For instance, let's say that Fred has a full soffit and ridge vent system installed. But thinking that more is better, Fred decides to install a couple of vents about halfway up the slope. Instead of improving his ventilation, he has now hampered it because air is now exiting out the vents in the middle of the roof, before it reaches the ridge leaving the attic partially unvented. Depending on wind pressure, air will also be taken in at the intermediate vents reducing the intake at the eaves where it should be. There is also the problem of weather infiltration.
Wind blowing across a roof surface creates a negative air pressure.
Nature will automatically try to compensate for it by moving air from a location of higher pressure, such as inside the attic. When the air is then removed from the attic in this manner, it has to be replaced. If the proper intake ventilation isn't used, then air will be brought into the attic through the exhaust vents and will at times bring moisture with it. More at the eaves is better if it can be attained. No attic vents should be installed between the intake and exhaust vents.