Photoshop Tutorial - Rainy Day Rain Effect

Photoshop lab has a nicely in depth tutorial on how to turn a fine day into a wet and miserable one - in photoshop.

I agree with their analysis that it's 'semi-realistic'. It's not perfect, but it's the best example of this I've seen.

The great thing about tutorials like this is that you learn techniques for individual effects, and in doing so, you add to your own library of techniques, giving you much more freedom to create the image in software that you can see in your head.