Distance of horizon in miles = 1.22 * square root of height above horizon in feet, for heights which are small in comparison to the radius of the earth. If you're standing on one hill and looking for another hill, add the two distances together e.g if you're on a 1600-foot hill then in optimal conditions you should be able to see a 2500-foot hill (40*1.22) + (50*1.22) miles away = 110 miles. Anything above this is a mirage or cloud.
 
Or from the beach near here, you should be able to see the top of a 30,000-foot cumulonimbus 200 miles away i.e well into France

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From a point on the South Rim of the Grand Canyon you can see Navajo Mountain some 90 miles (145 km).

On a crystal clear day you can see Lundy Island from Clevedon Pier - 127km.

Also, there's a point on the M5 just south of Weston-Super-Mare where you can see the Malverns - 100km.

When I was on the Balcon De-Europe in Nerja on the Costa Del Sol I could see North Africa, not sure how far that would be, some of the storms I could watch over North Africa also which was pretty amazing.

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Almost exactly 100 miles coast to coast, measuring on Google Maps 

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I often see Mont Blanc from the Maconnais and Beaujolais hills, for example from the Roche de Solutre. According to Kevin's link it's 197km from Mont de St Cyr so take off 20 or so and that means I'm looking a good 175km plus. It's clearest in winter when there is an inversion layer with mist and we are above.

But from those viewpoints I can sometimes also see the Vanoise and Ecrins massifs. That would be well over 200km, whatever that is in miles.

There is a wonderful moment in Nicholas Crane's book clear waters rising, where he is standing on one of the high points of the Cevennes. To the North East he can see the wall of the Alps and Mont Blanc. To the South West beyond all of the peaks of the Southern Massif Central he can make out Canigou. There can't be many more extreme examples out there than this one.

Do views from planes count? I once had a stunning view from somewhere above the Cotswolds all the way down the South West peninsula to Lands End, through Wales to Pembrokeshire and across the Irish Sea. On a Ryanair flight no less.

Those panorama links Kevin posted are amazing!

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Sicily seen from Malta about 60 miles away



A letter by Victor Gatty of Preston, Lancs written on the 2nd October 1921

"September 3rd 1921 was a day of exceptional clearness. The view from the summit of the tower at Blackpool (about 500 feet high) was remarkable, not only from the extent of country covered but also from the immense amount of visible detail at great distances. It was exceptional, too, in that the view was equally in every direction.
The Isle of Man, visible from the promenade only as a series of deatched islets, stood high out of the water 70 miles away and dark blue. The hills of Wales, 60 to 70 miles away, were equally clear.
To the east the view is more restricted, but to to the north, the hills of the Lake District were visible in great detail; every mark and gully on Black Combe, 33 miles away, could be seen with the naked eye and Saddleback, 57 miles away, was clearly distinguishable through the gap of Dunmail Raise. Further east of Howgill Fells, which rise to the north of Sedbergh in Yorkshire could be made out.
To mention smaller details, Great Orme's Head, above Llandudno, 48 miles away, could be recognised and the white wall of the lighthouse enclosure on its northwest point could be made out with glasses; with the same aid the colours of fields and woods nearer Welsh coast, 40 miles away could be distinguished; the buildings in Liverpool and the top of the transporter bridge over the Mersey at Runcorn, 35 miles away, were also visible.
Northwards, even from ground level, the colours of cornfields, pastrues and woods on the nearer hills, some 27 miles away beyond Morecambe Bay and the purple of the heather above could be made out with the naked eye.
From the tower with glasses, I could recognise the tower on Eller Horn, a hill to the northeast of Grange-over-sands, 29 miles away and could see the houses in Grange itself.
A little to the west of Black Combe, an isolated islet rose dark blue from the sea. This I took at the time to be a hilltop in Scotland but the map showed it to be the top of St Bee's Head, the most westerly point of Cumberland, 52 miles away.
I have only mentioned some of the more distant features; a view which included a great deall of detail in 9 different counties and the whole of the coast line from Llandudno round to Barrow is so exceptional that it seems worth recording. Probably visibility was at its maximum and everything which the sphericity of the earth allowed to be seen was distinguishable."

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Have a fair idea of the general height of each cloud type, although this of course depends on the upper and surface air mass and what the season is. EG, cloud bases in summer tend to be higher than that in winter and so forth.

 

Also, looking at satellites help. Could see a very distinct anvil cloud this evening, with what looked like an brief overshooting top, that was located off the SW coast of Donegal, all the way from South Connacht!

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I took this photo from the slopes of Mount Tongariro in New Zealand: that's Mount Taranaki, 134km (83 miles) away.

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It looks much closer.

Saying that, when we were driving out of Calgary toward the Rockies, I was struck by just how close they seemed, when in reality they were around 40/50 miles away. I was gobsmacked with the enormity of them.

 

 

Stood on Great Gable, I've seen Scotland & Man  (lovely mountain, is Great Gable)

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I also saw this one on holiday in Utah

The low humidity in the desert climate lets you see further I imagine

This is Navajo Mountain from Bryce Canyon about 100 miles away on the Arizona border

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