When I have rain water on the hood and I’m driving on the highway, the water does run toward the windshield. Yes it moves faster in some spots and doesn’t want to move much in others. So there isn’t 100% dead airspace on the hood.
Actually calling it dead air isn't really the proper designation for it,there is a proper term for the syndrome of air being more or less semi stagnate with-in a couple inches of the hood.Here's a bit of reading on scoops,pay attention to the shallow scoop that is not a NACA duct,which applies to our trucks scoops,they are not NACA ducts,lol.
Scoop design[edit]
To be effective, a functional scoop must be located at a high-
pressure area on the hood. For that reason, some functional scoops are located at the rear of the hood, near the vehicle's
cowl, where the curvature of the
windshield creates such a high-pressure zone, and may be placed so that their opening faces the windshield (a
reversed scoop).
The scoop will be most effective if it is either mounted high enough to clear the
boundary layer (the slow-moving air that clings to the surface of a moving object) or if it is a
NACA duct, mounted below the surface and designed to draw the faster moving air outside of the boundary layer into the duct. A shallow scoop that is
not a NACA duct may not admit a useful amount of air even if it is open.
Under the hood, an effective scoop must funnel air into the engine's intake in as short and direct a path as possible, preferably through a tube or channel that is insulated against underhood heat.
A scoop may be part of the hood, or may be part of the engine's air cleaner assembly, protruding through a hole cut into the bonnet. Such a scoop is called a
shaker hood, because the scoop vibrates noticeably when the engine is running, especially under power.