Nighttime temperatures in Yosemite National Park dip way, way down in the wintertime, but despite the chill, Yosemite and other campgrounds draw a good share of hardy winter campers. In fact, each month during the winter about 3,000 people camp at Yosemite's four year-round facilities, according to a park spokeswoman. As their reward, they will enjoy the park in relative quiet, as compared with the experience shared by the 100,000 campers a month who visit during the summer.
But staying healthy during winter camping requires preparation. While heat exhaustion and some other dangers of hot-weather travel are not a problem, there are other concerns to be aware of, including hypothermia (a dangerous drop in body temperature) and frostbite (freezing of the tissues).
Paying attention to bedding and other gear, clothing, wind conditions and especially to early symptoms of cold injuries can help winter campers stay in good health, according to Dr. James A. Wilkerson, a pathologist and wilderness-medicine authority as well as the author of "Hypothermia, Frostbite and Other Cold Injuries" (the Mountaineers Books, $11.95 paperback) and editor "Medicine for Mountaineering & Other Wilderness Activities" (the Mountaineers Books, $16.95, paperback).
Winter gear, including cold-weather masks and battery-operated heated socks and gloves, is sold at sporting goods stores. But some say old-fashioned attire can keep you just as warm.
No matter what the destination, cold-weather campers "have to be prepared for zero degrees," Dr. Wilkerson says. "You're not going to get it very often, but be prepared for it."
That means having a good sleeping bag, and most experts advise buying one rated for zero-degree environments. In addition, plan to use an insulated pad between the sleeping bag and the ground, Dr. Wilkerson advises. Two are even better.
For cold-weather sleeping bags, "nothing beats down," Dr. Wilkerson says, "but polyester is probably just as good [for warmth]."
Clothing should be thick and insulating, Dr. Wilkerson says. "Wear down-filled or polyester jackets, long underwear, pile pants and waterproof outer pants," he suggests. Don't forget gloves.
Dress in layers. The layer closest to the skin should be made of a material that can wick away body moisture -- polyester blends or wool, for example. The middle layer should ideally be of fleece or down for warmth. The outer layer should be water resistant and should "breathe" to protect from the elements -- Gortex, for example.
No matter how cold the evening, change clothes before going to sleep, Dr. Wilkerson says, to rid the body of perspiration-soaked clothes that could cause a chill while you rest.
Battery-operated socks and gloves generate warmth. "But the batteries don't last that long [perhaps a few hours, used continuously]," Dr. Wilkerson says, "and should not be depended upon. Keep your body warm and your hands and feet will stay warm, too."
Another option for keeping warm is a cold-weather mask, similar in appearance to those worn by gardeners to avoid breathing dirt and pollution. But a wool scarf pulled up from the neck that can also cover the cheek area is probably just as good for maintaining body heat, Dr. Wilkerson says.
And a good winter tent is essential. Four-season tents are usually constructed with more poles than other tents, allowing them to better withstand rain, snow and wind.
Staying warm can minimize the risk of hypothermia, a potentially fatal condition that can affect functioning of the heart, intellect and muscles.
At its mildest, Dr. Wilkerson says, hypothermia usually involves a body temperature below 98 but above 90 degrees; in severe cases the temperature may drop below 90 degrees.
He has nicknamed the first symptoms of hypothermia the "umbles" -- a person fumbles, mumbles, stumbles and bumbles, as the temperature decline affects muscular and intellectual functioning.
In mild stages, people with hypothermia sense skin numbness and chilliness, Dr. Wilkerson says, and may notice trouble performing fine hand movements, lack of coordination, stumbling and mild confusion.
As hypothermia becomes more severe, shivering may stop and the person may be unable to walk or stand and finally may lapse into unconsciousness.
Treat hypothermia as soon as symptoms become apparent. Warm the body by covering it and take the person inside. "If someone is so cold they have lost consciousness, it's an almost desperate situation," Dr. Wilkerson says. In severe hypothermia, he adds, the heart is prone to the life-threatening condition called ventricular fibrillation -- irregular heartbeat. Moving and jarring can set it off, so in recent years, rescue teams have begun trying to warm victims before evacuating them.
The body temperature must rise to at least 90 degrees before the heart will beat normally, Dr. Wilkerson says. Getting professional medical help immediately is essential.
Another danger is frostbite. Pain is usually the first symptom, but then subsides, usually for one of two reasons, Dr. Wilkerson says. "Either you have warmed up or the tissue has become frozen."
Most commonly affected are hands, feet, ears and face, and, in particular, the tip of the nose. Boots and other garments tight enough to inhibit circulation are an often-overlooked cause of frostbite.
Winter campers must be doing something right, though. "Frostbite is more common in cities than in the wilderness," Dr. Wilkerson says.