Originally Posted by Jax11
But just as a starter for you, discs tend to behave more understable when thrown down hill, and they tend to behave more overstable when trying to throw up hill. I am not aerodynamically intelligent and likely can't properly explain why this happens, but this is how it works with elevation.
Assuming that you throw level to the surface you are fighting gravity when throwing uphill, and gravity is helping your disc gain speed down hill. Same principal as rolling a ball.