Depending on what kind of weather system is producing the rain (whether the weather sticks around) it could affect track temperatures, which might hurt Mercedes slightly more than others. If it's much cooler, Button's setup might not work as well, either, while the likes of Hamilton could benefit. I doubt it will be a deciding factor but it could have some effect on strategies.
Doesn't look rainy on Sky's coverage. Is it live? The sky in the background looks dark but the shadows are quite sharp.
It's probably just cloudy but the water cannons being used a mile down the road is making it look like its raining.