Why a small bag matters more than you think

There is a particular British optimism that says a two-hour walk round a reservoir does not need a rucksack. Usually it is vindicated. Occasionally it is not, and that is the day you remember: the sky turning a bruised grey, the wind picking up, and the temperature dropping several degrees in the time it takes to reach the far side of the valley. British weather does not do dramatic. It does sudden.

The hills exaggerate everything. Air cools by roughly one degree Celsius for every hundred metres climbed, so a modest 900-metre summit can sit six or seven degrees colder than the car park — and that is before wind chill, which on an exposed ridge can make a mild spring day feel like February.

None of that is a reason to stay indoors. It is simply a reason to carry five kilograms of well-chosen kit, even on a low-level walk you have done a dozen times before.

Build your layers before you think about waterproofs

The old advice still holds: several thin layers you can add and remove beat one thick jacket every time. You want to feel slightly cool when you set off — you will warm up within ten minutes of walking.

• Base layer: a synthetic or merino top that moves sweat away from your skin. Leave the cotton t-shirt at home; once cotton is wet it stays wet and pulls heat out of you.
• Mid layer: a fleece or light synthetic jacket for warmth when you stop or the cloud comes in.
• Shell layer: a waterproof jacket with taped seams and a hood that fits over a hat.
• Spare warm layer: a small synthetic insulated jacket lives permanently in the bottom of my bag. It weighs very little, works even when damp, and turns a miserable lunch stop into a pleasant one.

Waterproofs, and why July is no excuse

A waterproof jacket and waterproof trousers belong in every day bag from October to March, and in most bags for the rest of the year too. Look for taped seams, a hood with a decent peak, and a fabric rating of around 10,000mm or better. Trousers are the piece people leave behind and then regret: wet walking trousers will chill you for hours, and full-length side zips let you pull them on over boots without stopping to undress.

Inside the rucksack, line everything with a dry bag or a thick plastic liner. Waterproof jackets leak at the seams eventually, and a soaked spare fleece is no spare fleece at all.

Water, food and a hot drink

Carry more water than feels necessary — around a litre for a half-day, two or more on a full day in the hills, more still in a heatwave. It is easy to become dehydrated in cold wind because you never notice yourself sweating.

Food should be calorie-dense and eaten before you are hungry. Flapjack, oatcakes, nuts, cheese, a decent sandwich, something salty. A small flask of tea or soup is a genuine morale item on a cold, damp day and weighs far less than most people imagine. Keep one sealed high-energy item at the bottom of the bag as an emergency ration; you will probably never need it, which is exactly the point.

The kit everyone forgets

• Map and compass — a paper map does not run out of battery, and phone signal in the hills is patchy at best.
• Headtorch with spare batteries. Dusk arrives faster than any of us plan for.
• Whistle and an emergency shelter — a lightweight survival bag or bothy bag costs little and weighs almost nothing.
• Spare hat, gloves and socks, kept dry in a sealed bag.
• A small first aid kit: blister plasters, tape, painkillers, any personal medication.
• Sun cream and sunglasses — you can burn badly on a cloudy day above the treeline.
• A charged phone in a waterproof pouch, and a note of your route left with someone at home.

If something does go wrong, dial 999 or 112 and ask for the police, then mountain rescue. In poor signal a text message sometimes gets through when a call will not, so register your phone for emergency SMS before you go.

Packing it so you will actually carry it

A 20 to 25-litre rucksack is ample for a British day walk. Aim for five to seven kilograms including water: heavy enough to be useful, light enough that you forget it is there.

Put the heaviest items close to your back and high up. Keep waterproofs and a warm layer near the top where you can reach them without unpacking, and stow the emergency kit at the bottom so it is always there. Clip the whistle to a shoulder strap where your hand falls naturally.

The real trick is doing this the night before. A bag packed in the hallway, checked once, is a bag that goes with you. And on the day the weather turns — because it will, eventually — you will be the one still smiling at the far end of the valley.