I am writing a science fiction story that would greatly benefit from a plot device that causes a large localized darkness effect. I'm aiming to get at least 3 days of darkness that covers a significant land-area. Think the size of a large state, or small country. This effect can't be caused by intelligent life. Something rather natural is required. I was hoping to have it be an extremely irregular event, if it had to happen more than once. Things I've considered (but may not work):
- A multi-day solar eclipse
- Extremely dense cloud-cover/weather phenomenon
- Insect swarm
I would like to keep the side-effects down, however. Most insect-related effects would gum up the works and preoccupy the characters. What ways can I create such a large-scale, long-lasting darkness effect on an alien planet?