Have you ever gone on vacation on one of those gloriously clear topaz days where all the colors are brighter and vibrating with life? Have you ever gone on vacation only to have the temperatures plummet, the wind blow clouds and misery in?
You probably felt a certain way about each of those experiences. And weather in books is a tool I can exploit to bring those feelings back to you while you read.
I can either juxtapose lovely weather against a character's misery, or I can have it mirror their misery or joy or sorrow or rage. Weather offers an easy-to-deploy threat, yet another obstacle to be overcome if I really need to test how well my characters have learned a lesson, or how badly they want a goal.
Because I'm writing mostly science fiction, environmental conditions take on outsized importance. Ships in space live and die by environmental factors. Weather provides great color for alien worlds. It's also a great plot point. It shows up in every single book I write. In one book, a key scene hinges on how sweltering a noxious swamp is. Another book describes a planet having its atmosphere being slowly blasted away by an expanding sun. In the current WIP, the weather is a ticking clock. Beat that clock and live. Fail and die.
Even in the urban fantasies, weather gets mention. It's such a constant part of our lives. Weather determines our survival as individuals and as a species. Without some kindly weather, we'd starve. So while it might be something we don't always consciously think about, we all experience it. Day in. Day out. It's familiar. It helps me take you someplace you've never been, but still anchor you with an experience you can relate to.
Besides. If I didn't use weather in my books, how else could I get my emo on?