Its a great word, and unfortunately one of the most often misspelled. It has nothing to do with meteorology. A wether is a neutered ram, chosen to lead the flock, because, being neutered, he's less likely to go far afield in search of ewes. Tie a bell around his neck, and the others follow him. The bellwether is, thus, a leading indicator, predictive of the direction in which the flock will go.
Yes, it is a great word. Not overused but almost always misspelled. Thanks for the etymology!
Thank you. You learn something every day.