I wish you could, but unfortunately, you can't. It's strange how some iPhone apps never made it to the iPad, but considering they just made the clock app universal, maybe we'll see them coming in future updates. I hope so at least! If you have Siri, though, and ask her the weather, the info she gives you is pretty much the same as the native iPhone weather app. I do wish there were a standalone app though - sometimes I just want to check something manually without having to speak (like when I'm in public or something).