In reference, and expansion on, This Question How does one tell where that point in the air is. Meaning If I target a place above a wall, and there just so happens to be a Mountain behind it, How do I pick the point I want to hit instead of that piece of the mountain that is behind the invisible air. How would I handle this?
To cast fireball you pick a point. How you describe this point to your DM is up to you (“half way between me and the tower, 20m I the air”), the important thing is that you know where the point is. Your character knows where the point is too. When the fireball arrives at that point it explodes.