This is a myth I made up, not one I have heard before.
Once there was a powerful witch that roamed the earth named Storm. She was the most powerful witch on the planet, but she could not seem to be satisfied with her powers. All of the other inhabitants of the earth, from the lowly peasant to the most powerful of the Gods, feared her and revered her at the same time.
One day, while starving for more power, Storm came up with a spell that would make her more powerful. She would steal the power from the God of the weather, Sunshine.
She waited until the perfect moment to get Sunshine alone, then she cast her spell. The spell, however, did not go as planned. It killed Sunshine. It gave Storm the power to control the weather, as she predicted, but it also gave her Godlike powers that her mortal body could not handle.
Within a few minutes, she began to have power surges. Her body shook violently and electrical charges shot forth from her fingers. She was out of control. Every time she opened her mouth to scream in pain a loud rumble escaped her lips.
The other Gods were alerted and came to investigate. After seeing the death and destruction Storm had caused, they decided to banish her from the Earth into the atmosphere. There she can still control the weather, but not be as harmful to others.
Now with each time there is to be a change in the weather, Lightning shoots from her fingers, Thunder escapes her mouth and the Storm begins.