Anyone who has ever stood frozen at a mountain summit or sweated through a long ascent knows the problem: dressed wrong. The onion principle – also called the layering system – is the solution. Applied correctly, outdoor layering keeps you warm, dry, and comfortable – whether in sunshine or snowstorms.
Layer 1: The Base Layer
The innermost layer lies directly on your skin and actively transports moisture outward. Cotton t-shirts absorb sweat and cool you down – the classic beginner mistake.
The best choice: Merino wool regulates temperature in both directions, naturally inhibits odors, and doesn't itch. Choose weight based on activity:
- Summer / active: Merino 150–200 g/m²
- Fall / trekking: Merino underwear 200–260 g/m²
- Winter / extreme: Merino 260–300 g/m² or double-layered
Layer 2: The Insulation Layer (Mid Layer)
The middle layer traps body heat. This is where fleece or down jackets come in:
- Fleece jacket: Ideal for active hours, insulates even when slightly damp – more in our fleece guide
- Down jacket: Maximum warmth during breaks and summit rest – compact and light in your pack
- Softshell jacket: Combines insulation and light wind protection for transitional weather
Layer 3: The Outer Layer (Shell)
The outermost layer protects against wind and rain. The hardshell jacket is the indispensable all-weather tool.
- Water column: At least 10,000 mm for alpine tours – all details in our water column guide
- Water repellency: PFC-free DWR coating protects the outer material
- Pack size: An ultralight hardshell jacket weighs under 300 g
The Onion Principle in Practice
- Spring tour (5–15 °C): Merino shirt + thin fleece jacket + hardshell in pack
- Fall trekking (0–10 °C): Merino long-sleeve + merino underwear + 200g fleece + hardshell
- Winter hike (-10–0 °C): Merino 260 g/m² + down jacket + hardshell
- Alpine climb: Merino + fleece + down + heavy hardshell
Don't forget the right equipment: how to use trekking poles correctly and how to adjust your backpack properly complete every tour.
FAQ: Onion Principle and Layering
How many layers do I really need?
For most tours, 2–3 layers are enough. In summer, base layer + shell suffices. In winter, all three. The advantage: you can remove layers during the ascent and add them at rest stops – avoiding both sweating and getting too cold.
Can I wear cotton as a base layer?
Cotton is the worst material for outdoor sports. It absorbs sweat and doesn't release it – it feels wet and cold. Always use merino or synthetic as your innermost layer.
What's the difference between hardshell and softshell?
A hardshell jacket is completely waterproof, a softshell jacket is only water-resistant. Softshell is more breathable and flexible – ideal for dry, windy days. In real rain, the hardshell is a must.