It's interesting to see they're pre-rendering the composition of the weather layer on top of their regular map. Where I work we have our own styled map tiles and probably would do a layer of transparent cloud tiles over our (unmodified) regular tiles. It just takes too long to re-render deeper scales.

That said, if I was serving up as many tiles as google does, I'd go for it this way too. You already need a buttload of resources just to handle the request volume, might as well exploit that to save the user on download time and yourself on bandwidth.