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.
During the witch hunts the belief in witches who could raise storms was not limited to the Tempestarii. Depending on a witch's preference, they were believed to cause tempests, hailstorms, and lightning. Witches struck homes and crops alike, sank ships, killed men and animals, and it was believed they took great delight in the process. Church authorities gave credence to the belief by stating that God permitted the Devil and witches to perform these acts as punishment for the wickedness of the world.
Since ancient times around the world, the ability to control elements - including the raising of storms and causing rain - has been attributed to magicians, shamans, sorcerers, and witches. As early as 700 A.D., the Catholic Church prosecuted sorcerers for causing storms.
The most famous storm believed to be caused by witches was recorded in 1591 during the North Berwick Witch Trials. John Fian and his alleged coven of witches were accused of raising a sea storm to drown James VI and Queen Anne on their way from Denmark.
On the Tempestarii and Magonia see Valerie I. J. Flint, The Rise of Magic in Early Medieval Europe, Princeton Univ. Press, 1991, p. 112.
Remedies against tempestarii
The Catholic Church prohibited superstitious remedies against witchcraft such as storm raising because the remedies themselves were of pagan origin. Prayer, sacraments, and the invocation of the name of God were prescribed instead with the belief that a person who had strong faith in God, kept the commandments, and revered the rites of the Church would be immune from storms and tempests raised by malicious witches.
Because many peasants were reluctant to give up their supersitions as being false, the church also sanctioned remedies like the ringing of church bells, believed to drive storm devils away, and placing charms made from flowers consecrated on Palm Sunday in the crop fields. It was believed that if a storm did strike after the charm was placed, the owner's crops would be protected even if the surrounding land and crops were destroyed.