Funny you should ask...
See my "got a few things done" thread for particulars, but at idle, I can feel a steady stream of hot air coming from my recently completed (rear facing) hood scoop project. And that's with a "diverter plate" in place under the actual hole to channel rain away from the top of the engine.

Also, I've read in posts from the long ago, about people having frost melt off their windshield during winter months using similar mods.