Airfield and weather
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Humidity and units
Adds the effect of humidity. Leave empty for dry air.
Density altitude
4,170 ft
The aircraft performs as if the field were 2,170 ft higher.
Behind the answer
- Pressure altitude
- 2,010 ft
- Density altitude (m)
- 1,271 m
- Rule of thumb (120 ft per °C)
- 4,280 ft
- ISA temperature at pressure altitude
- 11 °C
- Deviation from ISA
- ISA +19 °C
Uses the ICAO standard atmosphere. Take-off and climb figures come from the aircraft’s own performance charts.
What this means
- Thinner air than standard: expect a longer take-off roll and a slower climb.
- 5 °C warmer would raise the density altitude by about 540 ft.
- Without a dew point the air is taken as dry. Humid air is lighter and raises the density altitude a little more.
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Pressure and density altitude of your airfield from elevation, QNH and temperature, with what the thin air means for take-off.
How to use
- Enter the field elevation, the QNH from the METAR or ATIS, and the outside air temperature.
- Under Humidity and units, add the dew point if you know it, to include the effect of humidity.
- Read the density altitude and what it means for take-off. Pick other units there too if you need them.
Density altitude at a 2000 ft field
QNH 1013.25 hPa; dry air, with no dew point entered.
How it works
An aircraft’s wings, propeller and engine feel the density of the air, not the height on the map. Density altitude is the height in the standard atmosphere (ICAO Doc 7488) where the air is as dense as it is at your field right now. Hot, high and humid air is thin, so the aircraft behaves as if the runway were higher: it needs a longer take-off roll and climbs more slowly.
First the QNH gives the pressure altitude: what the altimeter would show on the standard setting of 1013.25 hPa (29.92 inHg). Then the air density ρ = p / (R · T), from that pressure p, the temperature T and the gas constant of air R, is found in the standard atmosphere. A dew point lowers the density a little more, through the virtual temperature (US National Weather Service method). Pilots’ rule of thumb from the FAA Pilot’s Handbook of Aeronautical Knowledge is close: add 120 ft for every °C above the standard temperature.
Example: a field at 2,000 ft with QNH 1013 hPa and 30 °C. The pressure altitude is about 2,000 ft, where the standard temperature is 11 °C, so the day is ISA +19 °C. The density altitude comes to about 4,170 ft (the rule of thumb gives 4,280 ft): the aircraft performs as if the field were more than 2,000 ft higher.
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Frequently asked questions
What is the difference between pressure altitude and density altitude?
Pressure altitude only corrects the elevation for the day’s air pressure: it is what the altimeter shows on 1013.25 hPa. Density altitude also corrects for temperature (and humidity), so it tells how the aircraft will perform.
Why does humidity raise the density altitude?
A water vapour molecule is lighter than the nitrogen and oxygen it replaces, so humid air is less dense than dry air at the same pressure and temperature. The effect is usually a few hundred feet on a hot, humid day.
Where do I get the QNH?
From the airport’s METAR or ATIS, or by setting the altimeter to the field elevation on the ground and reading the setting window.
Why is the rule of thumb different from the exact answer?
The 120 ft per °C rule is a straight-line approximation of the standard atmosphere. It is good to about a hundred feet on ordinary days and drifts further on very hot or high fields.
Can density altitude be below sea level?
Yes. Cold, high-pressure air can be denser than standard sea-level air. A negative result describes air density, not the physical elevation of the field.
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