If you're looking for jeans, Aerostich sells some now. THey have arborwear pants, heavy Carhart style canvas pants with double layers in the lower front legs and seat, and optional knee pads, and they sell a style of denim jeans with designed in pockets to fit knee pads in- when you get where you're going, you can unzip them and pull the kneepads out in a few seconds.
Aerostich Protekt Riding Jeans
Aerostich Arborwear canvas pants
Not that either of those will feel like an awesome "hot weather" solution.
There are also new RoadCrafter Light and Ultralight suits for urban riding.
Personally, I've got some Olympia mesh gear that I'm pretty happy with for what we call "hot weather" in Oregon, which is anything above 80F.