Hypothermia & Cold Exposure
The biggest backcountry killer you cannot see coming
Hypothermia kills more backcountry travelers at moderate temperatures than at extreme ones. The fatal cases are not climbers caught in a blizzard at 20 below. They are dayhikers in cotton t-shirts in a 50-degree rain at 5,000 feet in September. The reason is the mechanism — hypothermia is not about how cold the air is. It is about whether your body is losing heat faster than it can produce it. Both sides of that equation can shift quickly, and the conditions that do the shifting are not the ones that look dangerous.
This article covers how hypothermia actually happens, how to recognize it in yourself and your group before it becomes life-threatening, how to prevent it through layering and pacing decisions, what to do at each stage if it sets in, and how frostbite fits into the broader cold-injury picture. The information is field-level and reflects current Wilderness Medical Society practice guidelines for evaluation and treatment of accidental hypothermia. For deeper protocols — rewarming logistics, advanced assessment, evacuation decision-making for severe cases — take a wilderness first aid course.
How Hypothermia Actually Happens
Your body produces heat constantly through metabolism, and loses it constantly to the environment. Hypothermia happens when the loss rate exceeds the production rate for long enough to drop your core temperature below normal. There are four mechanisms of heat loss, and understanding which ones are working against you in a given moment is the first step in doing anything useful about it.
Conduction. Heat moves from your body into something cooler that is touching you — the ground, wet clothing, cold rock, snow. This is the fastest mechanism by a wide margin. Sitting directly on cold ground loses heat faster than standing in cold air.
Convection. Moving air or water carries heat away from you. Wind multiplies the effect of any cold air. Wading a cold stream loses heat dramatically faster than walking through equally cold air, because water has far higher heat capacity than air.
Radiation. Heat moves from your skin out into the cooler air around you as infrared energy. This is what hats and gloves slow down. Your head, neck, and hands radiate a disproportionate share of heat because they are well-supplied with blood and often left uncovered.
Evaporation. Sweat and any other moisture on your skin evaporates and takes heat with it. This is why getting sweaty in cold weather is dangerous, and why wet clothing kills.
The killer combination is cold, wet, and tired. Not extreme cold. Moderate cold, plus rain or sweat, plus the calorie deficit of a long day, plus the wind that picks up as afternoon turns to evening. Each one alone is manageable. Stacked together, they cross the line faster than people expect.
Recognition: The Stages
Hypothermia is staged by what is happening to the body and the brain, not by an exact temperature you can measure in the field. The stages matter because the response is different at each one, and treating moderate hypothermia like mild can make it worse.
Mild hypothermia
Shivering, often vigorous. The person is alert, oriented, and can still take care of themselves. They may complain about being cold, fumble with zippers and buckles, slur words slightly, or get clumsy. Coordination starts to degrade before they feel meaningfully impaired. They can still respond to instructions and help with their own treatment.
Moderate hypothermia
Shivering becomes violent, then starts to come in waves rather than continuously. Speech slurs noticeably. Coordination is visibly poor — they trip on flat ground, drop things, struggle with simple tasks like opening a snack wrapper. Judgment degrades; they may insist they are fine when they are clearly not. Memory and orientation start to slip. They cannot reliably take care of themselves anymore.
Severe hypothermia
Shivering stops. This is the most dangerous transition in the entire process because the body has given up on its primary warming response. The person becomes semi-conscious or unconscious, pulse weakens and may become irregular, breathing slows. "Paradoxical undressing" sometimes occurs — the person removes clothing as if they are hot, because of failures in temperature regulation in the brain. Severe hypothermia is a medical emergency.
Profound hypothermia
The person may appear dead. Pulse may be undetectable, breathing imperceptible, pupils fixed. Wilderness medicine teaches a specific rule for this presentation: "no one is dead until they are warm and dead." Profoundly hypothermic patients have been revived after extended periods of apparent clinical death, because the cold itself protects the brain from oxygen deprivation. Continue resuscitation efforts and evacuate.
The Umbles: Your Early Warning System
The single most useful field-level skill in hypothermia recognition is watching for the umbles — stumbling, mumbling, fumbling, and grumbling. They appear in mild hypothermia, before the person realizes anything is wrong, and they are visible to other people in the group before they are obvious to the person experiencing them.
Stumbling. Tripping on small obstacles. Missing footing on terrain they handled fine an hour ago. General loss of foot precision.
Mumbling. Slurred or quiet speech. Becoming taciturn. Sentences getting shorter and less coherent. Slower response to questions.
Fumbling. Difficulty with zippers, buckles, drawstrings, pole baskets, anything requiring fine motor control. Dropping things they would not normally drop.
