We don't think about the weather much usually, it just is, unless it's doing something particularly violent. In fact we don't think about it to the extent that we may not put any of it in a book we're writing. How many times have you read a book and not even noted the weather?
Now you say, it's not important really, and in the end, literally, that's probably true, but it can add a very powerful element to your writing, provide sensory texture and contribute to the mood you're creating in a scene. With a bit of practice and a light touch you, the writer, can foreshadow coming events or enhance a character's emotional response.
Weather can be a powerful backdrop to the action in the novel you're writing. But, that said, caution must be used. Weather can exert a pressure on your character that is otherwise invisible. It can be used to project symbolism into a scene, and it can add conflict. It can be a natural barrier to your character moving forward or act as an excuse, a delay, an obstacle.
BUT, and you'll notice that was a big but, the weather shouldn't become a 'central character' in your book unless it is a very unusual book indeed. You don't want the weather to overtly tell emotion with weather cliches like a raging storm above a fight between lovers or the old cryin' in the rain cliche.
And remember there's lots of weather for you as the writer to play with - not just rain. There's heat and cold and wind and sleet and snow and ice and sun and cloud and tornado and blizzard and hurricane...well you get it. If it can add color and life to your story, use it, just be sure you bring it along and don't just stick it in where you think it would be cool. A well-written story is intricately woven, you can't just stick things in willy nilly.
So, as one example, what can weather add to your story? Well, what are the possibilities? Let's say a storm is approaching.
Rain is likely, but there's a lot more.
There's lightening. The sight of it can be amazing, forking and branching, or a single bolt hitting the ground. It can travel amongst the clouds. It can be a single strike or it can be repeated with strikes hitting nearly one after another seeming nearly continuous.
But there's more. There can be smells associated with rain. The freshness of it falling on grass and trees, the electrical/ozone smell and burning that can accompany a lightening strike. The heat a nearby strike can generate. Lightening can kill or stun a person it strikes, drop him or her like a rock or toss them a great distance. It can splinter a tree, burn a patch in the grass or sizzle overhead like a passing UFO.
Rain itself can be gentle or pounding, slashing or drumming. It can pool and flow or it can sprinkle and soak into the earth.
Thunder can roll, howl or sing through the trees. Or a breeze and the gently falling rain can quietly rustle the leaves of the trees and patter on the ground at their feet.
All of this can contribute to the mood of your characters and test the writer abilities in you. Some people fear storms. Some are exhilarated by them. Lightening is insanely fast and often fills people with a sense of impending doom or fear or awe.
Think about the weather next time you write. Don't hesitate to use it to add contrast to your story. It can add color, flavor and tension. Think about it. A dark mood for a character could be made to appear more intense when contrasted with a beautiful sunny day, perhaps especially with one where the sun sparkles off the snow on the ground as it does outside my window right now. Or the opposite, a feeling of joy can be damped down by a frigid wind or darkly overcast day spitting icy slush. And that can foreshadow a dark day to come.
Think about the weather, play with it when you write. Don't make it your star (again, unless you're writing a very unusual tale), but let it pose as backdrop, emphasize emotion, contrast with what your characters are going through. Trust me, it'll add a lot to your stories.
