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Clouds and Lingering
Cloud effects (aka: Lingering effects) apply 1/3 of their listed damage several times per turn in the squares they affect.
To be precise, they apply 1/3 of the listed damage to the squares they affect every 1280 ticks. Since a combat round (aka: turn) is 7500 ticks, a cloud can usually apply very slightly less than 6 times per turn. A cloud will trigger four times per point of Lingering listed on the spell, so each point of Lingering can also be thought of as making the cloud last on field for 68% of a round.
Notes on Interactions
Additional effects not part of the initial spell (aka: nextspells) are applied on a spell landing/being cast, lingering clouds will NOT reapply the nextspell every time they apply damage. Notably, this does effect Sulphur Haze and Freezing Mist: they only apply their additional fatigue damage on the initial cast.
A cloud "remembers" who cast it, so spells like Damage Reversal, blessings like Blood Vengeance, and items like The Protection of Geryon will work correctly against Lingering effects.
To mod a spell to have Lingering, add 1000(multiplied by the value of Lingering you want) to its #effect. For instance, to make a Lingering 6 capped damage spell you would have "#effect 6109", 109 for the capped damage effect and 6000 for Lingering 6. The Dominions 5 Modding Manual does not mention this.