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Altitude – Pressure
See ‘Pressure Altitude’.
Barometric Pressure - Absolute
See ‘Absolute Barometric Pressure’.
Barometric Pressure - Corrected to Sea Level
In order to provide a standard method of stating barometric pressure, the National
Weather Service (NWS) adjusts the absolute barometric pressure observations to sea
level elevation (0 feet). Without this standard the barometer readings would be
meaningless at different geographic elevations around the world. The correction adds
about 0.001 Inches Hg per foot of elevation to the absolute barometric pressure number.
Corrected barometric pressure cannot be used for performance calculations without
subtracting the local elevation from the NWS value. Even then it would be an
approximation because the barometer is at another location.
Barometric Pressure - Dry
See ‘Dry Barometric Pressure’.
Correction Factor
The correction factor is a Society of Automotive Engineers (SAE) equation that uses the
calculated dry barometric pressure and the measured local temperature to compensate
for changes in dynamometer observed horsepower readings due to air density differences.
This method allows performance comparison of observed readings taken on different days
or in different parts of the world. The correction factor can also be applied to track
performance. There are 2 correction factor equations in common use in the industry - the
current SAE and STD or the old SAE standard. The STD and SAE methods also have
different reference points (where correction factor 1.000 occurs). Therefore, values
corrected with one method cannot be compared to values corrected by the other.
Density Altitude
Density altitude is a value in feet that relates the local measured atmospheric conditions
(absolute barometric pressure or pressure altitude, temperature, and relative humidity) to
the aeronautical industry defined standard day atmosphere. The density altitude for an
absolute barometric pressure of 29.92 Inches Hg or a pressure altitude of 0 ft, a
temperature of 60F, and a humidity of 0% is 0 ft. A good air day in the spring or fall might
be below 0 ft (i.e. –831 ft). Conversely, a hot summer day might have a density altitude of
4500 ft. (equivalent to 4500 ft of altitude on a standard day).
Dew Point
The dew point temperature is the calculated temperature at which the measured partial
saturation of water vapor in the air will turn to liquid and form dew. Moisture can cause
inconsistent traction and, in certain circumstances, effect engine performance. The
warmer the air is the more water vapor it can hold. See ‘Relative Humidity’ for more
information. An air sample has a specific amount of water vapor present. As the
temperature drops and reaches the dew point, the air can no longer hold the water vapor
suspended so it condenses and turns to water. This condensation will occur on cooler
surfaces first. Computech research with methanol fuel, and to a lesser extent with
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