Calebais Qustions

Here my take on designing a spell that would fit the bill

Rego Auram, duration: ring, target: structure, +1 or +2 size
Prevents rain and fog from passing into the covenant (I've no guidelines here to use in order to calculate a level)

I have some reservations regarding the use of target circle for this spell as target circle effects things that are within the circle when the spell is cast, so rain that shows up an hour or two later may not be affected. You could argue that the ring itself is the targetor you could draw a second ring inside the first but I'd rather use target structure and avoid all of the controversy.

You'd need to put a big ring around the covenant and make a bevy of concentration checks to cast the spell, then you'd have to keep your ring intact to avoid water damage and the pain of recasting the spell. All sorts of story potential there.

I love the moment in a game when someone sees a magical effect and is boggled by the amount of work that went into it. In this case to do the same thing a tarp would do. You gotta love wizards!

And then comes the "Hey we could do something like that!"

Playing devil's advocate, the spell could, in theory, be done with a single cerimonial casting not requiring labwork at all (with the caveot that the ring would need to be made somewhat durable or the effort would be in vain).

For comparison think about the effort that might go into making a tarp big enough to cover Calebais using 13th century technology, and how effective that tarp would be in the long run.