What does 'free air' mean?

[Deyar]

What does 'free air' mean?

[Sidle Jinks]

Free air is the one at 15.5 C and with atmosphere pressure corresponding to that at the sea level (if I haven't completely forgotten my Physics course at the University ).
There's another definition, Free air is the air at atmospheric conditions at any specific location. Because of variations in altitude, barometer, temperature, and humidity, this does not mean air under identical or standard conditions.

Just in case if you mean free air anomaly, it is the difference between observed gravity and theoretical gravity that has been computed for latitude and corrected for elevation of the station above or below the geoid, by application of the normal rate of change of gravity for change of elevation, as in free air.

And, finally, free air overpressure is the unreflected pressure, in excess of the ambient atmospheric pressure, created in the air by the blast wave from an explosion

Oh, and this is also a novel by Sinclair Lewis about Claire Boltwood, who, in the early days of the 20th century, travels by automobile from New York City to the Pacific Northwest, where she falls in love with a nice, down-to-earth young man and gives up her snobbish Estate.
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Factum non fabula