TC AIM MET 8.2 and MET 8.4.3(d); AIR 1.5.2
Why Does Density Altitude Hurt Performance?
Density altitude is "the altitude in ISA at which the air density would be equal to the air density at field elevation at the current temperature" (TC AIM MET 8.4.3(d)). Heat, a high aerodrome and low pressure raise it, and humidity raises it further. The higher it is, the thinner the air, and the worse your aircraft performs.
What density altitude is
Your altimeter reads pressure. Your engine, propeller and wings respond to density: how much air there is to burn, to bite on and to lift with. Density altitude puts today's density in altitude terms: the height in the standard atmosphere where the air is as thin as the air you actually have.
The standard atmosphere it is measured against is fixed: 29.92 in Hg and 15°C at sea level, temperature falling 1.98°C per 1,000 ft, and air that is "a perfectly dry gas" (TC AIM AIR 1.5.2). When the air is thinner than standard, your aircraft behaves as if the aerodrome were higher than it is.
“Density altitude is the altitude in ISA at which the air density would be equal to the air density at field elevation at the current temperature.”
Hot, high, humid and low pressure: four pushes the same way
- Hot. Warmer air is less dense. Every degree above the standard temperature raises density altitude.
- High. A high aerodrome starts with thinner air.
- Low pressure. Density altitude is worked from the aerodrome's pressure altitude (TC AIM MET 8.4.3(d)), and the lower the altimeter setting, the higher the pressure altitude: about 1,000 ft for each inch of mercury below 29.92 (TC AIM AIR 1.5.3).
- Humid. Moist air is less dense than dry air at the same temperature and pressure. The standard atmosphere is dry, and the density altitude a METAR reports is the dry-air figure (TC AIM MET 8.2), so on a humid day the air is thinner still than the report says.
The pushes add up. A hot, humid, low-pressure afternoon at a high aerodrome is one thin-air day, and you plan for it as one.
TC AIM gives a rough estimate for the temperature part. Work the figure: set a hot day at a high aerodrome, then a cold one.
“A rough value of density altitude can be approximated by adding 118.8 ft to the aerodrome pressure altitude for every degree Celsius the temperature is above ISA.”
Thinner air than the elevation suggests: less power, less thrust, less lift. Plan with your aircraft's own performance data.
Where you will see it: the METAR
You do not always have to work it out. When density altitude is 200 ft or more above the aerodrome elevation, the METAR prints it in the remarks, after the sea level pressure: for example, "SLP134 DENSITY ALTITUDE 2500FT" (TC AIM MET 8.2). Automated stations (AWOS and LWIS) add it on the same basis, once the density altitude, rounded to the nearest 100 ft, is 200 ft or more above the elevation (TC AIM MET 8.4.3(d)).
No remark does not mean no effect. Below 200 ft above elevation it is not printed, and the reported figure is for dry air.
“Density altitude will be indicated after sea level pressure when the density altitude is 200 ft or more than the aerodrome elevation.”
What gets worse, and why it matters on take-off
In thin air, the engine makes less power, the propeller less thrust, and the wing less lift at a given speed; and your true airspeed is higher for the same indicated airspeed. So the take-off roll is longer, the climb is shallower, and the landing roll is longer. The same aircraft, at the same weight, on the same runway, does less on a thin-air day.
This page gives no take-off or climb numbers on purpose. Those belong to your aircraft's own performance data, worked from the pressure altitude and temperature of the day. A generic number is not your aircraft's limitation.
Where candidates lose marks
- Humidity raises density altitude. Humid air feels heavy to a person and is light to an engine.
- Low pressure raises it too. It raises the pressure altitude that density altitude starts from.
- A cold day lowers it. Density altitude can be below the aerodrome elevation; TC AIM says so, though a METAR does not report it.
- It is not an instrument reading. No instrument in the cockpit shows density altitude. You work it out, or read it in a METAR remark.
- Thin air means less of everything: power, thrust and lift, and a longer roll on take-off and landing.
Keep practising free
Studying for the commercial written, and you can recite "hot and high" but not explain it? Density altitude, and its effect with humidity, is on the PPL and CPL syllabus (TP 12880E and TP 12881E), and three steps cover it:
- Learn the four pushes, then work the figure above until you can predict which way each slider moves the answer.
- Answer the four questions below, then one a day with the free Question of the Day.
- Before your next flight on a hot day, ask your instructor to work one take-off calculation with you from your training aircraft's own performance data.
How much worse, in numbers, for your aircraft? Only its performance data can say.
- What the commercial written covers, and how it is marked: the CPL exam guide. For the private licence, the PPL exam guide.
- Which altitude do you cruise at? VFR cruising altitudes.
- Fuel planning is a separate calculation from performance planning: VFR fuel requirements.
Practice
Original questions in the style of the Transport Canada written exams. Tap an answer to see why it is right or wrong.
Question 1 of 4
All else equal, which day gives the highest density altitude at an aerodrome?
Tap an answer to check it.
Question 2 of 4
A METAR's remarks end: SLP134 DENSITY ALTITUDE 2500FT. What does this tell you?
Tap an answer to check it.
Question 3 of 4
An aerodrome's pressure altitude is 3,000 ft and the temperature is 9°C above ISA. Using TC AIM's rough estimate, what is the density altitude?
Tap an answer to check it.
Question 4 of 4
On a day of high density altitude, which change in take-off performance should you expect?
Tap an answer to check it.
Taught in
The chapter goes further than this page, and ends with a quiz.
Other topics
Checked 29 October 2026. TC AIM MET 8.2, MET 8.4.3(d), AIR 1.5.2 and AIR 1.5.3 checked against TC AIM, October 1, 2026. Density altitude and its effect with humidity are on the syllabus of TP 12880E and TP 12881E (revised April 2026). No take-off or climb figure is given: those come from an aircraft's own performance data. TC Exam Prep is independent and not affiliated with Transport Canada.