Wikipedia
In medieval lore, Tempestarii were magicians, specifically weather-makers, dwelling amongst the common people, who possessed the power to raise or prevent storms at will. For this reason, anyone reputed as a weather-maker was the subject of respect, fear, and hatred in rural areas. One bishop, Agobard of Lyons, writing in 815 on the subject of the irreligious beliefs of his flock, complained that villagers resented paying tithes to the church, but freely paid a form of insurance against storms to village tempestarii; but, it was also noted, whenever a supposed weather-maker failed to prevent a storm, he or she would generally suffer the wrath of the populace, being victimised or killed.
Agobard of Lyons also referenced a related belief amongst his parishioners--a belief that tempestarii were in league with a mythical race of cloud-dwellers who came from a land named ' Magonia' ("Land of Magic", "Land of Thieves"). The Magonians were supposed to sail the skies in storm clouds; they would then pay Frankish tempestarii to summon up storms over farmlands, during which the Magonians could swoop down and steal the corn from the fields. On the particular occasion which prompted Agobard to write, several supposed Magonians had been taken prisoner by irate villagers shortly after a bad storm; the Bishop had been forced to intervene and debate with the villagers in order to save the prisoners' lives.