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Density & pressure altitude
Work out pressure altitude and density altitude from elevation, QNH and temperature.
How it is worked out
Pressure altitude comes from the standard atmosphere: about 27–30 ft for every hPa (1,000 ft per inch) the pressure is below 1013.25 hPa (29.92 inHg). Density altitude then corrects for temperature, and humidity if you add a dew point, using the full ISA equations rather than a rule of thumb.
Why it matters
On a hot day at a high field, the aircraft performs as if it were thousands of feet higher. Always check take-off and climb performance in your POH at the density altitude.