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KDR Confusion

  •'s Avatar Topic Author created the topic: KDR Confusion

Hi All,

I am sitting down to write some notes on my MET KDRs, so that I can revise them prior to the flight test and show the notes to the ATO, in the hope that he gives me a big gold star. However, I'm a little confused as to what to write down for the following syllabus points:

KDR: - Visibility, RVR, Factors Effecting (Part 61 MOS Unit 1.8.3 CMTC, 2.5)
Related Syllabus point: 2.5.3 Give reasons for differences between ‘inflight’ and ‘reported’ visibility.

KDR: - Inversions and Fog (Part 61 MOS Unit 1.8.3 CMTC, 2.8.1 [vii])
Related Syllabus point: (b) recognise signs which may indicate the presence of each phenomenon (inversions and fog);

If anyone could help me out, that would be awesome, as I'm stumped!


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Richard replied the topic: KDR Confusion

Hi Duncan,
Did you get the email I sent in repsonse to those KDR questions on the 10th? I have this sinking feeling it has vanished into a scatter of electrons in the aether...

In case it has, here's a quick summary or the points you mention above:

"Reported Visibility" is a ground-based measurement whereas "in-flight visibility" is a statement of the visibility expected in flight. In-flight visibility can vary because of factors such as slant visibility, whether you are in smoke, haze, cloud etc.
Interestingly, visibility is an expression of atmospheric transparency, independent of lighting. Therefore theoretically, the same conditions will give the same visibility value during the day and at night.

Signs of an inversion:
There are different types of inversion such as radiation, subsidence and frontal inversions but all share the same feature: there is a region where temperature is increasing with height rather than decreasing. So, on a cold night with a gentle wind, you are likely to get a radiation inversion forming. Underneath a high pressure system you will get a widespread subsidence inversion and there is always a frontal inversion associated with cold fronts in the upper atmosphere where the cold air is pushing in underneath the warmer air mass.
Fog will form in a radiation inversion if the air in contact with the cold ground cools to below its dewpoint. The closer together the temperature and dewpoint are to each other, the lower will be the cloud base. If they are identical, you may have cloud on the ground i.e. fog. So, if a METAR reports temperature/dewpoint pairs with little separation and there is a gentle mixing wind, you may well be facing fog developing.
So, for starters, when looking for fog or inversions, think about:
  • increasing temperature with height,
  • a layer close to the ground with poorer visiibility compared to the clear air above. Turbulence may also be present underneath as the inversion layer prevents convection and there may also be windshear at the inversion boundary. There may also be a sudden shift in wind direction and an increase in speed above the inversion.
  • Clear skies at night with a gentle wind. With no wind you will get an intense inversion very close to the ground and likely end up with dew or even frost. The stronger the wind the deeper the inversion will spread up into the atmosphere but the weaker it will become.
  • A high pressure system will produce a subsidence inversion
  • A cold front
  • Close temperature/dewpoint pair and a gentle breeze for the formation of fog

It's a great idea to write up your KDR's for the examiner. It shows you are on the ball and committed to doing a good job. Have fun with it and good luck with your exam!



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  •'s Avatar Topic Author replied the topic: KDR Confusion

Hi Rich,

I haven't seen an email from you - would you mind re-forwarding it?

Thanks very much for replying in such detail, it is really appreciated. The study material you guys put together and the support you provide is fantastic!


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Richard replied the topic: KDR Confusion

Hi Duncan,

The email pretty much said what I have written above. I've had a look for the original email but it is gone, vanished from Drafts, Sent items, Trash etc. Either Outlook is playing an evil trick on me or I dreamt the whole thing :D

I'm glad you are finding the forums helpful. That's what they're there for. Have fun!



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