There’s a tipping point on every autumn sailing when the pool is still open, the sun loungers are still neatly lined, and yet everyone onboard knows the jig is up. Summer has quietly left the building. Suddenly, the ship is full of people who look as though they’ve stepped straight out of a National Trust car park: layers, sensible shoes, and a faint determination to enjoy themselves whatever the weather throws their way.

This is the moment we call jumper weather on deck. It’s not a bad thing, in fact, it might be the most underrated season at sea. The air is fresher, the views sharper, and you can finally order hot chocolate without guilt. But the signs are unmistakeable. Blankets multiply, the solarium becomes prime real estate, and deck chairs take on the chill of a church pew in February. Spotting the shift is a sport in itself, and embracing it can make for some of the cosiest days you’ll ever spend at sea.

The wake is frothing, the sky is cinematic, and the pool looks wonderful from a respectful distance. Here are the tell-tale signs it is officially jumper weather on deck, with a few calm-headed tips to keep you cosy...

1) The blanket pyramid appears

It starts as a tasteful basket. By mid-morning it is a woollen cairn. Veterans drift past, weighing thickness and pattern as if their entire holiday rests on choosing the right one. By lunchtime, possession looks permanent: a blanket across the knees is an unspoken “this lounger is mine until further notice.” If you see someone carrying two, they are either kind or selfish. Assume the latter and get your own.

Smart move: stash a thin scarf in your pocket or handbag. It works as a headscarf, neck warmer or an extra wrap when the breeze getting brisk ideas.

2) Hot chocolate develops a waiting list

It starts with one person asking for whipped cream and suddenly the entire ship remembers cocoa exists. The queue forms without discussion, a gentle snake of knitwear and good intentions. Marshmallows become a moral question. The barista stops writing names and switches to nods. By mid-afternoon the whipped cream to cup ratio has increased exponentially. Coffee loyalists defect mid-queue, muttering something about antioxidants.

Smart move: slip in just after lunch or mid-morning once the latté crowd has gone. Bring a reusable cup with a lid and you have both a winter warmer and a hand heater for the walk back to your lounger.

3) The solarium becomes the hottest ticket on the ship

Outdoors is for photographs. Indoors is for unadulterated year-round relaxation. By ten o’clock the solarium is a quiet, contented microcosm of humanity at rest: novels face down on chests, headphones at whisper volume, towels tucked with the precision of hospital corners. The air is soft, the pool is politely warm, and every available lounger now has the legal protection of a cardigan. Outside, heroic souls attempt a brisk lap of Deck 12 and come back looking like they've just stepped out of a wind tunnel.

Smart move: stake your claim at odd times. Right as lunch opens you might find two seats together without negotiating. If it is wall-to-wall towels, try the observation lounge or a quiet bar with big windows (similar sunshine minus the humidity).

4) Deck chairs develop a metallic chill

They look inviting from a distance, all neat lines and sunny promise. Then you sit down only to realise the frame is conducting temperatures straight from the North Atlantic. Crew might try to soften the blow by layering every lounger with towels, so the pool deck begins to resemble a patchwork quilt. Meanwhile, the jogging track suddenly feels busy: apparently the only way to stay warm is to keep moving, and the walkers circling in fleeces wear the satisfied expressions of people who’ve outsmarted the elements.

Smart move: pick a chair on the sheltered side of the ship, ideally with something solid blocking the wind. A folded jumper works as an impromptu seat pad, and a hot drink buys you another 20 minutes of outdoor dignity.

5) The daily schedule begins to embrace the indoors

The programme reads like a rainy-day wish list. Trivia becomes competitive sport. The lecture you meant to skip turns out to be oddly fascinating. The spa’s thermal suite sells out as if it were a concert, and the matinee gets a crowd that claps on the beat. The captain’s noon update includes words like fresh and lively, which is maritime code for put on a layer and come inside.

Smart move: prebook the one thing you will kick yourself for missing, then keep the rest of the day soft around the edges. Claim a window table in a lounge and let the weather be the show while you win a quiz by knowing something obscure about lighthouses.

The honest verdict

Jumper weather is not a downgrade. It is permission. Permission to read for entire afternoons. Permission to order hot chocolate as if it were a medical necessity. Permission to watch the sea in its moodier outfits and claim the solarium like a cat. Pack the right layers and you will come home with cheeks the colour of ship’s flags and a phone full of blue-grey horizons that look suspiciously like peace.