When the weather turns on the tops

Most hill days go to plan, and that is exactly why the alternatives catch us out. A turned ankle on a rocky descent, a squall that arrives twenty minutes earlier than forecast, a group slowing to a crawl in wet snow, a broad plateau where the cloud comes down and every feature looks the same — none of these are emergencies in themselves, but any of them can turn into one if you are standing still in wind and rain with no way to get out of it.

Wind is the quiet thief here. Air at 5°C with a 40 mph wind on an exposed ridge feels closer to minus 5°C against bare skin, and that is before rain or sweat-soaked clothing enters the equation. A fit, well-equipped walker can go from "a bit chilly" to genuinely struggling in less than an hour. A group shelter, a bivvy bag and a spare warm layer kept dry in the pack are not luxuries for winter mountaineering only. They are the small, light insurance policy that buys you time while you work out what to do next.

The group shelter: the best square metre on the hill

A group shelter — often called a bothy bag — is a simple fabric bag you pull over yourselves and sit inside. That is the whole idea, and it works extraordinarily well. It cuts the wind entirely, traps body heat from everyone inside, and turns a miserable, dangerous wait into something manageable.

• Sizing: buy one rated for more people than your usual group. A four-person shelter that is genuinely comfortable with three weighs around 300–500g; a twelve-person version is still under a kilo and worth carrying for larger parties.
• Using it well: sit on your rucksacks or mats so you are off the cold ground, put your backs to the wind, and keep one corner or vent slightly open for airflow. Condensation builds quickly, and a sealed shelter soon turns into a warm, wet fug.
• What not to do: do not treat it as a tent and settle in for the night, and do not let everyone fall asleep without someone keeping an eye on the clock and the conditions. It is a waiting room, not a bedroom.
• Storage: keep it at the top of your pack, not buried under your lunch. If you cannot reach it in thirty seconds with cold hands, you will not use it when you should.

Practise getting inside it on a calm day in the garden or at a low-level picnic spot. Groups who have done this once get in fast and calmly; groups who have not fumble for five minutes in the rain, and someone always ends up standing outside getting cold.

The bivvy bag: personal shelter when you are on your own

A group shelter assumes there is a group. If you walk alone, or if someone has to be left with a casualty while others go for help, a bivvy bag is the answer. A simple, breathable hooped or non-hooped bivvy weighs 200–400g and packs down to the size of a flask.

For genuine emergency use, a breathable fabric bivvy is far better than the cheap foil-lined plastic sacks. The plastic versions are better than nothing, but they trap condensation so effectively that you end up wet from the inside, which rather defeats the purpose. A breathable bag used with a mat and a sleeping bag will keep a casualty warm for hours if needed, and it is also the right place to splint a leg or treat a injury without exposing someone to the wind.

Spare warm layer: dry beats clever

The single most effective thing you can carry is a warm, dry layer that has never been worn on the move. A lightweight synthetic belay jacket — the sort with 100–200g of fill — is ideal, because synthetic insulation keeps trapping heat even when damp. Down is warmer for its weight but loses almost all its loft when wet, so if you carry down, it goes in a dry bag and stays there.

Round it out with the small things that make a big difference:

• A warm hat and two pairs of gloves — one thin pair for walking, one thick dry pair for stopping.
• Spare dry socks in a plastic bag, and a spare buff or scarf.
• A sit mat — 60g of foam that stops the ground sucking heat out of you.
• A flask of something hot in winter, and a few hundred extra calories that need no cooking.

Making the call, and making it early

Shelter buys time; it does not replace judgement. The moment to get the group shelter out is when someone is shivering, when hands stop working properly, when the wind is stripping your ability to think clearly — not an hour later when you are all exhausted and the decision has been made for you. Stopping early on a ridge, getting warm and fed, and then walking out by a lower, easier route is not a failure. It is good mountaineering.

Tell someone where you are going and when you expect to be back, carry a torch and a whistle, and know how to give a grid reference. Then carry the shelter, the bag and the dry layer every time, even on a sunny July morning in the Pennines. The days you need them are exactly the days you did not expect to.