Graham Easterling wrote:

On Saturday, April 11, 2020 at 7:54:34 PM UTC+1, Metman2012 wrote:
On 11/04/2020 16:18, Nigel Paice wrote:
1500Z: 09005KT 12KM 1AC100 18.1/9.5 QFF 1021.5
SYNOP: 42962 10905 10181 20095 40215 57011 81030 333 81360=
Beaufort letters: by
Patchy Ac floccus around this morning, dispersing to zero for a
time before more floccus appeared; currently scraps of thin Ac to
the south. Medium size Cu tops visible NW-NE but too far away to
see their bases. Visibility improving inland but the horizon
remains 'fuzzy' over water. Light breeze from due east. RH 57%.
Max temp so far 19.3

Nigel (Niton, Isle of Wight)
101m amsl

Dependant where you are in Niton it's quite high up (we visited
Niton last year and stayed at the Enchanter Manor).
So viewing the horizon at sea level (well we'll say 1.7m as that
might be one's height) the horizon is 4.5 to 5km away. At 100m
(about Niton's height) it will be ~36km. If the viz is say 20km,
the horizon will 'fuzzy'. The lower you go, the closer the horizon
is and eventually it will be closer than the visibility so it will
then be sharp. In my day (and probably now) coastal observing
sites didn't report visibility over the sea, only landward,
presumably because of this (and lack of viz points. The equation is
=SQRT(m)*3.57 - where m is the figure in meters.

Luckily, there are various places visible from west Cornwall that can
give a good idea of visibility. Scilly is 29 miles from Sennen, quite
useful when I lived there. I have on a few occasions seen Lundy from
the top of the moors, around 80 miles. Lizard point from Penzance is
a useful landmark, 18 miles, if it's clear enough to see the rocks
off the point, then it's probably 30 miles. On very hazy days (like
today) headlands 5 miles away are hard to see.

I find landmarks around the coast very useful in judging visibility,
Google Earth helpful in knowing the exact distance.

Graham
Penzance

Furthest visibility point from our garden is 4km. Even so, we can see
maritime Cb over the Irish Sea and the North Sea. For long range
visibility I rely to some extent on the Cat & Fiddle webcam which looks
NNW across Greater Manchester. On clear nights the lights on the Winter
Hill TV mast near Bolton can be clearly seen.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
https://peakdistrictweather.org
twitter: @TideswellWeathr