The private pilot was conducting a Title 14, CFR Part 91 personal flight between two remote communities with one passenger. The pilot said that as the flight arrived in an area of a 650-foot elevation mountain pass, weather conditions consisted of low ceilings, fog, and light snow showers, which made it difficult to discern topographical features on the snow-covered terrain. The airplane entered the pass, and the pilot stated he initiated a climb to 700 feet msl in order to fly over the summit of the pass. As the airplane neared the saddle of the pass, weather conditions worsened, and he said he lost all visual reference with the terrain below. He reported that he thought he was through the pass, and had started a gradual descent, when he caught a quick glimpse of terrain and attempted to climb. The airplane subsequently collided with the snow-covered saddle of the pass, nosed over, and slid down the back side of the pass. The airplane continued to slide for about 1,200 feet, and came to rest in the valley floor below. Neither the pilot nor the passenger reported any injuries. The pilot reported that there were no preaccident mechanical anomalies with the airplane.