Can a cylinder-shaped spell be cast on air?

Question

The description of a cylindrical area of effect is

A cylinder’s point of origin is the center of a circle of a particular radius, as given in the spell description. The circle must either be on the ground or at the height of the spell effect.

This seems to indicate that the cylinder is always touching the ground, and is consistent with some chat comments I've had. So, for example, consider you have a 50ft high cylindrical spell. You either cast the point of origin on the ground and it grows 50ft up, or cast it 50ft up and it descends 50ft.

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That being said... What if you're on top of something?

Is the height at which you cast the spell based on the height at which you are when you cast it (and so, if you're on top of an expertly drawn tower, can you hit enemies above you, but way above the spell' range from the ground) or is the height always in relation to the ground (meaning it doesn't matter where you are, but only where you cast it)?

Answer 1

What exactly constitutes the ground is left up to the GM. Everyone has a sense of what would constitue "the ground" at a given position. A GM ruling based on that will most likely satisfy anyone if made in advance (so no "your casting fails as that is not the ground"). I have never had problems with rulings like this, most players do not even question these as it is within their "sense of common" too. Even if a player contests it, they also have nothing to base that on and you trump them by being the GM.

The following is how I would rule:

• Being "at the height of the spell effect" is relative to "the ground".
• Casting is dependent on the ground at/under the target, not on the position of the caster.
• Something can only be "the ground" if a small or larger creature can stand on it for some time without balancing. So no throwing rocks up in the air and casting on them. Also no casting based on any tree you can climb with branches to stand on (unless it is the "World Tree" or something else on that level).

So in the masterfully illustrated example you cannot include the pterodactyl in the AoE while it flies around the tower at 75 feet. The ground is at the base of the tower in those positions. If he moves over the tower (to attack you, for example) you can include him in the cylinder as the tower is a stable platform you are able to stand on and thus is the ground there.

Answer 2

No, this would not work as illustrated.

The spell effect has a height, specified in the description. The purpose of specifying the placing of the circle at the "height of the spell" effect is likely intended to circumvent cover.

In other words, if you wanted to affect a party of orcs hiding behind a wall, you could start your cylinder at the top of the effect (since you have a clear path to it), and extend it downward to the ground.