Mammatus clouds are a relatively rare and dramatic variety. One advantage of living in Colorado is that I see them somewhat often, especially during our stormy springs and summers. This video by Mike Olbinski features a dramatic skyscape of mammatus clouds (here in Colorado, natch) at sunset.
Although they’re often associated with stormy weather, there’s no widely accepted theory as to how mammatus clouds form. Their lobe-like protrusions form from cold, sinking air, but this is about as far as theories agree. It’s even unclear what their relation to extreme weather may be since these short-lived cloud formations can appear around, before, or even after such weather. (Image and video credit: M. Olbinski)