Found in medieval lore, the Tempestarii were thought to be witches whose main act of magic concerned raising storms for the mythical inhabitants of Magonia, a land in the sky.
The big storm clouds that came over the land where thought of as the ships of the Magonians. The Tempestarii would help their passage by whipping up wind, thunder, and lightning.
The Magonians were not particularly nice - they would dump their cargo overboard (thus creating hail) to ruin the crops, which they would then steal with the aid of the Tempestarii. They would also frequently leave without paying the witches, who would give chase. This was seen by the wispy clouds that sometimes follow a storm.
People in medieval times used the Tempestarii to understand the weather, much like people now will explain storms to scared children by saying that 'angels are crying' or 'giants are playing'.