Can I just wear one heavy jacket instead of layers?

No. You will overheat while hiking up a steep incline and freeze once you stop to take a photo. You need to be able to peel off and add layers throughout the day.

Do I really need waterproof pants?

Yes. When the wind picks up and the rain starts, it often comes at you sideways. Waterproof pants are essential to keeping your base layer dry.

How to layer for Patagonia's unpredictable weather

You need a three-layer system to handle Patagonia's 'four seasons in one day' weather. Start with a moisture-wicking base, add an insulating mid-layer, and finish with a windproof and waterproof outer shell.

Select your base layer. Wear a synthetic or Merino wool long-sleeve top and bottom. Avoid cotton entirely, as it holds sweat and will make you cold the moment the wind hits you.
Add an insulating mid-layer. Use a fleece jacket or a lightweight down 'puffy' jacket. This traps your body heat. Ensure it is slim enough to fit comfortably under your outer shell.
Choose your outer shell. Invest in a high-quality, seam-sealed GORE-TEX (or equivalent) jacket. The Patagonia wind is aggressive; your jacket must be 100% windproof, not just water-resistant.
Prepare your extremities. Bring thin glove liners and a warm beanie. Even in summer, you will want these when you stop for lunch at a high-elevation pass where the wind chill is significant.

Can I just wear one heavy jacket instead of layers?: No. You will overheat while hiking up a steep incline and freeze once you stop to take a photo. You need to be able to peel off and add layers throughout the day.
Do I really need waterproof pants?: Yes. When the wind picks up and the rain starts, it often comes at you sideways. Waterproof pants are essential to keeping your base layer dry.