Grumbling. Mood shift toward irritability, withdrawal, or apathy. Becoming reluctant to communicate or to eat. The person who was chatty an hour ago has gone quiet.
The point of the umbles is that you can spot them in other people, and they can spot them in you, well before anyone is officially hypothermic. They are not subtle once you know what to look for. If you find yourself struggling with a zipper, stop and warm up — do not push through it. If your partner has gone quiet for the last twenty minutes after being talkative all morning, ask them how they are doing and watch their face when they answer.
Prevention
Almost all hypothermia is preventable. The prevention work happens in three places: what you wear, how you pace, and how you fuel.
Layer with air, not bulk. Insulation is the air trapped inside and between your clothing. Loose layers that trap air outperform a single thick layer. The standard system is a moisture-wicking base layer, an insulating mid layer, and a wind/waterproof outer shell. Add and remove layers as you change pace and conditions — the moment you start to sweat is the moment you have too many layers on.
The cotton question, honestly. "Cotton kills" is the slogan and it is true in cold and wet conditions, where cotton loses nearly all its insulation value when wet and takes days to dry. In hot dry conditions, cotton is actually a good choice — the evaporative cooling is exactly what you want. The honest rule is not "no cotton ever." It is "no cotton in any environment where you might get wet from rain or sweat in cool or cold conditions." Wool and modern synthetics retain most of their insulation when wet and dry far faster, which is why they are the standard for backcountry base and mid layers in temperate environments.
Stay dry from the inside, too. Sweating in cold weather is how you get hypothermic without any rain falling. Pace down before you feel hot. Vent layers. Strip the mid layer if you are working hard. The discipline is counterintuitive — you are cold when you start moving and that is okay, because you will warm up. If you start fully bundled, you will be sweating within twenty minutes and wet for the rest of the day.
Eat and drink before you need to. Calories fuel the furnace. Cold weather increases calorie expenditure by 25-50% in some conditions, and appetite often goes the wrong direction — you stop wanting to eat at the exact moment you most need to. Force food in. Sugar and fat hit fastest. Dehydration accelerates hypothermia, because the body needs water for circulation and metabolism. Drink even when you are not thirsty; cold suppresses thirst.
Protect head, neck, hands, and feet. These are the highest-loss surfaces relative to their size. A warm hat does more per ounce of weight than almost anything else you can carry. Gloves with a windproof outer shell beat insulated gloves without one in any wind. Keep boot laces loose enough to allow full circulation; tight boots cause cold feet that no amount of sock layering can fix.
Watch your group. Most hypothermia victims do not recognize what is happening to them. The person experiencing the umbles is not in a state to self-assess. The job belongs to teammates. Look at each other periodically. Ask direct questions. Do not let "I'm fine" pass for an answer if their face tells you otherwise.
Field Treatment by Stage
Mild hypothermia
Stop the heat loss. Get the person out of wind, rain, and snow. Replace any wet clothing with dry layers. Add insulation, including a hat. Get them off the ground onto a sleeping pad, pack, or anything that breaks the conductive contact. Feed them sugary, easy-to-digest food and warm fluids if they can swallow safely. Light activity helps generate heat once they are dry and protected — jumping jacks, walking in place, anything that gets the muscles working without causing them to sweat.
Mild hypothermia, caught early, usually resolves within an hour with good field treatment. If they are not improving within 30 to 45 minutes, treat as moderate and consider evacuation.
Moderate hypothermia
Same priorities as mild, more aggressive. Get them into a shelter. Use the "hypothermia wrap" or burrito technique: insulate beneath them, wrap them in a sleeping bag, layer additional insulation around that, and add a windproof or vapor barrier outer layer. Body-to-body contact with another person inside the wrap helps significantly — this is one of the original reasons sleeping bags are sized for two people in mountaineering teams.
Hot drinks and food are useful if the person can swallow safely without choking. Sugary content is more valuable than the actual heat of the drink — the calories matter more than the warmth of the liquid itself. Never use alcohol; it dilates surface blood vessels and accelerates heat loss while making the person feel temporarily warmer.
Moderate hypothermia is also where exertion stops helping and starts hurting. The person is no longer reliably able to generate heat through movement, and moving them too vigorously can disturb the redistribution of cold peripheral blood into the core. Keep them still, keep them insulated, and warm them slowly.
Severe and profound hypothermia
Handle gently. This is the single most important principle. Severe hypothermia patients can go into cardiac arrest from rough movement, including from well-intentioned attempts to actively rewarm them. The cold heart is electrically unstable, and any sudden mechanical or chemical shock — CPR compressions started incorrectly, jostling during a carry, a sudden return of warm peripheral blood — can trigger ventricular fibrillation.
