Every time I select an example (can't get no dumber than that), I find almost immediately, through another rule making or rule interpretation by the FAA, that the quoted example is far obsolete (no longer near sufficient).  So, with that danger in mind...

There are about 200,000 airplanes in this country, total; give or take.  The only collision avoidance system we have is saturated at about 3,000.  That aside, the overwhelming vast majority of those have piston engines.

Consequently, the airplane has a service ceiling.  As you go up, the air gets thinner - less oxygen molecules per cubic meter.  Less oxygen with which to burn fuel, per cylinder volume, the less power the engine can generate.  If you "push more on the gas peddle", you just dump unburned fuel through the "tail pipe" (chemistry 101).  Power declines as you go up, maximum altitude above which you can't climb no more.  Service ceiling.  It's an air thing - it gets thinner as you go up.

The amount is extremely well known.  On the surface of the planet (mean sea level) the average pressure is 29.92" of (column of) mercury (pulled up by a vacuum).  Near the surface of the planet, the amount of change is by 1" of mercury per 1000' of altitude increase.  It is so known, that we measure altitude for airplanes by measuring "static" air pressure.  The altimeter dash board mechanical gauge, and the electrical sensor that reports altitude over the transponder, both measure inches of mercury.

How dumb are they?  The FAA figured that, since there is a service ceiling for an airplane, and it is codified, there should also be a service floor, and it should be codified.  You are not allowed to legally fly if the air gets denser than, and they picked an amount: 91.144 [Amdt. 91-240, 59 FR 17452, Apr. 12, 1994, and 59 FR 37669, July 25, 1994]. Click on and read the green box  Enough said?