How do you tell what is high air, low air, and medium air wihout testing all the different types of terrain? Is there just some sort of formula to tell?
High ground == Low air
Medium ground == Medium air
Low ground == High air

Just assume that the plane is always the same height from "sea" level. Then when it flies over some high terrain, it's lower to the ground.
So what about jungle where there is sea level, dirt, temple, and high temple?
Sea level and dirt are low air, temple is mid air, and high temple is high air.
Legacy is wrong. I've tested the air detection in a map I made. High air = high ground, and low air = low ground
Another thing - you can see air units fly higher and lower as they ascend and descend in terrain level. It's most noticeable with overlords.
But suppose that on badlands you have structure, dirt and water. Does dirt become medium, or does it stay low?
I think badlands has just two levels. medium and high i would assume.