The pilot reported that the ground visibility was about 4 miles at the time of his departure. The intended destination was the pilot's farm, located about 17 miles northwest of the departure airport. While en route, about 2 miles from the intended destination, the airplane encountered fog and the pilot decided to return to the departure airport. The pilot stated that he "lost direction" and performed a precautionary landing in a field. During the landing rollout the gyrocopter impacted a fence and nosed over. The vertical fin, rudder, horizontal stabilizer, rotor head, rotor blades, and propeller were damaged. The weather conditions at the departure airport were visibility 4 miles with mist, broken ceiling at 200 feet above ground level (agl), overcast ceiling at 2,800 feet agl, temperature 4 degrees Celsius, and dew point 3 degrees Celsius. The accident occurred at night, about 12 minutes after civil twilight had ended. The pilot noted that the accident could have been prevented if he had called a neighbor prior to departure to determine what the actual weather conditions were near his intended destination.