People bet on anything and everything, and the less controllable the better, afaict. "I'll bet you $5 that snail will eat the cucumber before the broccoli" - > "fuck yes, make it $20" is 100% a thing I could see happening. Weather is significantly easier to do every day though, unless you know a snail farmer with a twitch stream (if so, hook me up!).

There are legitimate reasons to bet on the weather. If you're a farmer, betting on drought can be a hedge against a bad year.