The pilot reported that prior to the night departure he checked the weather conditions for airports along his route of flight, each of which reported visual meteorological conditions existed. The pilot also observed that the night sky was clear during his preflight. After performing a maximum performance takeoff, and when about 65 feet above ground level, he encountered a thick layer of fog that had moved in during the 7-10 minute warm-up period and he lost all visual reference to the surrounding area. Having determined that he was able to keep the helicopter in a level attitude, the pilot decided to continue his ascent in an effort to get above the fog. A few seconds after initiating the ascent the helicopter impacted the top of a cedar tree. The pilot initiated an emergency descent and the helicopter sustained substantial damage after it impacted terrain and rolled over on its left side. The pilot reported no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.