More boost in the rain?

Rain usually means cooler temps, so ya denser air.

ID89GT is correct.

Cooler denser air means more PSI in the intake. It can be cooler or denser or both at the same time. Roots blower only turns X amount of RPM based on crankshaft speed, so if the air going in is denser, it will be denser in the intake as well after the blower.

Case in point: My Lightning (2003). Normally 8 psi boost. Took it on a long trip out to Colorado about 5 years ago. At the top of Raton Pass, I was at 2 psi and on the floor pulling the trailer. 7900 feet above sea level...92 degrees actual air temp means really thin air. Thin air going into the blower means there is less air to be compressed therefore less PSI. That's where turbochargers shine-to an extent. The WG only allows a certain PSI. If air is heavy sea level (or below in some cases), it might only make 10 psi. At altitude in thin air, the WG is still going to allow 10 psi although the compressor has to spin quite a bit faster to achieve the same manifold pressure. Some turbos are out of their efficiency range at altitude with thin air, which then helps reduce power even though PSI stays the same.