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Thread: high icing index above freezing level

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    Default high icing index above freezing level

    Hi again,

    in a night raid report is said "High icing index above freezing level at 10.000 ft."

    So I understand that the possibility of icing problems would be quite high during ops. But what is actually meant by "index". Is it just a word for a high possibilty of icing or does it refer to some real index, percentage or something.

    Sorry for that question, but I am really not familiar with this wording.

    Thanks.

    Marcel

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    Marcel,

    Without going into the whys and wherefores a basic lack of information would have made it difficult in the extreme to apply a specific number to the risk of icing during WW2. That said the probability of icing in a forecast would have been given as Nil, Low, Medium or High depending on the weather situation; thus on nights with no cloud the probability of icing would have been Nil, whereas on a night of thick clouds extending to high altitude the probability could have been High. (I've taken the two extreme cases.) This covers the probability, but not the severity of icing.

    The severity of icing in a forecast would have been given as Light, Moderate or Severe, depending on the expected thickness of cloud, the proximity of weather fronts or deep convection (cumulonimbus cloud extending to high altitude).

    Thus as it stands your quotation ("High icing index above freezing level at 10.000 ft.") is ambiguous in that it could mean there is a high risk of some icing above 10000 ft, or there is a risk of severe icing above 10000 ft. Equally ambiguous is that the report does not say if this is a forecast of icing, or of the icing experienced during the operation.

    Because it is mentioned in the report I suspect it means severe icing was experienced during the operation, but if you could email the report to me I might be able to help further.

    I've a feeling I've missed something here in terms of how a forecast is presented, but no doubt someone (Resmorah?) will put me right.

    Brian

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    Hi Brian,

    thanks a lot for answering my questions.

    The quoted text is from the weather forecast. I will email you the report.

    Thanks.

    Best wishes.

    Marcel

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    The problem was somewhat intractable in WW2. It remained so after WW2 (introduction of much turbo-prop and pure-jet aviation). And it seems not to have changed since (see https://aviationweather.gov/static/a...g_Forecast.pdf)!!!! Too many unknown variables! Even if Met get the cloud, temperature, and absolute/relative humidity right it seems that the shape of whatever bit of the aircraft going through icing clouds can affect the accretion!! Too difficult!!
    Sorry not to make it any easier!
    HTH
    Peter Davies (c/s Resmoroh)
    Meteorology is a science; good meteorology is an art!
    We might not know - but we might know who does!

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    If you could provide the date and the numbers of a couple of the squadrons which took part that would be a great help. I'm trying to cast my mind back to the forms we used, and I think one of the items listed was Icing Index (Nil/Low/Medium or High) but that was usually followed by the type of icing (Light/Moderate or Severe) - sorry it's a long time ago.

    Apropos to Peter's post an icing forecast always referred to airframe icing, there being no technique for forecasting carburettor icing at the time.

    Brian

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    Hi Peter,

    thanks for your comments on this difficult topic.

    Marcel

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    Hi Brian,

    I have sent the Night Raid Report to you. It was the raid of 11./12.6.1943 against Dusseldorf and the follwing squadrons participated:

    9, 10, 12, 15, 35, 44, 49, 50, 51, 57, 61, 75, 76, 77, 78, 83, 90, 97, 100, 101, 102, 103, 106, 109, 115, 149, 156, 158, 166, 196, 199, 207, 214, 218, 300, 305, 405, 408, 419, 426, 427, 428, 429, 431, 432, 460, 466, 467, 619

    Thanks for your help.

    Best.

    Marcel

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    Marcel,

    The Night Raid Report has not yet arrived, but having looked at the met situation and some of the ORBs I suspect the forecast of a "High icing index above freezing level at 10.000 ft." was over-pessimistic and did not apply to the whole route. The impression I get from the ORBs is there was extensive layer cloud from UK bases to the Dutch coast, after which the cloud largely breaks up. The layer cloud was associated with an eastward moving, and weakening, frontal system which was lying approximately N-S along about 2W just before midnight on 11 June.

    By chance I have notes from a forecasting course at this time (1943-44) plus a reference book containing advice on the formation of airframe icing. Neither of these would have supported a forecast of severe ice, but both would have indicated the presence of some light icing in the frontal cloud over a shallow layer between 10000 and, say, 14000 ft. I think that in this case the forecast of a high icing index is probably meant to indicate there would be some ice, but not severe ice. There's no doubt in my mind that the statement would have been amplified at briefings, and that the icing forecast referred to a narrow belt of cloud just ahead of the surface front rather than the whole route.

    Brian

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    Hi Brian,

    thanks a lot for thinking about this topic and giving me some hints how it was meant.

    I have resent the report to your normal email address which can be found via your Forum details, beginning with monbr....

    I hope that it works ...

    May I ask which kind of reference book it is?

    Best wishes.

    Marcel

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    Marcel,
    One publication is the “Forecasters’ Reference Book”, published by the Met Office, ref Met. O. 1012. First published 1970, 2nd Ed 1993 (my copy)
    Chapter 6 – Turbulence, Icing, Contrails, and Sea Waves.
    Sect 6.2 Ice Accretion.
    6.2.1 Types of Icing
    6.2.2 Airframe Icing
    6.2.3 Intensity of ice accretion
    6.2.4 Icing and liquid water content
    6.2.5 Estimating the maximum liquid water content of a cloud
    6.2.6 Icing and cloud type
    6.2.7 Cloud temperature and icing risk
    6.2.8 Icing on Ships.

    You should also be aware that, analogous to piston-engine carburettor icing, jet engine intake icing caused a lot of problems with the early RAF V-Force aircraft. The problem was known about by the aircraft builders. They had arranged that in intake icing conditions one could bleed some of the hot exhaust air back round and stuff it into the front to counteract the icing. Brilliant! But this reduced the thrust of the engines! At one stage a Valiant, with full fuel load and The Bomb on board couldn’t generate sufficient thrutch to get off the runway! [Send note to Cold War adversaries – “Pse don’t start a nuclear war while we’ve got icing conditions in UK”!!]
    HTH
    Peter Davies
    Last edited by Resmoroh; 21st October 2019 at 16:28.
    Meteorology is a science; good meteorology is an art!
    We might not know - but we might know who does!

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