My understanding of tule fog is a very dense, ground-hugging fog of relatively low depth with nearly clear visibility above a well-defined ceiling. A friend once related to me his experience with tule fog in the Sacramento valley. He was northbound somewhere outside Sacramento near the river, driving an old chevy pickup with a tall cab. He was following the tail lights of a car in front, whose driver knew the road. The fog bank was about 4 feet high, and above it was clear air. Roadside signs were clearly visible. Next thing he knew he was being pulled over by CHP. When the officer got out of his car and stood up, his head was out of the fog and he knew why the pickup driver was driving faster, he could see ahead much better! No ticket that day.