essentially an aneroid barometer. The difference is the scale.

FAA Aviation Weather Handbook (FAA-H-8083-28B), chapter 8, 8.4 Altimetry

A pressure datum which when set on the sub-scale of a sensitive altimeter causes the altimeter to indicate vertical displacement from that datum.

CASA Visual Flight Rules Guide v8.3 (CC BY-NC 4.0), Glossary, altimeter setting. Rules as at 31 October 2026

QNH is an atmospheric pressure adjusted to sea level and measured in hPa or millibars so that when QNH is set the altimeter will read elevation above mean sea level (AMSL).

CASA Visual Flight Rules Guide v8.3 (CC BY-NC 4.0), Altimetry. Rules as at 31 October 2026

Put simply, pressure altitude is the height above the ISA datum of 1,013 hPa.

CASA Visual Flight Rules Guide v8.3 (CC BY-NC 4.0), Pressure altitude considerations. Rules as at 31 October 2026

PREDICT FIRST

You are parked at an aerodrome 1,000 ft above sea level. Today's QNH is 1003 hPa. You wind the subscale to 1013 hPa. What does the altimeter read?

TRY IT

Turn the subscale knob

The same 1,000 ft aerodrome on another example day, and this time QNH is hidden. Watch the dashed line: it is the level the altimeter counts up from. Find the setting that makes the altimeter read the aerodrome's elevation. That setting is QNH.

Sea levelAerodrome, 1,000 ft1013 hPa level850 ft
01234567891013hPaALTFEET
Altimeter reads
850 ft
Elevation
1,000 ft
QNH today
Not found yet

Subscale on 1013: the altimeter reads 850 ft, the aerodrome's pressure altitude today. The aerodrome is at 1,000 ft.

That is the definition of QNH at an aerodrome: the setting that makes the altimeter show the aerodrome's height above mean sea level.

That pressure setting which, when placed on the pressure setting sub-scale of a sensitive altimeter of an aircraft located at the reference point of an aerodrome, will cause the altimeter to indicate the vertical displacement of the reference point above mean sea level (AMSL).

CASA Visual Flight Rules Guide v8.3 (CC BY-NC 4.0), Glossary, QNH altimeter setting. Rules as at 31 October 2026

Flying from a high to a low

A pressure setting is only accurate near the station that reported it. Leave the departure setting in the window while you fly toward lower pressure, and the altimeter reads too high. You hold the same reading, but the pressure level you are following sinks, and you sink with it.

Altimeter setting is defined as station pressure reduced to sea level, but an altimeter setting is accurate only in the vicinity of the reporting station.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Setting the Altimeter

when the actual pressure is lower than what is set in the altimeter window, the actual altitude of the aircraft is lower than what is indicated on the altimeter.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Setting the Altimeter

The aircraft steadily decreases true altitude while the altimeter is held constant through pilot action as discussed in the previous section.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Altimeter Operation

PREDICT FIRST

You depart with QNH 1023 hPa set and hold 3,500 ft on the altimeter. Where you are now, QNH is 1003 hPa, and you have not reset the subscale. Roughly how high are you really?

TRY IT

Fly from a high to a low

An example route with illustrative pressures and hills. Fly it with the departure QNH set, then use “Set QNH for here” along the way. Then make the air colder. The model uses 30 ft per hPa and treats the air below you as one column, evenly warmer or colder than standard. It counts the temperature error from sea level, as if every QNH came from a station at sea level. PHAK's chart counts it from the airport.

Higher pressure, QNH 1023Lower, QNH 1003Altimeter shows 3,500 ft3,200 ft clearDeparture100 NM
01234567891023hPaALTFEET
QNH here
1023 hPa
Real altitude
3,500 ft
Terrain below
300 ft

Subscale matches the QNH here.

The altimeter and your real altitude agree here.

The same thing works the other way. Fly from low to high pressure without resetting and you are higher than the altimeter shows.

Conversely, if an aircraft is flown from a low pressure area to a high pressure area without an adjustment of the altimeter, the actual altitude of the aircraft is higher than the indicated altitude.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Effect of Nonstandard Pressure and Temperature

Either way, the fix is a current setting. Which setting, and up to what height, is up to your country's rules. Pick your country to see them.

Air colder than standard puts you lower too

The subscale corrects for pressure at the surface. It cannot correct for temperature. Cold air is denser, so pressure falls faster as you climb through it, and a given pressure level sits lower than in standard air. Warm air does the reverse.

Adjustments to compensate for nonstandard pressure do not compensate for nonstandard temperature. Since cold air is denser than warm air, when operating in temperatures that are colder than standard, the altitude is lower than the altimeter indication.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Effect of Nonstandard Pressure and Temperature

Since the total pressure decrease is the same in each column, the rate of decrease of pressure with height in warm air is less than standard, while the rate of decrease in pressure with height in cold air is greater than standard.

FAA Aviation Weather Handbook (FAA-H-8083-28B), chapter 8, 8.2.4.2 Temperature's Effects on Pressure

Set the subscale correctly in the simulation, then slide the temperature down. The altimeter does not change, but your real path drops toward the hills. PHAK puts a number on it: with a reported temperature of −10 °C, 1,000 ft above the airport you may be as much as 100 ft lower than the altimeter shows. At a sea-level airport, −10 °C is 25 °C colder than standard. The temperature slider in the simulation sets that difference.

Look at the chart using a temperature of –10 °C and an aircraft altitude of 1,000 feet above the airport elevation. The chart shows that the reported current altimeter setting may place the aircraft as much as 100 feet below the altitude indicated by the altimeter.

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Figure 8-4 caption

the sea level free air temperature is standard (+15 degrees Celsius (°C) or 59 degrees Fahrenheit (°F))

FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), chapter 8, Altimeter, Principle of Operation

Where you change to 1013.2 hPa

Below a set height you set QNH and fly altitudes. Higher up you set 1013.2 hPa and fly flight levels. Australia and New Zealand put the change in different places.

Which country do you fly in?

The rules differ. Pick yours to see them.

Lock it in.

5 Australian and 9 New Zealand practice questions on altimetry and cruising levels. The ones you miss come back sooner.

Practise altimetry and cruising levels (Australia)Practise altimetry and cruising levels (New Zealand)

SOURCES FOR THIS LESSON

Source-checked, awaiting pilot review. Learning only. Your aircraft's flight manual and your instructor come first. Sources and licences