Udall, 61, left June 20 from a trailhead 10 miles northwest of Pinedale in western Wyoming. He was due back last Wednesday. The search for him began Friday, the day he was reported missing.
The Wind River Range sprawls 100 miles southeast of the Yellowstone ecosystem and jags the horizon of the nearby sagebrush country like saw teeth.
Compared to other wilderness areas in Wyoming, the area's maze of trails is popular with summertime backpackers and day hikers. Many who explore the area venture off-trail to fish or climb, said Rick Rochelle, director of professional training for the Lander-based National Outdoor Leadership School.
A well-known wilderness training academy, NOLS currently has 29 groups of 14 students and instructors participating in courses in the Wind Rivers. Groups headed into the mountains were being told to keep an eye out for Udall. Half a dozen NOLS employees joined the core search party Tuesday, he said.
The search area includes dense pine forests.
"You can't see through the forest canopy to see the ground, if you're searching from a helicopter," Rochelle said. "So you need people on the ground to search that kind of area."
Thin air and slick snowfields are challenges higher up, said Rochelle, who has taken part in search efforts in the Wind River Range over the years.
One thing going for Udall: The weather has been warm, with nighttime temperatures dipping to freezing only at the very highest elevations.
"It's been pretty comfortable conditions out there," Rochelle said.
Sublette County authorities described Udall as an experienced hiker and no stranger to exploring the Wind River Range over the past 30 years. Even so, the rugged winds claim even experienced hikers and climbers with disturbing frequency.
In November 2007, NOLS employee Clay Rubano, 46, apparently fell and died of exposure during a hike in Sinks Canyon in the southern end of the mountain range. A hiker found his remains the following spring.
In 2006, Wyoming's chief state public defender, Kenneth Koski, 56, died on 12,585-foot Mount Bonneville about 60 miles southeast of Pinedale. Koski was alone and apparently fell more than 400 feet during a climb.
In 1998, Idaho pastor Mike Turner, 48, became trapped between two boulders while backpacking on the east side of the Wind River Range. He used a cup attached to a rope to try to fetch water from a nearby pond. He wrote at length in a diary about his ordeal and survived nine days before he perished.
Udall is an author and helped found the Community Office for Resource Energy Efficiency, a nonprofit group that promotes the use of renewable energy in the Aspen, Colo., area.
Under a program Udall helped create, people building large homes in the Aspen area pay an energy fee which is then used to pay for rebates for energy reducers like efficient appliances or solar power systems, said Mona Newton, the office's current director.
Newton said her staff members are optimistic and hopeful Udall will be found healthy and safe.
"He's a very capable, strong individual," she said.
Colleen Slevin in Denver contributed to this report.