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Hi,
Velocity means that air is being deflected, or pushed up and over the front of the hood, and this is where drag is most easily created. The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.
So, the diagram makes sense if you think about velocity (changing direction), and this also means pressure against the surface of the vehicle. Low velocity is closer to still/stagnant air, and therefore is lower pressure against the surface of the car (because it is closer to the way it was before the car pushed it out of the way).
The vectors are showing lift, I guess. They only shade the parts that have a positive Y value. I guess it's the X values that I have a problem with -- why are the vectors below the neutral point (on the front bumper) going the right (positive X) and the vectors above the neutral point go to the left? Is this area of the car pulling the car forward?
I'll draw a picture later that is more helpful, to me at least.
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