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Horizon Astronomy 6

Unaided long-distance observing and weather.

1.6.1   Some critics of Alexander Thom's work do not accept that the unaided eye can detect details on a distant horizon further than 30 miles (48 km) or so, believing that lack of resolution and opaque atmosphere cannot allow reliable seeing over this distance. However many years of photography and observation in the high ground of Wales and Scotland have shown that horizons at more than 50 miles (80.5 km) may be identified on many days of the year and, under special weather conditions, detail may be visible at these distances even to the inexperienced observer. From the higher parts of Llananno and Dolfor parishes in Mid Wales regular sightings, often simultaneously, are made of the Brecon Beacons in South Wales at 45 mls, (72 km), and Mount Snowdon in North Wales at 55 mls (88.5 km). 1.6.2   Visibility. Obvious conditions for good seeing are present during cloudless and mist free weather but several other patterns of local conditions may also give rise to crisp viewing at long distances. Obstruction to visibility in the atmosphere is caused by particles suspended in the air. These particles are either water droplets or fine solids as dust and often a mixture of both. In the west of Britain, where the prevailing weather movement is west-to-east, industrial pollution is a near negligible factor but naturally occurring vapour and dust are not. Sequences of weather which enhance visibility. 1.6.3   Dust. In dry, sunny conditions accompanied by steady breezes the highlands of Britain will dry very quickly due to steep drainage profiles and thin soil and clay cover. Conditions for the raising of dust into the air may result in less than a week after a prolonged period of rainfall allowing the build up of a dust haze in the lower atmosphere. In rural Wales this fine dust is being raised by wind and animal and vehicle movements and may occur in winter also when fine open weather with below freezing temperatures freeze-dries soil surfaces. This dust layer will persist until it is laid and dispersed by rainfall and wind.

Moonset on Drygarn Fawr at 17.7 miles, (28.5 km). For more on this alignment see S1. Llananno/Drygarn Fawr.

Dust haze may soon reduce visibility to less than 30 miles. Several days of showers or continuous rain will lay this dust. During the several days drying out period, after rain, evaporation of water from damp vegetation can also cause greatly reduced visibility but there is usually a window of extreme clarity when evaporation stops and before dust begins to rise again. Here is a photograph taken during a fine example of this type of weather window which occurred during the prolonged drought of 1989. The long spell of fine weather persisted into August and had been punctuated by a week of light showers. The wet weather had, in effect, rinsed the dust from the air and ceased 5 days before this photograph was taken. The drought had re-established and rapid evaporation under sun and wind had reduced visibility for these 5 days but, on the night this photograph was taken, the vapour haze had disappeared and this extreme clarity prevailed for about two days before dust haze again arose. This weather window gave a fine opportunity to catch the Moon at it's monthly southerly extreme.

1.6.4   Frost. Sudden frost after a prolonged spell of wet weather may cause another weather window offering high visibility conditions. This photograph was taken in early November after several days of rain followed by the first sharp frost of the season. The prolonged rain had again rinsed the atmosphere of dust then open weather followed overnight with a sudden drop in temperature which laid the moisture content of the air as hoar frost on the ground giving an exceptional clarity at sunrise. The hues of the green flash, only visible in very clear conditions, persisted in the solar disc as a bluish tinge.

Rhoscrug II at 5.5 mls, (8.8 km),

1.6.5   Heavy rainfall. The western parts of Britain are well known to experience many times the annual rainfall of the east and the resultant greater cloud cover and occasions for mists and fog would be expected to reduce considerably the number of clear days of high visibility. However experience shows that periods of exceptionally high visibility are often, if not usually, associated with preceding rainfall. From Llananno details on Mt. Snowdon at 55 miles may often be as discernable after heavy rain as during bright, sunny weather.

1.6.6   Very long-distance seeing. This photograph was taken on an evening in August after a day of torrential rain in Cumbria. The rain ceased shortly before sunset and a break in the cloud cover over the Irish channel afforded back lighting to several far distant features visible to the unaided eye.

	Blakeley Raise stone circle , Ennerdale Bridge, Lake District. The Sun is within 8 arc minutes of the ideal declination for Megalithic Calendar Intervals nos.2 & 8. This stone circle may have been positioned to observe the final flash of the Sun settling in the notch between hill-tops in the Rinnes, Galloway on these two days of the year.

1.6.7    On the evening of this sunset the lower atmosphere had been scoured by the day's heavy rain not only of dust haze but also of mist and vapour haze. Blakely Raise lies on ground 800 feet, (244m), above sea level and has wide views over the Solway Firth to Scotland and across the Irish Channel to Northern Ireland. As the Sun approached the horizon small hill features became prominent on the landfall of the Mull of Galloway in Scotland at a distance of 70 mls, (113 km), To the west across the stretch of the Irish Channel another low coast could be seen and in the seascape a small conical feature. These distant objects are just discernable in this photograph. Magnetic compass bearings of these three features was taken and corroborated with subsequent calculations derived from the azimuth of the setting Sun verifying that the landfall to the west is the southern headland of the estuary of Lough Belfast in Northern Ireland at 94 mls, (151 km), and the conical feature in the sea is the 2220 foot, (677 m), peak of the Sperrin Mountains, Londonderry, which stand half-way across Northern Ireland at 156 mls, (251 km). 1.6.8   This was an unusually clear evening but equally as important a condition for this very far distant viewing was the position of the setting Sun. It seems that, given good weather conditions, visibility in the direction of the Sun on the horizon may be virtually unlimited. 1.6.9   It can be seen that common weather patterns in the wetter western districts of Britain can give frequent opportunities for long-distance observing today and in the 2nd millennium BC, which is thought to have been warmer but comparably as moist, the climate should have offered an even greater number of days per year of reliable observing.

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