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.