A leading cause of death in the outdoors, hypothermia occurs when your body's core temperature drops low enough that your vital organs can no longer function. Most cases of hypothermia occur at temperatures in the 50s, not at freezing, as you might expect. Often the victim has gotten wet, and/or is fatigued from physical exertion.

Initial symptoms of hypothermia include uncontrollable shivering, often followed by a complete stop in shivering, extreme lethargy, and an inability to reason. The hypothermic person will often want to lay down and rest or sleep. His or her hiking partners must jump into action to get the victim warm and dry immediately. Take off their wet clothes and put on dry clothes. Cover their head with a warm hat. If someone in the group has an emergency space blanket (see my Day-Hiker's Equipment list), wrap it around the hypothermic person. Get them to eat some quick energy food, even candy, and drink warm beverages--this helps the body produce heat. Do not give them alcohol, as this encourages heat loss.

HEAT STROKE:

Usually the result of overexposure to the sun and dehydration, symptoms of heat stroke include headache, mental confusion, and cramps throughout the body. Immediate action must be taken to reduce the body's core temperature. Pour water on the victim's head. Have them sit in a cool stream if possible. Make them drink as much liquid as possible. Heat stroke is easily avoided by staying adequately hydrated and wearing a large-brimmed hat for protection from the sun.

ALTITUDE SICKNESS, AND ADJUSTING TO ALTITUDE:

Many hikers experience a shortness of breath when hiking only a few thousand feet higher than the elevation where they live. If you live on the California coast, you may notice slightly labored breathing while hiking at an elevation as low as 5,000 feet. As you go higher, it gets worse, sometimes leading to headaches and nausea. It takes a full 72 hours to acclimate to major elevation changes, although most people feel better after only 24 to 48 hours. The best preparation for hiking at high elevation is to sleep at that elevation, or as close to it as possible, the night before. If you are planning a strenuous hike at 7,000 feet or above, spend a day or two before doing easier hikes at the same elevation. Also, get plenty of rest and drink plenty of fluids. Lack of sleep and alcohol can contribute to your susceptibility to "feeling the altitude."

Top photo: Mono Lake after a winter storm

Serious altitude sickness typically occurs above 10,000 feet. It is generally preventable by simply allowing enough time for acclimation. But how do you acclimate for a climb to the top of Mount Whitney or Shasta at more than 14,000 feet? The answer is you can't, at least not completely. Spending a few days beforehand hiking at 10,000 feet and above will help tremendously. Staying fueled with food and fully hydrated will also help. But if you've never hiked above a certain elevation--say 13,000 feet--you don't know how you are going to feel until you get there. If you start to feel ill (nausea, throwing up, severe headache), you are experiencing altitude sickness. Some people can get by with taking aspirin and trudging onward, but if you are seriously ill, the only cure is to descend as soon as possible. If the altitude has gotten to you badly enough, you may need someone to help you walk. Remember that fatigue and elevation sickness can cloud your judgment in the same manner than hypothermia does, so take action before your symptoms become too severe.

LIGHTNING:

If you see or hear a thunderstorm approaching, avoid exposed ridges and peaks. This may be disheartening advice when you're only a mile from the summit of Half Dome, but follow it anyway. If you're already on a mountain top, stay out of enclosed places such as rock caves or recesses. Confined areas are deadly in lightning storms; hikers seeking refuge from lightning have been killed inside the stone hut on top of Mount Whitney. Do not lean against rock slopes or trees; try to keep a few feet of air space around you. Squat low on your boot soles, or sit on your daypack, jacket, or anything that will insulate you in case lightning strikes the ground.

GETTING LOST:

If you're hiking with a family or group, make sure everybody knows to stay together. If anyone decides to split off from the group for any reason, make sure they have a trail map with them and know how to read it. Also, ensure that everyone in your group knows the rules regarding what to do if they get lost:

• Whistle or shout loudly at regular intervals.
• "Hug" a tree. Or a big rock or a bush. That means find a noticeable landmark, sit down next to it, and don't move. Continue to whistle or shout loudly. A lost person is easier to find if they stay in one place.