PKPF68-77
I was trying to describe how you get a layer of mist, of limited depth, following the slope. This is a special case of up slope fog occurring when the on shore wind is strong and the slope profile suitable such that the layer nearest the ground is compressed, speeds up, and hence forms mist in advance of the bulk of the air mass. It is a common enough phenomenon in those parts, thin layer of mist running up the slopes – a classic view can be had when a “Levante” blows against the water catchment slopes at Gib – a layer as thin as one metre can be had the full length of the slope.
It has perhaps only recently been understood with the study of windfarm placement on hills – a theoretical paper that quantifies the effect is:
BASIC METEOROLOGICAL CONCEPTS AND
RECOMMENDATIONS FOR THE EXPLOITATION
OF WIND ENERGY IN THE
ATMOSPHERIC BOUNDARY LAYER
by
Hartwig Dobesch
Central Institute for Meteorology and Geodynamics (ZAMG)
Vienna, Austria
and
Georg Kury
ENAIRGY
Vienna, Austria
This is heavy going but there are lighter explanations available on the web if you look around.
The significance here is that even a thin, whispy such layer blurs surface detail – thicker obscures objects such as trees and buildings – making visual judgment of range difficult.
The orographic cloud higher up denies larger topographical features as references.
My experience in the NW of Scotland is that in the conditions you describe <<In a humid southerly, say, Ailsa Craig and the hills on the Mull are forming their own weather.>> such effects are common, to be expected, and with a forecast predictable.
Local witnesses I have spoken to support the view that these expected conditions were the case that day.
I believe that they were not in fog as they approached the high ground until the last seconds but had a problem judging the range to go – at their height in the crucial period they would have been below the local orographic cloud which would have extended a significant way before the landmass.
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You are not the only one to wonder why the fishermen were not called as witnesses to the weather – a helo pilot who had been flying around the Mull that day and who attended the scene 45 mins after the crash had seen the fishermen and likewise wondered why they had not been called – he was also amazed that he himself had not.
The only explanation that seems logical is that it suited the MOD to stick with the generally bad weather scenario in which the pilots had made the wrong decisions – easy to sell to the public.
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You (and others) say they continued straight on after the waypoint change – this was not so:
If you refer to the map in my post 3095 (and there’s a few following that may interest you), they made a significant turn to the right to 035 mag, which appears deliberate and planned as this was the course setting found on the handling pilot’s Horizontal Situation Indicator (HSI) course selector.
035m takes you from the position of waypoint change to the crash site.
From Aldergrove to the position of waypoint change was a straight line of 027m.
As the handling pilot would surely have had 027m on his course selector for a journey of nearly 40 miles (including many miles over the Antrim Hills where weather was patchy) it seems reasonable to say that the setting of 035m was deliberately set and not just coincidentally at that figure.
People familiar with HSIs would probably be puzzled by this – by setting the course selector on a particular bearing, this would be your preferred track to a navaid or a waypoint in your nav computer – you would then have the track bars to help keep you on track – there would not be any point setting such a course if you had no navaid or waypoint to refer to.
No here’s the rub – there was no fixed navaid in that direction that they could have worked off – nor a waypoint in the SuperTANS.
So what were they working off?
Whatever it was sure screwed up their distance judgment – and in the conditions described above the Mk1 eyeball couldn’t over ride it.