Why layering beats one big jacket

British hills are brilliant at changing their mind. You can leave a valley at midday in warm sunshine, shirt sleeves and midges, and be leaning into a wind that carries rain sideways within the hour. No single jacket copes with that range, because the moment you start climbing you overheat, and the moment you stop you go cold in your own damp. Layering solves this by letting you adjust in small steps rather than all or nothing. You regulate your temperature instead of enduring it, and you arrive at the tarn or the bothy far more comfortable than the person who set off in a heavy coat.

The system, in order

Three layers do the real work: a base layer next to your skin to move sweat away, a mid layer to trap warmth, and a shell to keep wind and rain out. Everything else — the spare fleece at the bottom of the rucksack, the synthetic jacket for stops, the dry clothes waiting in the bothy — is an addition rather than a replacement. The system works because each layer has one job, and you can shuffle them independently as the weather shifts.

• Base: thin, close-fitting, wicking. Merino or synthetic.
• Mid: fleece or light synthetic fill, easy to pull off without taking your pack apart.
• Shell: windproof and waterproof, with a proper hood and pit zips.

Base and mid: managing sweat and warmth

Most people who feel cold on a hill are actually wet. Cotton is the classic mistake: it soaks up sweat and holds it against your skin, so the first gust on an exposed ridge chills you fast. A thin base layer moves moisture outwards, and two thin layers will often beat one thick one, because you can shed half the warmth without stripping off.

For the mid layer, a light 100-weight fleece is the workhorse for moving. It breathes well, dries quickly and weighs almost nothing when stuffed in a side pocket. For stops and summit sandwiches, something thicker — a 200-weight fleece, an Alpha-style layer or a light synthetic jacket — adds a surprising amount of warmth for very little bulk. A gilet is underrated if you run warm but your core complains the moment you stop.

The shell: wind, rain and breathability

Even on a dry day, wind is the biggest thief of warmth on a summit. A windproof shell can add the equivalent of several degrees, which is why so many people carry a lightweight one in summer as well as a full waterproof in winter. Look for taped seams, a hood that will sit over a hat without drowning your face, adjustable cuffs you can cinch with cold hands, and pit zips you can open on the climb and close on the descent.

The knack with a shell is to use it as a vent, not a coffin. Open the zips and the front when you're working hard uphill; close everything down when you turn into the wind or drop onto an exposed col. A shell worn fully zipped on a steep pull will leave you as wet inside as the rain would have made you outside.

Adjusting on the move

The old saying — be bold, start cold — is worth trusting. Set off slightly cool and add a layer within the first ten minutes if you need it, rather than starting warm and stewing. Beyond that, a few practical habits make the biggest difference:

• Add before you need it. Layer up at the bottom of a summit push or a windswept col, not once you're shivering on top.
• Strip before you sweat. If you're about to climb for twenty minutes, take the mid layer off first and put it back on at the top.
• Use stops as reminders. Any break longer than two or three minutes is a cue to add a layer and pull your hood up.
• Keep your spare layers accessible. A mid layer buried under a rolled mat is a mid layer you won't use.

Dry layers for tarns, bothies and camp

The layers you walk in should never be the layers you sleep in. Pack a small dry bag with a spare base layer, dry socks and a warm synthetic jacket, and treat it as sacred — it's your insurance if the weather turns properly foul or someone in the group gets cold. Synthetic fill keeps working when damp in a way that down does not, which matters enormously in a bothy on a wet November night.

If you're swimming in an upland tarn, get dry and dressed quickly, out of the wind if you can, and pull on a warm layer before you start rummaging for the stove. Windchill after a dip is far sharper than the water temperature suggests. In a bothy, hang wet kit near the fire, keep damp things away from your sleeping bag, and accept that yesterday's base layer may still be clammy at breakfast. Rotate between two base layers if you can — it's a small luxury that transforms a multi-day trip.