How exactly do cylinder spells and effects "ignore obstructions" in their areas? Or what kinds of obstructions they can realistically ignore?

Question

So, Spell descriptions on Area entry says:

When casting a cylinder-shaped spell, you select the spell’s point of origin. This point is the center of a horizontal circle, and the spell shoots down from the circle, filling a cylinder. A cylinder-shaped spell ignores any obstructions within its area.

I feel like "any" is a bit to strong claim for how it does actually work.

Could someone elaborate step-by-step, how do you actually target such a spell when there are:

• a low wall in a spell's area;
• a wall, higher than the height of cylinder's point of origin, selected by a caster;
• ceiling above one of desireable "targets", but lower then a mentioned point of origin?

Should all of them really be ignored?

Answer 1

I think this is a poor description, because it is trying to describe a method of drawing a particular volume (by drawing a horizontal two-dimensional shape on the \$XY\$ plane and projecting it downward on the \$Z\$ axis), but then also using that as the description of what the spell itself is doing. The reason that’s a problem is because the combination of two factors here undercut any concept of the spell being “projected” at all. The first issue is that spells come into effect instantaneously; there is no time at which an effect is just the circle and then it travels downward. The other issue is the last quoted line, “A cylinder-shaped spell ignores any obstructions within its area,” which says we have to ignore anything blocking the projection on its way downward.

The sum of those two effects is that the spell isn’t “projected” at all, but simply comes into existence at all points within the volume simultaneously, without regard for what else is in that volume or where any point within is in relation to any other point in it. This is a valid way to imagine magic happening, of course, and probably appropriate for many spells. Moreover, even when it isn’t precisely appropriate, the top-down projection is very specific and definitely isn’t going to apply in a lot of cases, so it makes sense to avoid implementing that as a global default. All of which they could have easily just said, and relied on the “projection” description to talk solely about how you should draw the shape/know you have the right shape, but they didn’t do that. Instead, they talk about the “spell” doing that, and then need to apply the “hack” of having it ignore obstructions.

Anyway, yes, the long and short of it is, the official rule is that, by default, cylinder-shaped spells simply fill the entire volume without regard for what’s in it, or where any point is in relation to another or the existence of anything between them. Which is kind of nonsensical for some spells, but appropriate for others, so we should look to individual spell descriptions creating exceptions for those spells that actually want effects to come down from above. (Of course, I imagine, a great many authors of individual spells didn’t double-check the default rules and think through these concerns, and probably many spells that should have such an exception lack it—that’s where the DM comes in.)

Answer 2

The wording "A cylinder-shaped spell ignores any obstructions within its area" really is obscure - especially if we take into account that there has to be a line of effect from the spell's origin (which is the cylinder's top circle) to any square within the spell's area in order to affect them.

A burst, cone, cylinder, or emanation spell affects only an area, creatures, or objects to which it has line of effect from its origin (a spherical burst’s center point, a cone-shaped burst’s starting point, a cylinder’s circle, or an emanation’s point of origin).

A line of effect is a straight, unblocked path that indicates what an ability, effect, or spell can affect. A solid barrier cancels line of effect.

So the rules demand "a straight unblocked path" and at the same time say we should "ignore any obstructions".

A way to deal with this discrepancy would be to rule that cylindrical areas work similar to spreads which Skip Williams suggests in his rules of the game article on Reading Spell Descriptions (5).

Cylinder: A cylindrical area is something like a spread. The area extends from the point of origin in a horizontal circle, and then extends downward. The rules say a cylinder ignores obstructions in its area, which isn't too helpful. In effect, this means that line of effect for a cylinder isn't blocked so long as you can trace an unbroken line from the point of origin without leaving the confines of the cylinder. (You still need line of effect from you to spell's point of origin, however.)