Drops of highly corrosive acid pours down from the sky in a 30 ft. radius, 150 ft. high cylinder centered on a point within range.  Each creature within the cylinder must make a Dexterity Saving Throw.  On a failed save, a creature takes (6d6) acid damage and another (4d4) acid damage at the start of its next turn.  On a failed save, a creature takes half as much the initial damage and no damage at the end of its next turn.  A creature that takes the later damage also takes (4d4) acid damage at the end of its next turn, unless it uses an action to wash the acid off of itself.  The creature can't wash off the acid if it remains in the area of effect.  

For each consecutive turn that a creature takes damage from this spell, the later damage a creature takes increases by 1d4.  When a creature ends its turn without taking any damage, the damage resets to 4d4.  

As a bonus action, you can move the acid rain cloud up to 60 feet.  If you move the acid rain cloud over a creature, so that creature is now in the cylinder's area of effect, that creature must make a Dexterity Saving Throw against the spell's damage, and the cylinder stops moving this turn. 

Acid droplets falling from the sky lightly obscure the area. 

If the spell goes for its full duration and remaining in the same space the entire time, a pool of acid is created.  The pool of acid is 2 feet deep, and creatures moving through it must spend 3 ft. of movement for every 1 ft. it travels.  A creature that enters the area for the first time, starts its turn there, or ends its turn there, takes 4d6 acid damage.     

At Higher Levels.  When you cast this spell using a 6th level spell slot or higher, the spell's initial damage increases by 1d6 for every spell slot level above 5th.  The spell's later damage increases by 2d4 for every spell slot level above 5th.  The spell's radius also increases by 10 ft. for each spell slot level above 5th.