Projectile modifier spells that add an arc of material between multiple projectiles. Arc spells require a minimum of two projectiles in order to function
Fire Arc is a modifer that causes flaming particles to flow between fired projectiles. Unless the spell naturally produces more than one projectile per cast, like Triplicate Bolt, it must be used with a mutlicast modifer to work. Creatures hit by the particles will be set on fire.
Fire Arc is, funnily enough, the "safest" of the Arc-series to use due to the counteracting effects of being soaked in water,