THE SIGNS ARE SUPPOSED TO remind hikers of their vulnerability. Especially those who, emboldened by cellphones and global positioning systems, set off into the Presidential Range in New Hampshire's White Mountains carrying little else besides day packs stocked with PowerBars. The message, in black lettering on yellow, is blunt: "STOP. The area ahead has the worst weather in America. Many have died there from exposure, even in the summer. Turn back now if the weather is bad." And each year, many people do stop, long enough to pose for a picture. Some photographs are uploaded to Flickr and other websites - including one of a pink-faced man gleefully acknowledging the warning with upraised middle fingers. A caption reads, "Tom showing the White Mountain National Forest what he thinks of their sign." Todd Bogardus, the search and rescue leader with the New Hampshire Fish and Game Department, is accustomed to such cavalier attitudes. For a new breed of hikers, he says, high-tech gadgets have replaced common sense, even though cellphone service is spotty in the mountains, and many people do not know how to operate their GPS devices. "Technology is good when it's used with proper basics and education, but it also gives a false sense of security," says Bogardus. "They figure, 'This is the weekend I took off , and, by God, I'm going to climb that mountain.'" He is talking about hikers like Tom, who probably returned home unscathed. Most of the 5 million visitors to the 800,000-acre national forest each year do, even if they have spent less time preparing to navigate its wilderness trails than they would their local supermarket. "People don't start the day thinking, 'Oh, I'm going to get hurt,' " says Rebecca Oreskes, spokeswoman for the national forest. "They might not have the proper equipment, and they underestimate how difficult the White Mountains can be. They're starting from the valley, where it's 80 degrees. They're in shorts and T-shirts, and they get up high, and there's sleet. It's a really different world." ABOUT TWO HOURS NORTH OF BOSTON, INTERSTATE 93 CURVES SOFTLY to reveal the indent of Franconia Notch, etched by Cannon Cliff on one side and Franconia Ridge on the other. It is so seemingly benign, so accessible. Seventy million people live within a day's drive. Early one June morning, sun filters through haze, draping the vista in a blue-gauze light. On the radio, a traffic report details a backup in the O'Neill Tunnel; on the horizon, there is clear road to wilderness. Twenty minutes up the highway, cars filled with climbers pull into the lot at the Old Bridle Path trail head. The Franconia Mountains are one of the national forest's premier destinations, along with the Presidentials, crowned by 6,288-foot Mount Washington, 20 miles northeast.

Old Bridle Path begins a 9-mile route that ascends mile-high Mount Lafayette, continues south along a skinny, treeless ridge, and heads down via the rugged Falling Waters Trail. Eric Pedersen, who works for the conservation and education nonprofit Appalachian Mountain Club, is headed to AMC's Greenleaf Hut. It is perched most of the way up Lafayette, 3 miles from the highway at an elevation of 4,200 feet, where scrub surrenders to ledges. Pedersen oversees all eight of the club's huts in the White Mountains, which host 42,000 overnight visitors annually. He also coordinates search and rescue missions for the club, in conjunction with about a dozen other agencies and groups. The guidebook estimates a two-hour-and-40-minute trip to Greenleaf; Pedersen, 24, can crunch it to 45 minutes. He has long legs, wears trail-running shoes, and sips water from a tube attached to a backpack reservoir so he doesn't have to break pace. A man sporting a tank top and sneakers checks his cellphone - no signal. A family of four douse one another other with insect repellent and with deliberate step fade into the greenery. The temperature is 72 degrees, the humidity rising. There is a chance of thunderstorms later. Pedersen watches a girl of about 12 dressed in a fluffy sweat suit. He says she's going to be warm until the tree line, where the temperature could skid into the 50s. Normally, it drops 3 to 5 degrees for every 1,000 vertical feet. Add a cloudburst, fierce winds, and dehydration, and suddenly hypothermia becomes a risk. The condition, cooling of the body core, can be fatal. "You're cold, you start making poor decisions, and you don't know why. Your temperature goes down, and you start losing motor and muscle skills," Bogardus says. "As it worsens, there are deep chills to violent shivering, and eventually, unconsciousness and death." Chris Thayer, the AMC's White Mountain facilities director, says winter-climbing hazards are well documented. Generally, only hardened climbers attempt an ascent that time of year. There is less publicity about "shoulder seasons," he says, like the weeks from late August through October, when the inexperienced explore the outdoors in droves. Days that begin balmy can end brutally. "The weather patterns are tricky," Thayer says. "But you might have a hard-charging group of guys whose goal is the summit, no matter what. And I've seen many families where the parents push the kids to make it up in bad weather. Once, I took this incredibly heavy pack from a family. They had a toaster they thought they could plug in at the hut." LAST YEAR, THE NEW HAMPSHIRE FISH AND GAME Department, national forest, AMC, and other groups assisted in 164 incidents, about 75 involving injured or lost hikers. Search and rescue missions in 2007 cost the state $150,000, plus thousands of volunteer hours. About $42,000 of that was spent aiding people who were later deemed "negligent." Under a New Hampshire law that took effect in June, hikers who fall into that category and refuse to pay rescue costs can lose their licenses to drive, fish, and hunt. The definition of "negligent hiker" may be open to interpretation, but Bogardus says he knows one when he rescues one. If a mountain outing goes awry in the Whites, he gets called - in the dead of night, in rain and snow, in temperatures cold enough to freeze skin in minutes. "It's my job," he says flatly. "When there's no explanation for an accident other than fate, those are hard to explain. But it's frustrating when somebody was aware of a substantial danger and disregarded it. Something that was so avoidable." For example, he says, hikers who do not tell anyone where they are going, ignore weather reports and admonitions from rangers, or knowingly lack adequate clothing, food, water, and survival gear.

