Will my phone die in -40°F?

Yes. Lithium-ion batteries drop dead almost instantly in that cold. Keep your phone in an inside pocket against your body heat and use an external battery bank stored inside your jacket.

Should I wear cotton?

Never. Cotton absorbs moisture from sweat, stays wet, and causes rapid heat loss. Once cotton gets wet in -40°F, you are at immediate risk of hypothermia.

How do I prevent frostbite on my face?

Use a balaclava or neck gaiter that covers your nose and cheeks. Apply a non-water-based moisturizer or heavy petroleum jelly to exposed skin to provide a barrier against wind.

How to Dress for -40°F Survival

To survive -40°F, you must wear three distinct layers: a moisture-wicking base, a thick wool or fleece middle layer, and a windproof, waterproof outer shell. Never wear cotton, as it traps moisture and will freeze against your skin.

Master the Base Layer. Wear 250-weight merino wool or synthetic thermal leggings and a long-sleeve top. This layer must fit tight against your skin to trap body heat while moving sweat away from your body.
Add Mid-Layer Insulation. Add a thick, chunky wool sweater or a heavy-duty fleece jacket. If you are extremely sensitive to cold, add a lightweight down vest over this.
Seal with Outer Shells. Your outer layer must be windproof and waterproof. Use expedition-weight down parkas with a fur-lined hood to prevent frostbite on your face. Wear insulated ski pants or bibs as your bottom layer.
Protect Extremities. Wear a thin liner glove inside a heavy-duty insulated mitten. For your feet, use wool socks and boots rated for at least -50°F; ensure they aren't too tight, or you will restrict blood flow.

Will my phone die in -40°F?: Yes. Lithium-ion batteries drop dead almost instantly in that cold. Keep your phone in an inside pocket against your body heat and use an external battery bank stored inside your jacket.
Should I wear cotton?: Never. Cotton absorbs moisture from sweat, stays wet, and causes rapid heat loss. Once cotton gets wet in -40°F, you are at immediate risk of hypothermia.
How do I prevent frostbite on my face?: Use a balaclava or neck gaiter that covers your nose and cheeks. Apply a non-water-based moisturizer or heavy petroleum jelly to exposed skin to provide a barrier against wind.