What is density altitude?
Density altitude is pressure altitude corrected for non-standard temperature and, in more complete calculations, humidity. It represents the altitude in the standard atmosphere where the air would have the same density as the current conditions.
A high density altitude means the airplane behaves as though it is operating at a higher altitude than the airport elevation suggests. The result can be longer takeoff distance, reduced propeller and engine performance, and lower climb capability.
Density-altitude formula
The common pilot rule of thumb starts with pressure altitude and compares the actual outside-air temperature with the International Standard Atmosphere temperature at that altitude:
All temperatures in this approximation are in degrees Celsius. ISA temperature decreases by roughly 2 °C per 1,000 ft from 15 °C at sea level.
Pressure altitude versus density altitude
| Term | What it describes | Main inputs | Why pilots use it |
|---|---|---|---|
| Field elevation | Physical airport elevation above or below mean sea level | Airport data | Starting reference for atmospheric calculations |
| Pressure altitude | Altitude in the standard atmosphere corresponding to current pressure | Field elevation and altimeter setting/QNH | Performance-chart and density-altitude input |
| Density altitude | Standard-atmosphere altitude corresponding to current air density | Pressure altitude, temperature, and sometimes humidity | Performance awareness in hot, high, humid, or low-pressure conditions |
Worked METAR example
This representative example is for education, not current flight planning:
- Decode the METAR. The relevant atmospheric inputs are temperature 29 °C, dew point 17 °C, and altimeter 29.92 inHg.
- Use airport elevation. With standard pressure, pressure altitude is approximately the 177 ft field elevation.
- Estimate density altitude. ClimbCheck’s Precise model produces approximately 2,070 ft; the temperature-only rule of thumb produces approximately 1,900 ft.
- Interpret the result. Although the runway sits near sea level, the aircraft experiences air density resembling a substantially higher standard-atmosphere altitude.
For a calculation focused specifically on each METAR field, see Calculate Density Altitude from a METAR.
Same airport: cool morning versus hot afternoon
The following rule-of-thumb training comparison holds pressure altitude at 3,000 ft and changes only temperature. The 1,200 ft baseline and takeoff adjustments are generalized ClimbCheck examples, not aircraft-specific POH values.
| Condition | OAT | Approx. density altitude | Generalized TO increase | Adjusted 1,200 ft baseline |
|---|---|---|---|---|
| Cool morning | 10 °C | ≈ 3,110 ft | ≈ +47% | ≈ 1,760 ft |
| Hot afternoon | 35 °C | ≈ 6,110 ft | ≈ +92% | ≈ 2,300 ft |
How density altitude affects aircraft performance
- Wing: more true airspeed is normally required to produce the same indicated lift condition.
- Propeller: thinner air reduces the mass of air the propeller accelerates.
- Normally aspirated engine: less oxygen enters the cylinders, reducing available power unless compensated by the engine system.
- Takeoff: acceleration and liftoff can require more runway.
- Climb: excess power and climb rate can be reduced after liftoff.
Read the dedicated explanation: How Density Altitude Affects Takeoff Distance.
Does humidity matter?
Yes. Water vapor is less dense than the dry-air molecules it replaces, so warm moist air is slightly less dense than warm dry air at the same pressure and temperature. Temperature and pressure usually dominate the calculation, but humidity can still raise density altitude enough to matter in marginal conditions.
FAA reference material
These references explain the concept and chart-based estimation. The aircraft AFM/POH remains the primary source for aircraft-specific performance.
Density-altitude questions
Can density altitude be high at a sea-level airport?
Yes. Hot temperatures, low pressure, and humidity can create a meaningful density-altitude penalty even when field elevation is near sea level.
Can I calculate without fetching a METAR?
Yes. Enter all weather and airport values manually. A METAR is only a convenient source of observations.
Does a density-altitude number tell me whether takeoff is safe?
No. It is one input. The decision also depends on aircraft-specific performance, weight, configuration, runway surface and slope, wind, obstacles, aircraft condition, pilot technique, and suitable safety margins.