A proper mountain peak is, of course, one that you could fall off on any side. As the high point of the Cuillin, Sgurr Alasdair certainly meets that description, says self-confessed exposure-phobe Simon Ingram, in this extract from his new book The Black Ridge: Amongst the Cuillin of Skye.

The thing that worried me about Sgùrr Alasdair was its summit. One of the old local names for this peak above Coire Làgan was Sgùrr Biorach, which literally means 'Pointed Peak' – an interestingly specific christening in the context of the Cuillin ridge, where everything is pointed. It made me wonder what it was about this one that deserved to be singled out. Lots of descriptions referred to its top as being 'airy', 'compact', 'tiny' even – but these terms could mean vastly different things from person to person. It's only when someone pins a recognisable quantity to a summit that you get a true measure, but people rarely do, despite it being, to me at least, the immediate question that springs to mind when considering an ascent of a mountain. (As an aside, I once asked an Everest summiteer to describe the very highest point on the planet in relatable terms; his response, after a long pause, was that it was like standing on an upturned bathtub.)

When it came to Sgùrr Alasdair, for me it was a description in W. Kenneth Richmond's Climber's Testament, where he wrote of this summit that 'a point it undoubtedly is: no room for a doll's house.' Now, I don't know how big a doll's house was in 1950, but the footprint of the one in my house isn't of a size I'd comfortably stand on atop a summit with drops of a thousand feet all around. J. Hubert Walker – I'm not sure what it is with these mountain authors with their keenness to abbreviate their first name – described mounting his camera tripod on the summit and finding himself unable to walk around it. 'The top of Alasdair is very sharp indeed,' he wrote. 'There is barely room for two people to sit down.' For someone trying to negotiate the Cuillin and manage a keen fear of exposure at the same time, I don't mind saying that I didn't like the sound of this place one bit.

A chasm had opened beneath us as we climbed out of the chimney. A long way down it, I could see a snake of corries leading down towards Coruisk. It was dizzying, but the footing was good, and before long I was comfortable enough to start enjoying and scaring myself just a bit. This was apparently the point, after all. I could sense the weather was breaking, possibly badly. But just for now I didn't want to think about it. Matt was here, he knew the ridge, could judge the weather and would say if we needed to change tack. You trust your guide.

Above Matt the chimney kinked a right, and as I came up beneath him a tickle of air on the back of my neck indicated that we were on the ridge once again. Soon the skyline began to lower towards us as Matt led the way up steep, broken ground, this way and that, up corners and steps; pathless, but secure. Cloud had begun to swirl around us, masking the space below my feet. Matt had stopped, and I found myself looking at his ankles and listening to my own breath. The rock had changed from gritty and black to smooth edges and blocky breaks, with tinges of green lichen. I knew what that meant.

'This is a basalt summit,' Matt said. I looked up, and he appeared as a black hole against a sky bright and filled with cloud vapour alive on the wind. 'It's been struck by lightning a lot. So it might be slippy, there might be some loose bits, so. . .' He stopped short of saying, 'Be careful,' as there really was no need.

He began to move again, and I followed before the rope went tight. And then very quickly the last block underfoot became the last block on the mountain. We stepped out onto the highest rocks of the Cuillin ridge, of Skye, and the Hebrides.

The clouds were thin and painfully bright. Behind them I could see the ghosts of mountains, shadows beyond the grey-white. There was no real view, just impressions of shapes: slender ridgelines, steep drops, the occasional glimpse of a summit, everything white and deep. It could have been three thousand feet high or twenty thousand feet – there was no visual anchor, just us, the summit, the cloud and its shadows. This mountaintop, 3,255 feet above the sea, was a weather-sharpened point in silhouette, hard- angled and solid, against empty air. The summit of Sgùrr Alasdair, as I had read, and feared – fell away briskly on all sides. Slip into a fall, and you'd die. But it wasn't a fearful moment. For the first time that morning, the air seemed motionless. And I was grateful to discover that there was in fact a secure place to stand, and a bit of rock to hold on to. Somewhere to just stop, and soak it in.

We were in a little dip, like a gap in a stone wall, between two outcrops. To the right, perhaps ten feet away, was a rough apex. One side was a sloping slab tattooed with white lichen and furry with moss.

Matt took out his water bottle, wandered slowly forward and stood on the platform. He took a long drink, replaced the lid, then placed the bullet- shaped bottle on the very top rocks. I didn't know if it was a conscious thing, but as he did it the mountain suddenly had an informal little summit marker – a shape that visually drew together the slopes of the rocks either side into a human-augmented point. Matt, his feet below but head above, stood there for a moment. Beyond him, far off in the distance, the sun burned through the cloud and lit the sea in bright gold lashes. The scene was completely other – one of those feelings of unreality you sometimes get in the mountains. I thought back to the times I'd seen Alasdair from afar. Then I looked at the mossy rock underfoot and the summit just there, and thought: I'm on top of that. It was a moment. We didn't speak.

And then, just as quickly, it faded. The invisible sun dimmed. The shadows of the mountains around and beneath lost their veil-like appearance and disappeared into the grey. Matt looked around, picked up his bottle, and the summit was once again a naked, unadorned top. His movements had lost their unhurried slack, and became purposeful and brisk.

'Time to go,' he said. The sky had darkened, as if somewhere a huge pair of curtains had just been swept shut and – with frightening speed – bright white became a heavy, cindery grey. In less than thirty seconds, we were on a different mountain, on a different day.

• The Black Ridge: Amongst the Cuillin of Skye by Simon Ingram is published by William Collins.