Cold is rarely the problem on a hill day. Wet clothing, wind, and being stopped when you are hot are the problems. A layering system exists to manage those three, which is why it is worth thinking about layers as adjustable stages of a day rather than as a shopping list.
The principle is simple: several thin layers, worn in the right order, allow you to change your insulation and protection without stopping for long. A single very warm garment offers no adjustment at all, and it is usually either too little during the ascent or too much on the summit.
The base layer manages moisture, not warmth
The job of a base layer is to move sweat away from your skin and into the layers above, and to feel tolerable when damp.
Polyester and polypropylene base layers dry quickly, feel cool and are cheap. They also hold odour. Merino wool regulates temperature across a wider range, feels comfortable against the skin and resists odour, but it is more expensive, slower to dry, and it wears through faster in high-friction areas such as under a pack harness. Blends are a reasonable compromise.
Fit should be close but not tight, with no folds that will rub under a strap. Cotton in any layer you intend to sweat in is a mistake; it soaks and then holds water against the skin.
One piece of advice that is repeated because it works: start cold. If you are warm and comfortable in the car park, you will be soaked before the first summit. Being slightly cool for the first twenty minutes is normal and it saves the rest of the day.
Mid layers provide the insulation
The mid layer is where the warmth comes from, and it is the layer you will add and remove most often. There are three broad families, and they fail in different ways.
Fleece is breathable, cheap, tolerates damp and dries quickly. Gridded fleeces and high-loft versions move sweat better than the dense fleece of twenty years ago. Fleece is not very warm for its bulk, and wind goes straight through it, which is why it works best under a shell.
Synthetic insulation - short-staple fills in a quilted shell, or continuous-filament fills knitted into a sheet - provides more warmth for the weight, blocks more wind, and holds a useful amount of warmth when damp. It is less compressible than down and loses loft over many seasons, especially at the elbows and shoulders.
Down offers the best warmth for the weight and the best packability, which is why it dominates for camp and for cold, dry conditions. It also loses most of its insulating value when wet, dries slowly, and needs careful storage.
Down or synthetic, in practice
The choice is easier than the marketing suggests, because it follows from how wet the garment will get.
Choose down when the insulation will spend most of its life dry - a belay jacket, a sleeping bag, a cold and dry climate - and when packed size matters. Higher fill power means more loft per gram, which is a real measure: a garment of the same weight will be warmer, or the same warmth can weigh less. Higher fill power also tends to cost disproportionately more, and the benefit shrinks once the shell fabric is the limiting factor.
Choose synthetic when the garment will be worn during hard effort in wet conditions, when it may get rained on, or when you want it to survive many washes without special treatment. It is heavier and bulkier for the same warmth, and it degrades with compression more than down does.
A common and sensible arrangement for wet, cold climates is synthetic for the layer you wear while moving and down for the layer you put on when you stop. That way the wet risk sits with the forgiving garment.
The shell is protection, not warmth
A shell blocks wind and rain, and it traps some warm air as a side effect. Treating a shell as insulation is how people end up cold: the shell is not generating warmth, so under it you still need the right mid layer, and when you stop you still need to add insulation rather than huddle in a jacket that is only blocking wind.
Windproofing deserves its own mention. Wind strips away the thin layer of warm air around your body, which is why a modest fleece under a windproof feels far warmer than the same fleece in the open. A light windproof layer is the most underrated item in the system, and it weighs almost nothing.
Adjusting through the day
Layering only works if you actually use it, which means stopping before you need to.
- Add a layer, or open vents, before the steep section rather than after you have started sweating.
- Open a zip down the front and open the cuffs when you begin to feel warm, rather than removing a layer.
- At every stop, add insulation immediately. Cooling down in wet clothing is where most day walks go wrong.
- Keep one dry layer, ideally the insulation, in a dry bag. Its job is to be worn when everything else is wet, at the end of the day or in an emergency.
Gloves, a hat and a neck gaiter belong to the system too. Extremities lose heat fastest, and a hat or a hood changes how warm the rest of your clothing feels within seconds.
Ratings, and why we ignore them
Garments increasingly carry comfort ratings - a temperature, a warmth level, a season. These are not produced by a single standard method, and they cannot account for your effort level, the wind, your dampness or your own metabolism. Treat any single figure as a marketing label rather than as information, and compare garments by loft, fabric, fill weight and construction instead.
The test of a layering system is not how it performs in the shop. It is whether you can climb a long ascent without soaking your base layer, stop for ten minutes in the wind without shivering, and finish the day with one layer that has stayed dry the whole time. Everything else is detail.
