At about 1830 Pacific time, a Cessna 140 struck trees during a forced landing near Vacaville. The pilot and passenger were seriously injured. The flight had departed 15 minutes earlier on an 18-mile flight from Napa. While the airplane was airborne, fog closed in over the area. The pilot looked for a hole so he could descend. He contacted Travis Approach on the emergency frequency and was given vectors to the nearest airport. While en route, the airplane apparently ran out of fuel.