At first we thought this was a lighting detector (for photographers), but upon closer inspection we realized it was a lightning detector. You know, the type that tells you if there's lightning. The thing has a convenient belt clip and tells you whether lightning's 20-40 miles away, 12-24 miles away, 6-12 miles away, or less than 6 miles way.
We're not sure how well this works, but it's probably more useful than relying on your own eyesight to tell you whether Storm's coming. Yes, we're talking about the X-Men character. She's very angry.