Remember ST is a TV show and it has to be produced to a budget, as for it always being sunny, isn't that more a case of were the show was filmed.
Was there even one episode of TOS where it rained--on Earth or an alien world?
Not that I can think of, off hand. Los Angeles is generally a dry region.
I just thought of one---sort of--the street is clearly wet in the final exterior scene of All Our Yesterdays as Kirk goes back to the portal access.
It was filmed on backlot and in real world it could have been a case of studio people just hosing the location off.
But in the context of the episode it had to be evidence of it having rained.
McCoy mentions 'rainy day' in 'Devil in..'and sandoval mentions, 'moderate rainfall' in This side..'
that's as close as we get I suppose.