Before commencing an analysis of the subject matter at hand, it will be as well to present some definitions which we have adopted, in our own work, even if they represent in part arbitrary interpretations of our own.

(a) Pressure altitude is, of course, nothing but barometer pressure at any given elevation, converted into equivalent altitude (feet) units; hence a variable.

(b) Sea-level in connection with altimetry is only a theoretical reference value which does not necessarily conform with mean-high water; in terms of pressure altitude, needless to say, it is also a variable.

(c) True altitude (above sea-level) cannot be obtained with satisfactory accuracy by merely correcting for sea-level barometer pressure. On the other hand, we must dispel the notion—not uncommon—that one can obtain more accurate altitude values by correcting for temperature and humidity at a particular flight level. There are such things as inversions and convection currents; we do not need cold weather to have a high barometer, and sometimes it is warmer a few thousand feet up than at ground-level.

(d) Performance of pressure instruments is properly expressed in parts of the pressure range—not in per cent of altitude—and the reason is simple: at, say, 60,000 ft. you have about 16 times as may feet per unit of pressure as at sea-level.

(e) Sensitivity is—technically speaking—the smallest part of the pressure range that will produce a measurable response.