According to Dictionary.com, this is the word that means “a person or thing that shows the existence or direction of a trend”.
But it’s nothing to do with the way the wind is blowing. So it’s nothing to do with the weather.
“Bellwether”: a sheep (wether: a castrated ram) with a bell around its neck that leads the flock and allows you to find it in the dark or mist.
“Bellweather”: the word that should describe ideal conditions for campanology. If it existed…