The field protocol is: stop further heat loss, insulate against more cold, handle as gently as possible, and evacuate. Do not attempt active rewarming (hot water bottles, heating pads, hot showers) in the field unless you have no choice. Passive rewarming — insulation that lets the body recover its own heat — is what the field can do safely. Active rewarming belongs in a controlled medical environment.
If the person has no detectable pulse or breathing, begin CPR if you have trained to do so, and continue. The cold-protected brain can tolerate far longer without circulation than a normothermic one, and revival from apparent clinical death from hypothermia is well-documented. The "warm and dead" rule means hospital-warm, not just out-of-the-rain warm.
For severe hypothermia, gentle handling matters more than fast rewarming. The instinct is to warm them quickly. The correct field response is the opposite — stop further loss, insulate, handle carefully, and evacuate. Active rewarming in the field can kill a severely hypothermic patient who would have survived passive treatment and transport.
Frostbite
Frostbite is the freezing of tissue, usually at extremities — fingers, toes, ears, nose, cheeks. It can happen at temperatures well above zero Fahrenheit when wind chill is significant, and the early stages are easy to miss because the affected area goes numb and stops complaining.
Frostnip is the earliest stage — cold skin that turns white or pale, feels numb, and rewarms fully and quickly without lasting damage. Catch it here. This is what regular face checks during cold-weather travel are for.
Superficial frostbite involves skin and tissue just below it. The area looks white or grayish, feels firm but not rock-hard, and may have a waxy texture. Rewarming produces redness, swelling, blistering with clear fluid, and significant pain. With prompt and controlled rewarming, full recovery is common.
Deep frostbite involves muscle, tendon, and sometimes bone. The area is hard, woody, and pale or blue-gray. Rewarming produces dark blisters, extensive swelling, and severe pain. Tissue loss is common and amputation is possible.
The field rule for frostbite is the most important thing in this whole section: do not rewarm frostbitten tissue if you cannot keep it warm. The thaw-refreeze cycle causes catastrophic, irreversible tissue damage — far worse than leaving the tissue frozen through the duration of the self-rescue and rewarming under controlled conditions. If you are six hours of walking from the trailhead and your toes are frozen, leave them frozen. Walk out. Rewarm in the car, at the hospital, anywhere you can guarantee continuous warmth.
When you do rewarm, it should be in water at about 100 to 104 degrees Fahrenheit (slightly warmer than body temperature, not hot enough to burn). The process takes 20 to 40 minutes and is extremely painful. Pain management, sterile handling of any blisters, and prompt medical evaluation are all important. The 2024 WMS frostbite guidelines cover the rewarming protocol and post-thaw care in clinical detail. This is hospital-level care, not trailside care.
Cold-Water Immersion
Cold-water immersion deserves its own brief mention because the timeline is completely different from gradual hypothermia. A person who falls into 40-degree water has minutes, not hours, before incapacitation. The initial cold shock response — involuntary gasp, hyperventilation, possible drowning — can kill within the first minute. After that, "swim failure" sets in within about ten minutes as muscles lose function. Actual hypothermic core temperature drop is the slowest of the three.
The "1-10-1 rule" is taught in cold-water safety education: one minute to get breathing under control, ten minutes of meaningful movement, one hour before severe hypothermia. If you fall in, focus on controlling your breathing first, then on getting out of the water by any means available, even if it means crawling onto ice that you broke through. Stay still in the water if you cannot get out — movement strips heat. Once out, treat for moderate-to-severe hypothermia. See the Water Safety page for more on cold-water travel.
When to Evacuate
Mild hypothermia caught early and treated well in the field usually does not require evacuation. The person warms up, eats, dries out, and the trip can sometimes continue with more conservative pacing. Use judgment about the rest of the day's conditions and the person's reserves.
Moderate hypothermia should be evacuated for medical evaluation regardless of how well the field treatment seemed to work. Cardiac stress, electrolyte disturbances, and delayed complications are real, and they are not detectable in the field.
Severe and profound hypothermia are medical emergencies. Activate rescue. Continue insulation and gentle handling during the wait and transport. Frostbite of any depth beyond clear-recovery frostnip should be evaluated medically as soon as practical, and deep frostbite is its own emergency.
Related Resources
For the gear and layering choices that prevent most cold injuries in the first place, see Wilderness Preparedness. For formal training in field treatment of cold injuries and the patient assessment that comes with it, see First Aid & Wilderness Medicine. For the specific cold-water hazards covered briefly above, see Water Safety. For winter-specific terrain hazards beyond the cold itself, see Avalanche Safety.