The phrase "dense fog" means that there is a thick and heavy cloud close to the ground that reduces visibility and makes it difficult to see far ahead. Full definition
Multiple car pile - ups have occurred because people drove too fast in dense fog. (drive-safely.net)
Next morning at 7:30 dense fog hung over the second - leg starting line. (si.com)
Confirmation of Voyager's interstellar exploits came after determining that the probe is surrounded by a relatively dense fog of galactic particles rather than a thin mist of solar ones. (sciencenews.org)