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The forecast's not good. North-westerly winds up to 30 miles an hour on the hills ‘will impede steady walking on higher exposed areas’. Risk of one or two flurries of hail and snow on the highest summits.

Mam Sodhail is 1,181 metres (3,862 feet old style) – you don't get much higher. But, if not now, when?

I'm on the track along Loch Affric, north side, at 8.20 in the morning. The sky's a welter of varying shades of grey with fast-moving clouds parting and swiftly closing over scant patches of blue. There's some hope but it's dark and sombre over by Kintail where the weather's coming from. The wind whips up waves across the loch and there's a line of thin foam where they break on the shore.

Seen from a distance, the path ascending the deep-cut glaciated valley appears as a long brown thread. The multitudinous burns are knotted strands of white water. Somewhere above is the hill, invisible as yet in a dense bank of mist. Stray shafts of wan sunlight cast pools of bright green on the hillside with theatrical effect. An embryonic rainbow flickers through the mist.

The way, washed by floodwater, is stony and steep. Loose stones clatter and roll under my feet – such attractive stones they are, rounded and veined and glistening in the wet.

I linger in the corrie, hoping for signs that the mist will clear for a decent interval. A sudden glimpse of the summit is all too brief – the veil falls again but there's sufficient promise to draw me onwards. In a grassy meadow under an abrupt scarp, an infant burn exits from a pool, whence the path climbs steeply in measured zigzags up to the ridge. It's a craftwork of canny engineering from Victorian days when stalkers’ paths were maintained and it still serves. Here Sergeant Winzer and his companions sweated in their serge uniform jackets in the summer of ’48. Here their ponies or mules picked their way up, labouring under loads of timber for building, the heavy theodolite and its associated impedimenta, food and cooking utensils, everything required for their encampment. Was there a regular traffic up and down this track during their occupation of the mountain top or did they hold out, isolated, cut off for the duration? I can only speculate.

Behind me, the glen unfolds below in a long bare corridor reaching towards the loch shore. There's no sign of life on the path though it's visible for most of its length. No other walker has appeared all day. It seems strange to have this hill all to myself when the summer's barely gone.

On the exposed ridge, the wind is less fierce than anticipated, buffeting me and chilling my hands but less troublesome than the forecast implied. It's easy walking now – a low-angled ascent over firm ground littered with stones. A corner of drystane wall emerges from the dimness, the remains of a substantial hut – two chambers linked by a low arch. I have to duck to enter.

The mist clears briefly. Views come and go in swift succession and suddenly the cairn built by the triangulators of ’48 appears on the crest – a squat circular wall like an Iron Age broch broken-off at the top. It's solidly built like some kind of fortification (army engineers knew their masonry defences) and about nine feet high, half its original height. When whole, it must have been awesome.