Frostbite and Hypothermia Signs to Know

Frostbite is skin and the tissue under it freezing in one spot. Hypothermia is your whole body's core temperature dropping below 95°F (35°C). On an aurora night the first usually reaches your cheeks, nose and fingers. The second builds slowly while you stand still for hours.

You need to know the signs because both conditions blunt your ability to notice them. Frozen skin goes numb, and the CDC's frostbite guidance warns that "a person who has frostbite may not know they have it until someone else points it out." Hypothermia slows thinking, so the person affected is often the last to see it. In practice, the person standing next to you is the one who decides when you go inside.

Hypothermia starts when, in OSHA's words, "exposure to cold temperatures causes the body to lose heat faster than it can be produced." Normal body temperature is 98.6°F. Below 95°F is a medical emergency.

Frostbite is the body defending that core. According to the National Weather Service, your body restricts blood flow to the extremities to protect the core organs, and the fingers, toes, nose and ears are left to cool. Frostbite generally occurs below 32°F (0°C). At wind chills of -15°F (-26°C) or lower it can develop in 30 minutes or less, according to Cleveland Clinic. The NWS advises avoiding all outdoor exposure at wind chills of -50°F or below. Those wind chills are well within the range of some aurora destinations, which you can check in typical aurora night lows.

Cleveland Clinic divides frostbite into three stages. The first is reversible. After that, the damage starts to stay.

Frostnip. The skin is cold, sore and painful, red to purple or paler than usual, with mild tingling. The damage is temporary. Small red bumps called chilblains may appear after thawing.

Superficial frostbite. You feel pins and needles while the skin still feels warm. This is the stage that fools people: ice crystals are already forming in the skin. After rewarming you can expect painful patches, bruise-like purple or blue skin, peeling like sunburn, and fluid-filled blisters.

Deep frostbite. The area goes completely numb and is hard to move. Large blisters appear one to two days later. Dead skin turns black and can form a hard shell called a carapace, which may need surgical removal. Severe cases can end in amputation.

The CDC lists the warning signs other people can see: redness or pain first, then white or grayish-yellow skin that feels firm or waxy.

Mild. Uncontrollable shivering, feeling cold and sluggish, trouble thinking clearly. OSHA says shivering should never be ignored. It is your body rewarming itself, and it is also the proof that you are losing heat.

Moderate. Exhaustion, fumbling hands, loss of coordination, confusion, memory loss, slurred speech and drowsiness, according to the CDC's winter weather guidance.

Severe. Shivering stops. The skin turns blue, the pupils dilate, pulse and breathing slow, and the person loses consciousness. When shivering stops in someone who was shivering hard, they have not recovered. The situation has gotten worse.

In any group, some people will cool faster than the rest. Cleveland Clinic names people under 18 or over 65 and smokers. It also lists several medical conditions: peripheral vascular disease, Raynaud's syndrome, diabetes, hypothyroidism, arthritis and malnutrition.

Older adults deserve extra watching. The NWS notes that their bodies are less efficient at signaling cold and they may not shiver enough to warn anyone. The CDC adds clothing that is inadequate for the conditions and heavy alcohol use.

The NWS cites CDC data attributing about 1,300 deaths a year in the U.S. to hypothermia. The sources do not break that figure down by activity, so there is no published rate for aurora watchers specifically.

Wetness. The CDC says hypothermia can develop above 40°F if you are wet from rain, sweat or water. CDC/NIOSH puts the effect plainly: "Wet feet lose heat 25 times faster than dry feet." Wet feet can develop trench foot at around 60°F. On an aurora trip, the likely sources of moisture are sweat from the walk to a viewpoint and snow melting into your trousers or gloves. Both go on draining heat after you stop moving. Getting the base layer that moves sweat right matters here.

Alcohol. The CDC lists excessive alcohol use as a risk factor for both frostbite and hypothermia. Every first aid source here says never to give alcohol to someone who is cold.

Sitting still on cold ground. None of these sources puts a number on heat lost by sitting on snow or rock. What they do establish is that wetness multiplies heat loss. Snow that melts under you turns a dry layer into a wet one.

Many people have both at once, so treat the hypothermia first. It is the condition that threatens life.

Pair up and look at each other's faces every so often. Pale or waxy patches on a cheek or nose are visible to your partner long before you feel anything. Ask each other simple questions, because fumbling words or answers are early hypothermia signs that the person affected misses.

Treat hard shivering as the signal to go to the car or cabin, not as something to push through. Check whether your fingers have gone from painful to numb, which is when chemical or electric warmers have stopped keeping up. Keep the wind chill forecast in mind: at -15°F the sources give you half an hour or less.