Abstract: In a window assembly including a panel mounted for pivotal movement relative to a casing in a roof sheathing, a moving rib attached to the panel and a guide rib operatively connected to the moving rib, actuating means for automatically pivoting the panel from a closed position overlying the casing to an open position disposed at a predetermined angle to the casing comprising a preloaded spring member, at least one actuating lever connected to said spring member, said actuating lever pivotally connected at its ends to the casing and guide rib and operable upon release of the panel from a closed position to effect pivotal movement of the panel to a predetermined angular position relative to the casing for a given linear displacement of said spring member and adjusting means for selectively varying the angular displacement of said panel by said actuating means.