More typical is the ignorant adventurer, a single slippery stone from disaster. Few hikers carry maps, and hardly any bring what should be essential reading - the AMC's White Mountain Guide. The guide, published since 1907, includes detailed trail descriptions, topographic maps, and trip-planning advice. To call attention to what should be obvious, the national forest and state fish and game department developed a program called hikeSafe. Its website, hikesafe.com, gives hikers lists of 10 essentials to bring. A "hiker responsibility code" is also posted. Its main message: Save yourself. NEARING A SECTION OF OLD BRIDLE PATH CALLED AGONY RIDGE, Pedersen points to a place where a woman fractured her leg two summers ago. Last October, a woman broke her back farther up Lafayette. She was placed in a litter and carried by volunteers to Greenleaf Hut for a sleepless night. Low clouds made a helicopter evacuation impossible. After daylight, Pedersen and a rescue crew climbed up to the hut, then carried the woman down the snow-covered path, a four-hour endurance test. "We started with seven people, barely enough," he says. The general rule is "12 to 18 people, six people carrying at a time, switching off every five to 10 minutes." On this day, Pedersen's two-way radio is quiet, but in May it crackled with grim news from a hut caretaker: A boulder had dislodged on Falling Waters Trail, careening 40 feet and striking Shu Qin, a 28-year-old tourist from Shanghai. No one could have predicted it. From his home, Pedersen instructed the caretaker on treating Qin. It was not enough. She was pronounced dead at a hospital. "Very traumatic," he says quietly before changing the subject. Greenleaf is empty at midmorning. Forty-eight hikers have reservations tonight, a full house. Breakfast and dinner are served family style, and bunk rooms are communal - Spartan by most standards, but luxurious up here. Exhausted hikers sleep deeply, and the sound of snoring can rival a gale. Sitting barefoot at a long wooden table in the common room, "hutmaster" Hillary Gerardi complains mildly to Pedersen about the menu - an order of ground beef didn't arrive, so tonight's main course will be vegetarian chili. Gerardi, 21, a Middlebury College student, has the robust complexion of someone who lives on a mountain from June through August. Like the other five Greenleaf crew members, she packs in food and other supplies, 30 to 70 pounds at a time. In addition to making meals and maintaining the huts, crews frequently participate in searches and rescues because of their proximity to trouble spots. They gently educate guests about safety, partly for selfish reasons. "If someone sprains their ankle, we may be the ones to hike out and find them," Gerardi says. "But we're not here to reprimand people. If somebody comes in unprepared and wants to do the whole ridge, we might say, 'You know, there's a chance of storms. I see you're wearing flip-flops. Maybe you should wear shoes next time.'" One evening, as Gerardi watched lightning bounce off the peaks, two hikers arrived at Greenleaf sopping wet. "One hand-held flashlight, wearing cotton, and with no tent, looking for a place to stay at 9 p.m. in a thunderstorm. They ended up camping on the floor."

ATOP LAFAYETTE, A MILE FROM Greenleaf, wind whistles through spiky outcroppings, the air chills, rain comes horizontally, and a sinister-looking cloud band is creeping in from the west. Summer has taken leave. For the next 1.7 miles to Falling Waters Trail, there is nowhere to hide. In February, a helicopter crew plucked two men from this ridge after they were overcome by subzero temperatures. One died, the other suffered severe hypothermia and frostbite. Bogardus says they chose to disregard an ominous weather forecast. The next week, another hiker became lost in the area. Fortunately, his cellphone, equipped with a GPS receiver, worked - and so did his headlamp. While Bogardus spoke with the hiker, the 911 system was able to home in on his position. A helicopter spotted the climber, as did a search party. He was rescued after spending 30 hours in winds of up to 90 miles per hour. It was the first time GPS coordinates located through 911 proved accurate in these mountains, according to Bogardus. Before long, clouds cover Lafayette in a shroud. A couple from Quebec, wearing nervous expressions and ponchos, can't locate the stone piles, called cairns, that mark the trail. They nearly go in the wrong direction. "We weren't expecting this," the woman shouts into the din. That a common refrain in the White Mountains, says Nicholas Howe, author of Not Without Peril, book chronicling 150 years of Presidential Range accidents. "The biggest mistake is not turning back," he says. WHEN THE SUBJECT TURNS TO WEATHER, Mount Washington's worldwide wind record is invariably mentioned - 231 miles per hour in 1934. But smaller numbers also impress: The temperature on Washington averages 27 degrees year-round and visibility often is less than 100 feet. "Snow and ice can occur any month of the year," says Brian Clark, a meteorologist at the Mount Washington Observatory. "In the grand scheme of things, we're a small mountain. But we sit at the conjunction of three major storm tracks." And when winds loaded with moisture smack into that granite wall, watch out. The ferocity accounts for many of the 140 fatalities recorded on or near Washington since 1849, including 33 since 1990. Weather permitting, Clark and his colleagues usually have company. Three hundred thousand visitors swarm the summit between spring and fall, about 220,000 via the auto road and the cog railway. For many, the only postcard views they see are on postcards in the gift shop - Mount Washington is wrapped in clouds 60 percent of the time. The visitor center's amenities also lead some hikers to mistakenly believe there is overnight shelter. This spring, Clark says, "a gentleman, his elderly father, and son came knocking on the observatory door right around sunset. No pack, no water."