Good discussion on BeechTalk about ATC separation of aircraft.
Separation is always provided between IFR aircraft.
Within the class B ATC is always responsible for separation, all aircraft are separated from each other, that’s why you need a clearance to enter if you are VFR. Within the class C IFR aircraft are separated from VFR aircraft. In all other airspace IFR aircraft are not separated from VFR aircraft. The actual weather is not relevant in that it does not change separation rules. – Scott Newpower, Air Traffic Controller