Why is cotton bad for the Andes?

Cotton dries very slowly and loses its insulating properties when damp. In the Andes, sweat or sudden rain will make you dangerously cold if you are wearing cotton.

Do I need a heavy parka?

No. A heavy parka is cumbersome to pack and difficult to vent. A thin base layer plus a down puffy and a wind shell is much more versatile.

How to Pack Layers for Andes Mountain Weather

Pack a three-layer system consisting of a moisture-wicking base, an insulating mid-layer, and a waterproof, wind-resistant outer shell. This allows you to adjust to the rapid temperature swings between the freezing mornings and the intense, sun-drenched afternoons.

Select a synthetic or merino wool base. Avoid cotton entirely, as it traps moisture and cools your body down once you stop moving. Bring two long-sleeve merino wool tops that you can rotate.
Pack a high-loft insulation layer. Choose a down or synthetic puffy jacket that packs into its own pocket. This is your primary defense against temperatures dropping below 32°F (0°C) at night.
Bring a technical wind-shell or rain jacket. The Andes are notorious for sudden, high-velocity winds. Ensure your outer shell is seam-sealed and features a hood with a cinch cord.
Test your layering sequence. Put on all three layers together. You should be able to move your arms freely without the jacket fabric bunching or restricting your range of motion.

Why is cotton bad for the Andes?: Cotton dries very slowly and loses its insulating properties when damp. In the Andes, sweat or sudden rain will make you dangerously cold if you are wearing cotton.
Do I need a heavy parka?: No. A heavy parka is cumbersome to pack and difficult to vent. A thin base layer plus a down puffy and a wind shell is much more versatile.