If you put a boost gauge on a NA engine, it'll read manifold vacuum. That'* what we read at idle. You go from there up to your max boost. It'* not a linear reading, but the gauge is accurate to what'* happening in the intake. With higher air density during colder weather, both NA and SC intakes benefit from higher input pressures, which are indicated by LESS vauum on the gauge. The really cool thing (no pun intended) is that you start out with a higher pressure (ie: lower vacuum) and then you get to compress it for a higher post-sc pressure. This is why you always feel more power in cold or wet weather. Foggy days are the best, as the moisture in the air decreases the air density at first, then as the water evaporates, and cools the intake the density increses for more efficient operation.
I've actually gone through this quite a bit lately, in several threads. Would anyobdy like to see all these interactions listed in the Techinfo section?
OK....a couple hours later....I just posted a long discourse on how the density is affected by water injection: