In the very first sentence of The Big Sleep, Marlowe tells us that, "It was about eleven o'clock in the morning with the sun not shining and a look of hard wet rain in the clearness of the foothills" (1.1). Of course, there's nothing unusual about mentioning the weather in a novel—especially at the beginning—but in this novel, it seems to be doing a bit more work than just setting the scene.
When it's about to rain, that means something is about to happen. Usually something bad. That's when we'll hear the sound of distant thunder. If you've ever lived or spent time in Southern California, you probably know that it doesn't rain that much. Sure there'll be some drizzly days here and there in the later months of the year, but the constant downpour that occurs in The Big Sleep is pretty implausible for the dry, desert-like climate of L.A.
So for Chandler, the motif of rain is used less for realistic purposes and more for adding symbolic value. And to figure out that value, we'll have to ask ourselves, what does rain traditionally symbolize? Dark clouds usually symbolize that something bad is coming round the bend, and rain is often associated with some sort of cleansing or purification. The washing away of sins and whatnot. So as you read the novel, try to keep track of when Marlowe mentions the weather and try to keep those themes in mind.