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).
  When you cast the spell, choose a point you can see under the cloud. A bolt of lightning flashes down from the cloud to that point. Each creature within 5 feet of that point must make a Dexterity saving throw. A creature takes 3d10 lightning damage on a failed save, or half as much damage on a successful one. On each of your turns until the spell ends, you can use your action to call down lightning in this way again, targeting the same point or a different one.
  If you are outdoors in stormy conditions when you cast this spell, the spell gives you control over the existing storm instead of creating a new one. Under such conditions, the spell’s damage increases by 1d10.