McKENZIE BRIDGE, Ore. -- The search for a missing University of Oregon mathematics professor continued Tuesday, but crews were treating the search as a "recovery" effort.

The searched was scaled back on Monday. Last weekend, dense fog and clouds grounded air support.

Daming Xu has been missing Sunday, November 4th, when he left for a solo day hike near Cougar Reservoir.

Search and rescue volunteers retraced trails on Olallie Mountain where Xu was last spotted, searching for any sign of the 63-year-old professor, possibly on one of the many game trails that crisscross marked routes, according to Lane County sheriff's Capt. Bill Thompson.

"Our fear and one of our specific areas of focus is Mr. Xu has gone down one of those trails inadvertently," Thompson said.

The weather, which had been relatively mild during most of the search, worsened over the weekend.

Snowfall can either help or hurt a search, Thompson said. If Xu is able to walk, his footprints will be much easier to spot, but if he is injured and immobilized, the snow will cover any clues.

Xu is believed to have begun his hike without warm clothing, food, water or a cell phone. A heavy jacket and a map were found in his car by search teams.

Xu's family asked that anyone without state search and rescue certification to stay out of the area to avoid hampering search efforts by adding footprints and other confusing signs, said Andrew Bates, Xu's son-in-law.

 Background: Footprints a "false alarm"

Bates said the family has taken comfort in stories of other people surviving for long periods of time in the wilderness, such as 76-year-old Dora Anderson of Sandy, who survived two weeks in the Wallowa Mountains in Eastern Oregon in August.

 Also: Grandmother survives being lost in wilderness for 13 days