For the reference, I measured the levels with my basic dosimeter on one flight. Measurements topped at 72 uSv/h. Common reading during the flight was in the 50-60 uSv/h range.
What I found very interesting was that the levels were slightly lower in the air near the ground than on the ground where I landed.
This is the correct answer. There're tons of microparticles of radioactive monzonite (and other radioactive minerals) in the dirt. And if you're near a granite body, you will show even higher levels of radiation.
What I found very interesting was that the levels were slightly lower in the air near the ground than on the ground where I landed.