On January 17, 2001, at 1845 mountain standard time, a Cessna P210N, N4817K, sustained substantial damage when it departed the side of the runway during landing roll at Stevens Field, Pagosa Springs, Colorado. The private pilot and his passenger were not injured. The flight was a local area flight operating under Title 14 CFR Part 91 and no flight plan was filed. The flight originated at 1815. Night visual conditions prevailed. According to the pilot, he was practicing night landings and on his second touch-and-go landing, the aircraft entered a fog bank during landing roll. The pilot said he lost visual reference and the aircraft departed the left side of the runway into approximately 15 inches of snow 500 to 700 feet from runway 19 threshold. Weather at the time was clear skies, calm winds, and a temperature dew point spread of 1 degree. During the excursion from the runway, the left main landing gear collapsed causing damage to the left main landing gear wheel well, and the left horizontal stabilizer was damaged.