In medieval lore, Tempestarii (or Tempestarius (singular)) were weather-making magicians who dwelt among the common people and possessed the power to raise or prevent storms at will.
A closer examination of Agobard's writing shows that he actually argues against the existence of weather-witches, but acquiesces that the saints of God are able to cause these things by praying with faith.
Some of them have the power to close up the heavens, lest it rain on days when they are prophesying, like Elijah; and to change water into blood and torment the earth with every plague as many times as they wish, as Moses and Aaron did to Egypt.
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.
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.