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The description of the Call Lightning spell says:

A storm cloud appears in the shape of a cylinder that is 10 feet tall with a 60-foot radius, centered on a point you can see within range directly above you. The spell fails if you can't see a point in the air where the storm cloud could appear (for example, if you are in a room that can't accommodate the cloud).

The range of the spell is 120 feet.

Other description says a point directly 100 feet above the caster.

Does that mean within 100 feet with room for the 10-foot cloud, or that I need a 100-foot-tall room?

At 11th level, going through the Waterdeep: Dungeon of the Mad Mage adventure, I haven't yet had combat in a room that is 100 feet tall.

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The minimum required room size in practice is 120 feet across and about 15 feet tall

If you want to cast Call Lightning indoors, we need to figure out how big the room needs to be to accommodate the cloud. Here is the relevant spell text as of the latest erratum:

A storm cloud appears in the shape of a cylinder that is 10 feet tall with a 60-foot radius, centered on a point you can see within range directly above you. The spell fails if you can't see a point in the air where the storm cloud could appear (for example, if you are in a room that can't accommodate the cloud).

So, in order to fit the cloud inside, the room needs to be 120 feet across and 10 feet tall. However, the cloud needs to be "above you", and since you are about 5 feet tall, the total height needs to be about 15 feet in order to fit 10 feet worth of cloud above you.1 Note that the room must be 120 feet wide in all directions. A narrow hallway 120 feet long will not suffice. Additionally note that if the room is exactly 120 feet wide, when you must stand in the exact center of the room in order to successfully cast the spell, since the center of the cloud forms "directly above you", and the cloud must be exactly centered in the room in order to fit.

In more general terms, you can only cast Call Lightning indoors if you have at least 10 feet of headroom above you and at least 60 feet between you and the nearest wall. Obviously, you are very rarely going to encounter a sufficiently large room to cast the spell, making it an outdoor-only spell about 99% of the time.

1Technically you can cast the spell in a room that is only 10 feet tall, in which case the cloud will extend all the way from the ceiling to the floor. However, in this case you may have difficulty targeting your lightning strikes, since you can only target a point you can see. The spell doesn't say whether the cloud obscures vision or not, but the DM could well decide that the cloud works similarly to a Fog Cloud spell and causes heavy obscurement within its area. If you don't want to leave this up to the DM's interpretation, you'll need to leave enough space for yourself under the cloud.

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  • \$\begingroup\$ Specifically, the cloud is a cylinder centred directly above you; as per the rules for point of origin of a cylinder, the cylinder can fill down or fill up from that centre point. You could make the point of origin on a ten-foot high ceiling above yourself and be included in the cloud that results, so the spell is strictly usable in a 10-foot high room. \$\endgroup\$
    – Carcer
    Commented Jan 25, 2020 at 18:28
  • \$\begingroup\$ @Carcer Oh, that's a good point. I'll fix that. \$\endgroup\$
    – Ryan C. Thompson
    Commented Jan 25, 2020 at 18:32
  • \$\begingroup\$ @Carcer Actually, while you could cast the spell in a 10-foot tall room, you wouldn't be able to target anyone with lightning, since targets must be under the cloud. So I'd say 15 feet is the minimum to cast the spell and have it be useful. Edit: actually, you don't target a creature, you target a point, so you could still target points on the ground. But you need to be able to see the point you target. But it doesn't say whether the cloud obscures vision. So... I need to think about this some more. \$\endgroup\$
    – Ryan C. Thompson
    Commented Jan 25, 2020 at 18:36
  • \$\begingroup\$ You don't actually target people with it, though, you target a point on the ground beneath the cloud and then things within 10 feet of that point get hit! \$\endgroup\$
    – Carcer
    Commented Jan 25, 2020 at 18:42
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    \$\begingroup\$ @Carcer I've added a footnote explaining that 15 feet is not absolutely required but is highly recommended for practical use. \$\endgroup\$
    – Ryan C. Thompson
    Commented Jan 25, 2020 at 18:48

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