In this race, the weather forecast is watched more closely than the clock. Held here in January near the Canadian border, the Arrowhead Ultra 135 is an event in which a change in temperature can have a demoralizing effect on the competitors.

Adding too many layers can overheat these ultradistance athletes, producing sweat that can later turn cold on rest breaks along the 135-mile course. But shedding layers risks hypothermia. Two years ago, the temperature dropped to 40 below, and few racers finished the course. This year was balmy, with daytime highs in the 20s.

“I’ve lived here for 30 years, and this has been the warmest winter I’ve ever seen here,” Ken Krueger, the race director, said. “It’s ridiculously warm. But even in perfect conditions, it’s tough. Getting to the finish line is tough. Our historic finish rate is 52 percent.”

Cardiovascular endurance and mental discipline are important, but mastery of one’s gear is an essential factor in the frozen northwoods. Many of the competitors are gear junkies and self-professed “weight weenies” who count every ounce as they try to stay light while maximizing the benefits of the gear they must haul to survive the grueling ordeal.

All racers are required to carry a minus-20-degree sleeping bag, a sleeping pad, a bivouac sack, a fire starter, a stove and pot, 8 ounces of fuel, a two-quart insulated water container, a headlamp or flashlight, two flashing LED lights, 10 square inches of reflective material on their front and back, a whistle and 3,000 calories of food. Racers can receive no outside help while on the course.

None of this is negotiable. The gear is checked by volunteers before the race, and racers must carry it with them. None of it can be ditched; the gear is double-checked at the end of the race.

Athletes can do the race by foot, ski or bike, so their choice of mobility reflects how they must carry their equipment. Foot racers either pull a plastic sled or push a kicksled, a kind of modified dog sled. Bikers keep their gear in panniers. Skiers tend to use backpacks.

Many apparel brands are represented at the race, but not all gear is high-tech. For an underlayer, Tom Lais, 61, of Minneapolis, buys wool sweaters at a Goodwill store and shrinks them to size in his dryer. “Wool is the only way to go,” he said. To keep sweat from soaking into the wool, he wraps a diaper mat around his belly. Once that is drenched, he tosses it into the trash at a checkpoint. He calls it his secret weapon.

John Bertram, 52, of Columbus, Ohio, uses an old Army surplus Alice pack. As a six-time veteran foot racer, he was the first person to finish a race carrying only a backpack. “It was bad,” he said. “What I found was that I almost needed to carry a little stool with me. Like a fold-up thing, because I couldn’t sit down. I was on my knees and trying to take the whole backpack off and taking the food out. It was more of a pain than anything else.”

He still uses the same pack, but now divides the gear between the pack and a sled. “I keep telling myself it has got to last me one more year,” he said. “Some of the rivets have popped out because I carry too much weight in it.”

Some of the gear is handmade. Helen and Chris Scotch, both 38, of Bend, Oregon, pull their sleds with bungee cords. The forgiving stretch of the cords makes the movement of the sled less jarring. They also have lightweight handmade ponchos, and to keep their water warm Chris Scotch used insulated foil Bubble Wrap to make covers for their water bottles. He found it so handy he ordered an entire roll from an industrial distributor. “I think it works better than neoprene because there is more air to insulate it,” he said.

Chuck Lindner, 44, of Warroad, Minnesota, wears an anorak made by Lure of the North, a company based in Ontario. The jacket’s material is of Egyptian sailcloth that is breathable and windproof. The hood is trimmed with coyote fur. “When it’s windy, the advantage of a fur ruff is it creates its own microclimate, so at 30 below you won’t need a face mask,” Lindner said.

Jared VanderHook, 32, of Minneapolis, had a gear bag made out of Cuben Fiber, a material used for yacht sails. The friend who built the bag also created a website called Faster Tracks, which processes the GPS signal from his SPOT tracker, which uses satellite technology and is mounted on his sled. His girlfriend follows him on her computer; other racers and their support teams were also using the software this year.

Cellphones are allowed, but taking one might not be worth it because coverage is spotty. Tracking devices are permitted, but as Krueger, the race director, pointed out, it doesn’t take away from the rugged independence of the race.

“We discourage snowmobile rescues,” Krueger said. “We encourage self-rescue. If you’re tired or cold or sore, we want people to get to a checkpoint. We don’t want them dependent on snowmobiles to get out. This is not a baby-sitting race. This is one of the hardest races in the world, and you’re supposed to be self-sufficient.”

Cash is one lightweight item that racers should carry. The first checkpoint is Gateway General Store, a gas station that sells groceries, gifts, hardware and fishing tackle. Racers take over the place like a locker room, and roaster ovens are loaded with soups, sloppy joes and macaroni and cheese.

It is here that Dan Vold was to meet his wife with his newborn daughter. Svetlana Vold, 33, of Minneapolis, gave birth to Yeva seven weeks before. She trained for the Arrowhead right up to the birth and was training again shortly after. The plan was to attach a breast pump to her midsection with a Velcro belt and drop off the breast milk to Dan and Yeva at the three checkpoints along the course.

In past winter racing experience, she found that batteries drained quickly in the cold temperatures so she designed a wrist pocket sleeve to hold spare ones. If the breast pump — or any other equipment — failed, she could swap out the dead batteries for new ones.

To keep her spirits up, she has two fake flowers attached to her bike, a signature that has made her recognizable to other racers. “Flowers are always on my bike because when everything is gray and there is no color, it makes you. ... ” she said as she shrugged, then broke into laughter. Originally a cross-country runner from Belarus, Russia, she keeps up her strength by eating Russian candy, cheese, bread and kielbasa.

Consuming calories, especially hot food, is essential for the body to fight off the cold. Wendy Drake planned to keep it casual yet efficient. Drake, 48, of Boulder, Colorado, had a Jetboil Flash stove: The bottom part is composed of the gas can and burner that mounts to the cook pot. The pot has a strap attached to the side so it can be held like an oversize coffee mug. Since she was on foot, her plan was to fire up the stove as she hiked along while sipping chicken noodle soup spiked with toasted sesame oil.

“The chicken noodle soup is comfort food while the oil will really give me a lot of nutrition,” she said. “I’m superstoked about that.”

Her friend Rebecca Hansen, 54, of Naples, Florida, used to be a brand manager for Snickers, so when her former co-workers heard she was doing the race they joked that she should take some of the candy bars along to fulfill the 3,000-calorie requirement. Snickers has a marketing campaign in which the main logo on the packaging is replaced by an adjective related to moods, like sleepy, impatient or grouchy. She carried three, labeled “confused,” “feisty” and “princess.”

Of those, two had back stories. In last year’s race, she got lost and ended up circumnavigating a frozen lake. She was disqualified for not staying on the course, hence the Snickers bar labeled “confused.” As for “feisty,” while she was training in Florida with a sled and a weight vest, she came across a 9-foot alligator blocking the trail. Exhausted and in no mood to hike back 20 miles, she picked up her sled and carefully stepped over the gator. “He didn’t flinch or move at all,” Hansen said.

At the prerace meeting at the Backus Community Center here, Krueger reminded the racers about one last item to have when they arrived at the finish line at the Fortune Bay resort.

“Last year was warm and we had some nasty, nasty trench foot,” he said. “So please take care of your feet, and I’m not sure I know how to tell you to do that, but I would start with dry socks. It’s not that I really care that much about your feet, but our hospitality room smelled awful last year.”