The pilot reported that he was conducting an aerial observation flight during night visual meteorological conditions. The pilot stated that as the airplane approached the ridge tops of a mountain range from east to the west at an altitude of about 800 to 1,000 feet above ground level (agl), the airplane rolled uncommanded into a 90-degree left bank. The pilot stated he corrected the roll and regained his original heading. Shortly thereafter, the airplane encountered “another gust” and rolled to the right. The pilot reported that as he was rolling back to level flight he realized that the top of the hill was higher than his altitude and remained in a right turn. He stated that during the turn, the ground became illuminated by the landing light and then the airplane impacted snow covered mountainous terrain and came to rest upright. Examination of the airplane by the pilot revealed that both wings and left horizontal stabilizer were structurally damaged. The pilot reported no mechanical anomalies that would have precluded normal operation prior to the accident. The pilot further reported that at the time of the accident, he estimated the wind to be from the west at 20 to 30 knots.