You're Not Thirsty in the Cold. That's the Problem.

Quick answer: Don't wait for thirst in the cold, because it won't show up on time. Drink 16 ounces before you leave, then take a warm drink on a schedule, about a cup every hour. Check your urine. Pale straw is right. Dark means you're behind.

The 6 a.m. thermos

A hunter climbs into a stand at 5:45 on a December morning. He's had one coffee. He's got a full thermos of water in his pack, and at noon it's still full. He isn't being careless. He just never felt thirsty.

By midday he's got a dull headache and cold hands. He's irritable and slow, and he's decided the climb down can wait another hour. None of that feels like dehydration. It feels like a long cold morning. That's why it works on you.

Why your body lies to you

Three things happen at once, and none of them announce themselves.

Cold suppresses thirst. In the cold your sense of thirst gets quieter. People reliably drink less in cold conditions than they would in the heat, even when they're losing the same amount of fluid. The usual alarm bell is muffled.

Cold makes you urinate more. When your skin cools, the blood vessels near the surface tighten and push blood toward your core. Your body reads that as extra volume and sends the surplus to your kidneys. Physiologists call it cold diuresis. You make more urine without drinking more, and you never feel it as a loss.

You breathe out water. Cold air holds very little moisture. Every breath you take has to be warmed and humidified before it reaches your lungs, and you lose that moisture when you exhale. The white plume in front of your face is water leaving. Work harder, breathe harder, lose more.

Add sweat to that. Climb a ridge in a puffy jacket and a shell, and you'll sweat under your layers. It evaporates or soaks into your clothes. You never feel wet the way you would in July, so you never count it.

What it does to you

Dehydration on its own rarely kills anybody in an afternoon. The danger is what it does to your head while the cold applies pressure.

A fluid deficit brings fatigue, headache, slower thinking and a short temper. In the cold, those are the exact symptoms that lead to the decisions that hurt people. You skip building the shelter because you're tired and it's only a few more miles. You press on past dark. You ignore numb fingers. You get clumsy with a zipper, a knife or a rifle.

That last one matters if you hunt. A foggy, cold, under-watered shooter is where the four rules get tested hardest: treat every gun as loaded, keep the muzzle pointed somewhere safe, keep your finger off the trigger until you're ready to fire, and be sure of your target and what's beyond it. If you're new to any of this, your first firearms class isn't a test you can fail. Go take one. Hunting laws and carry rules vary by state and province, so check your local regulations before you head out.

The routine that works

Keep it boring and keep it scheduled. You can't rely on feel.

• Before you leave: 16 ounces of water with breakfast. Coffee is fine in normal amounts, but it doesn't count as your water.
• While you're out: a few swallows every 15 to 20 minutes while moving, or a full cup at every hourly stop. Set a watch alarm if you have to.
• Warm beats cold. A warm drink gets you to actually drink, and it helps you feel human again. Broth, tea, or hot water with a little juice powder all work.
• Eat. Food gives you fuel and some water, and your body burns more in the cold.
• Skip the alcohol. It feels warming, but it makes you lose heat. Save it for the cabin.
• Don't dodge water to avoid peeing. Plenty of people drink less because stripping off layers in the wind is miserable. Plan for that. Pick a spot before you're desperate and go early.

Gear that keeps water liquid

Frozen water is a useless supply. A few pieces of kit, bought once, solve it.

• Insulated stainless steel bottle, 24 to 40 ounces. Roughly $25 to $45. It keeps water liquid for hours and holds a hot drink. Buy a wide mouth, because narrow ones freeze shut.
• Two bottles instead of one big bladder. Hydration bladder tubes freeze first, and you won't know until you're thirsty. If you use one anyway, blow the water back out of the tube after every sip and run the tube inside your jacket.
• Store bottles upside down in the pack, inside it rather than on the outside, so any ice forms at the bottom and not in the lid you're trying to open.
• A small canister stove and a 1-liter pot. A basic stove runs about $40 to $60 and a metal pot $20 to $30. This is how you make hot drinks and melt snow.
• A water filter, kept warm. A squeeze-style filter costs about $40. If it freezes with water in it, the internal fibers can crack and you won't see the damage. Sleep with it or carry it in an inside pocket. Tablets, around $10 to $15 a bottle, make a cheap backup.

Snow is water, but don't eat it

Eating snow pulls heat out of your body and costs fuel to warm it up. Melt it.

Put an inch of water in the pot first. Snow in a dry pot can scorch and taste burned, and you'll drink less of it. Add snow in small amounts. It takes much more snow than you'd think to make a cup, so start early.

If you're collecting from a stream, treat it. Cold clear water can still carry things that will put you on your back.

What it looks like when you're behind

Check these every hour, the way you'd check a fuel gauge.

• Urine darker than pale straw. This is your best signal, and it's free. Some vitamins turn it bright yellow, so know your own baseline.
• Headache, dry mouth, a cramp, or a sudden drop in energy.
• You're short with your partner or can't make a simple choice.

If you see those, stop. Get out of the wind, sit on your pack, and drink something warm and sweet. Eat something. Give it 20 minutes before you decide what happens next.

If someone is confused, stumbling, slurring, can't keep fluids down, or has stopped shivering while still cold, that's an emergency. Get medical help right away, and get them warmer and out of the wind while you wait. This is general information and not medical advice. Your doctor is the person to ask about your own health, medications or conditions.

Take the class

Reading about cold-weather fluid loss is useful. Practicing the response is better. A Wilderness First Aid course runs about 16 hours and a couple hundred dollars. NOLS, the American Red Cross and many local outfitters teach it. You'll handle a cold, dehydrated, confused patient in a field with someone watching, and that beats any article. If you've ever been lost, what you do in the first hour will decide how the rest goes, and a hydrated mind makes that call better.

The last swallow

Fill the bottle tonight. Put it in the pack, upside down. Set an alarm for the hourly cup.

The woods don't care how tough you are. A man who drinks his water at 9 a.m. is sharper at 3 p.m. than the one who skipped it. He's also the one who walks out.