The pilot stated that he had been established in cruise flight at 4,500 feet mean sea level for the approximate 45-minute cross-country flight. As he neared the destination airport, he descended to about 100 feet above ground level (agl) in order to view some winter wheat fields for the upcoming agricultural spraying season. As he leveled off, he observed increasing fog along his route of flight. He began a 180-degree turn to reverse course and intended to divert to an intermediate airport when the airplane impacted a high-tension power line. The airplane severed one of the lines and subsequently impacted an adjacent farm field. The pilot reported no failures or malfunctions associated with the airplane prior to the collision with the power line. The pilot reported weather conditions at the time of the accident as an obscured ceiling about 800 feet agl, with 2 miles visibility in fog. Winds were light and variable, and there was no precipitation.