I just wanted to share an idea since I'm a latecomer. Along the lines of the snowglobe, instead make the artifact an entire room or fortress. Within the artifact is a scale model of the entire world, which is magically able to generate the necessary detail to control the weather in a specific region. As a little bonus, it would probably work as a sort of Scrying tool as well. In order to activate the artifact, however, a specific item must be used to control the weather. Perhaps a garment, a ring, or a glove must be worn by the user. I'm imagining things like the controller's breath as icy wind, snapping fingers as lightning strikes, and a simple gaze causes freezing temperature. Something cinematic *grin*.
In this case, the heroes could simply find the ring that activates the artifact, or actually find the artifact itself and prevent the villain from controlling it. Just a little twist, at any rate.