A “bellwether” is an indication of what is to come (“Are rising home prices a bellwether for the economy?”) or a leader for others to follow (“Infosys is no longer a bellwether for the IT sector.”). Sometimes it’s spelled “bellweather,” perhaps because, as Garner’s Modern American Usage says, “like a weathervane, it shows which way the wind blows.”
But “bellwether” has nothing to do with weather, regardless of how many people misspell it. Though it does have to do with bells.
A “wether” is a castrated sheep or goat, around whose neck a bell would be tied. Said animal would then lead its flock to or from pasture. Since it was no longer, um, intact, it could be trusted to not “mess around” along the way.
A sci-fi book, Bellwether, uses the word as a verb to mean, among other things, to check classics out of libraries to make it look as if they’re in demand, thus saving them from the trash.
In the US, a “bellwether stock” is considered a leading indicator of a market segment. (In Britain, a “bellwether stock” is called a “barometer stock,” thus clouding the weather issue even more.) But bellwether also retains a woolly scent in some dictionary definitions of it as “a leader, esp. of a sheeplike crowd.”
In this case, be a sheep and follow the crowd that spells it “bellwether.”