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Re: Spark length dependence on air pressure.



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Jim,

If you get your hands on a pilots slide rule, you can easily calculate "density" altitude using temp and barametric pressure (temperature corrected to sea level - what the weather man or your local airport reports) for your elevation. Density altitude would be the equivalent altitude that has the same density at standard temp and pressure as your site at non standard temp and pressure.

Gerry R.


Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

His 7.7 km number may be more accurate than my 7 km number.  Would need
to check.
Another complication is that pressure also depends on temperature, which
varies with height also, and so the pressure
falloff with height is slightly different than for density.  But density
is the more fundamental parameter for breakdown, because it controls the
mean free path of the electrons, and it is over these mean free paths
that they gain enough energy to ionize.

Excellent point. I did use pressure to get my 7.7 km, and he's right, density is the controlling thing. Assumed lapse rate (temperature profile as you change in altitude) also affects it, because you're basically integrating. I just used ICAO standard atmosphere.