Summon Large Fire Elemental (Sp): Once per day, an adult mithril dragon can cast summon nature's ally V to summon one Large fire elemental, 1d3 Medium fire elementals, or 1d4+1 air elementals of smaller size.
Climate/Terrain says any but from their love of thermals, I'd say they prefer semi-arid and coastal regions because that's where thermals and updrafts are more prevalent. Of course, it would suit them if there are mesas or tall seaside cliffs or where they can lair.
