There is a particular kind of northern hill day that begins in warm sunshine, ends in horizontal drizzle, and passes through a tarn so inviting you simply have to get in. Packing for that day means accepting that the weather will probably do three different things before teatime. A modest 25-litre rucksack, properly filled, weighs very little and turns a good day into a safe one.

Start with a Rucksack You Barely Notice

For a summer day on the fells, something in the 20–30 litre range is about right. Big enough for layers, food, water and a towel; small enough that you do not fill it with nonsense. Look for a padded hip belt and a lid pocket, and give everything inside a home, because the moment you need your waterproof trousers is not the moment to be rummaging.

Pack soft items against your back, heavier items in the middle, and the things you might need while walking — hat, snack, map — in the lid or side pockets. Line the whole thing with a dry bag or a couple of lightweight stuff sacks. Even a "waterproof" rucksack will let water in through the seams in a proper downpour, and a wet spare fleece is no use to anyone.

Waterproofs: Non-Negotiable, All Year

A waterproof jacket and trousers live in the bag from April to October without exception. Not a shower-resistant windproof, not a poncho that flaps itself into a flag, but a genuine waterproof with taped seams. Northern Britain specialises in rain that arrives sideways with no warning, and on an exposed ridge there is nowhere to shelter while you change your mind.

Check before you leave that the jacket still beads water rather than soaking it up like a flannel, and that the trousers fit over your boots easily. If you are heading anywhere near a tarn for a swim, waterproof trousers also make an excellent, if slightly undignified, changing mat and windbreak.

Warm Layers and the Wild Swim Kit

This is where hill walking and wild swimming in the uplands overlap. The tarn you have been eyeing all morning, glassy and peaty-brown, will be several degrees colder than you expect. Standing around afterwards in wet shorts is how a lovely afternoon becomes a shivering one.

• Spare warm layer — a fleece or lightweight synthetic jacket, kept dry in a stuff sack.
• Spare base layer — a dry thermal top makes a huge difference after a swim.
• Hat and gloves — a thin beanie weighs nothing and saves an evening.
• Microfibre towel — small, quick-drying, packs to the size of a fist.
• Swim kit and a dry bag — so wet trunks do not soak everything else.
• Swim cap and goggles — a cap keeps the cold out of your ears and hair.
• Light shoes or sandals — tarn beds are stony, silty and sometimes unexpectedly deep.

Enter tarns slowly, stay shallow, and get out before you feel properly cold rather than after. A flask of hot tea waiting in the rucksack is one of the great small luxuries of the northern hills.

Food, Water and a Little Slack

Take more than you think you need. Hill appetite is sneaky, and a steep pull up a scree path burns through anything you ate at the car park. Aim for a mix of slow-release and quick-energy food: oatcakes, cheese, a pork pie, nuts, dried fruit, a couple of chocolate bars, plus a sandwich or something substantial for lunch. Keep a boiled sweet or a cereal bar in your jacket pocket for the last climb.

Water is the one thing people under-pack most often. Carry at least a litre and a half for a half-day in warm weather, more on a long route, and remember that tarn water should be filtered, boiled or treated before you drink it, however clean it looks. A small flask of hot drink counts towards your fluid intake and lifts the spirits on a cold, misty summit.

Navigation and the Headtorch You Hope Not to Use

A paper map and a compass, plus the knowledge to use them, are still the standard on the fells. Phone batteries die in the cold, screens get wet, and signal vanishes in the valleys. Fold the map into a clear case so you can read it without stopping, and keep the compass on a lanyard around your neck rather than buried in the bag.

A headtorch goes in every day bag, even in June. A twisted ankle, a slow group, a sudden bank of cloud, and a three-hour walk becomes a five-hour one. A torch weighing under a hundred grams is cheap insurance. Add a whistle, a small first aid kit with blister plasters, and a foil emergency bag, and you have covered most of what a day in the hills can throw at you.

The Final Shake-Down

Before you lock the car, run through it once: waterproofs, spare warm layer, food, water, map, compass, headtorch. Everything else is comfort and pleasure. Then hoist the bag, tighten the hip belt, and walk out towards the ridge, knowing that whatever the weather and the tarns decide to do, you are ready for it.