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Old 16th Nov 2010, 00:08
  #6996 (permalink)  
walter kennedy
 
Join Date: Sep 2003
Location: Perth, Western Australia
Posts: 786
WEATHER
Summarising the yachtsman's evidence (repeating parts posted a while back by John Blakeley which should have cleared this up):
<< … In his statement to the Board Mr Holbrook said that the aircraft was well below cloud level and visibility was about a mile limited by haze. ...
64. When he gave evidence at the FAI Mr Holbrook expressed the opinion that the pilot could have seen "the location of the Mull lighthouse" and described the low cloud as
"hugging the Mull" (Sheriff's determination, p 110 of HL Paper 25(ii)). He was criticised by the Ministry of Defence for having given different versions of his account to the Board and the FAI. In these circumstances we invited him to give evidence, an invitation which he willingly accepted.
65. Mr Holbrook's evidence to us began with a statement which he had asked leave to make (Q 594). He explained that the low cloud clung to the contours of the high ground so that the location of the Mull massif itself was not in doubt from sea level. He considered that the crew of the aircraft when he saw it could determine without ambiguity where the Mull was and could see the cliffs, beach and lower perimeter walls of the lighthouse complex. 66. Mr Holbrook reaffirmed that when he saw the aircraft he could not see the physical features of the Mull but he went on to explain that he was able to see the location of the lighthouse complex because the buildings and the white perimeter wall showed up as a colour change against the background of the land mass (QQ 594, 602). The top of the lighthouse was in cloud as the cloud level moved up and down (Q 606). Mr Holbrook went on to explain that the cloud was following the contours of the land and was very localised (Q 615). >>



The first bit makes the point that the general cloud in the area was not an issue for a LL VFR flight (which fits in with the forecasts) – the rest describes a localised weather system generated by the Mull itself (and therefore fixed on it on the windward side) and this, in my opinion, was to have been expected on such a headland at that time of year, at that time of day, with a strong onshore wind blowing.
In my opinion, there would have been two components to that localised weather:
1 A locally generated cloud base on the top of the Mull at 800-900 ft (known as “cap” cloud, orographic cloud, or up-slope stratus);
2 Local ground hugging mist following the slopes up, starting at the level of the base of the lighthouse at 270 ft (a less widely understood formation, dependent upon compression and speed-up of the lowest layer of air, – not up-slope stratus).
The local cloud (1) formed on the top of the Mull was typically orographic and had formed when the air mass reached its dew point at about 800 ft ASL (as far as can be gleaned from information available); it would usually have a defined base (in this case ~800ft amsl) at which point the moisture in the bulk of the air mass pushed up by the landmass condenses – covering its top and preventing geographic orientation from the large scale topography - it should not be confused with the mist on the lower slopes.
The mist (2) on the lower slopes of the Mull is rather special – in these conditions, the lower layer of the air mass moving against the slope is subject to streamline compression and results in a phenomenon called “speed up” – the speed of the air near the ground increases significantly and because of the Bernoulli effect the pressure in this layer is lower which effectively lowers the temperature of this layer and so this layer can reach its dew point in advance of the bulk of the air mass and so form that thin blanket of mist, which has the appearance of running up the slopes. The profile of the slopes facing the strong wind that was coming off the sea (moisture laden near the end of a summer day) was ideal for the formation of this mist and it was to have been expected at the time.
In fairness to those on this thread who have had difficulties finding authoritative references to this kind of fog formation, it is hard to find – I have often witnessed such (much sea time off the NW Scottish coast and hill walking) and so I dug deep to find the scientific explanation, the answer coming from recent wind farm studies and it is termed “speed-up”. It is not orographic or up-slope stratus that you would be familiar with but when you have orographic cloud on such a headland you can confidently expect it on the lower slopes running up to merge with the orographic cloud when a strong wind is blowing.
Here's that video I took at the crash site that I have posted on previous occasions (not so benign conditions for its formation but shows the process):
MullMistVideo video by grauniad - Photobucket
The relevance in this case is that this layer is close to the slope and so the resultant mist closely follows the slope up until the level at which the bulk of the air mass reaches the dew point and forms the orographic cloud (about 800-900 ft ASL in this case). Below 800 ft, one could be confident that the mist ahead was on the land and not closer – if your navigation (by whatever means) was telling you that the land was, say, ½ mile away then you would not enter the mist for another ½ a mile – the downside is that ground detail is obscured and, as the mist merges with the orographic cloud at 800-900 ft which covers the upper slope and therefore obscures the hill top profile, judgement of one’s distance off visually is extremely difficult and so remaining clear depends upon your other navigation.
Conversely, if you do have some reliable means of accurate navigation, it would not be unreasonable to plan a close approach to it as you could avoid running into it – here's a hypothetical scenario for you to consider:
You want to swing around the back of the light house at speed;
with the mist starting about the level of the light house, you are confident that you can do this while staying above the mist (they were approaching the shoreline several hundred feet higher than the elevation of the light house yet beneath the base of the orographic cloud);
having been there on previous occasions, you know that an approach to the LZ at waypoint A has a safe wave-off by initiating a banked turn (anything between 30 and 60 deg at high cruising speed given the turning radii for a Chinook) on crossing the shoreline – a perfect line-up for swing around the light house;
the tricky bit is identifying the required crossing point and judging one's closing range with the misty background, the ground to the right being higher sooner and you wouldn't want to overshoot the shoreline because you weren't expecting it just yet;
you couldn't go wrong if an exercise was set up involving someone with a PRC112 standing on that LZ, also toting a VHF radio perhaps for the HP to line up on using the VHF homer system, belt and braces, a walk in the park;
trouble is, you wouldn't have any visual cues to contradict your instruments if the chaps on the ground were ½ mile or so up the hill … until you entered that mist.


Oh and while we're at it, forget the ridiculous idea of them spiralling out of control – check out the data from the last steering calculation – fits in with a straight and level leg after the slight turn to the right after waypoint change – and of course, we have that final leg on a track that was set on the HP's horizontal situation indicator course select.

Last edited by walter kennedy; 16th Nov 2010 at 00:22. Reason: added link
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