A pedantic correction not for its own sake -- Patrick's meaning wasn't in any doubt -- but because I think it's quite interesting.

Not "bellweather" but "bellwether". A wether is a male sheep. A bellwether is a male sheep at the head of a flock, around whose neck a bell is hung so that you can hear where the flock is even when you can't see it. (The rest of the flock will follow its leader like, um, sheep.)

So a bellwether is a single thing whose behaviour is a leading indicator of that of a larger-scale bunch of other related things.