TAF - What's the difference between ...

[AerBabe]

I was training at Duxford last week and the weather wasn't particularly nice. In fact, the TAF was showing Cambridge as being OVC000 and FG. :confused:

Is the difference that 'FG' alone suggests there is no further cloud above, but OVC000 indicates there are further cloud layers? Unfortunately, I didn't keep the TAF, so I can't remember the rest of it.

[DFC]

Cloud on the ground reducing the visibility to less than 1000m is Fog.

Thus - 27015 0700 FG OVC000

In a TAF or METAR, you will not get cloud layers reported above the base of the layer that is OVC (perhaps except CB but I would have to check that). The reason is twofold;

a) When making an observation, it is impossible for the observer to see through an overcast layer and see wht is beyond - if they could then it is not overcast; and

b) When making an approach as you descend, you are interested in when you will break cloud with a posibility of seeing the ground - that would not happen by descending through a layer that is above an overcast layer - hense it is not useful information (unless it is a CB?).

When delaing with enroute reports and forecasts (F215) then there can be several overcast layers reported - you may want to cruise between them!

Regards,

DFC

[TheOddOne]

AerBabe,

The FG code is a qualifier for the type of obsuration of the visibility, which always immediately precedes it and may be smoke, sand, dust, etc. It is really coincidence that in this case, the cloud was what was causing the reduction in visibility.

A cloud report of OVC000 indicates that the cloud ceiling (what you & I might call the cloudbase!) was less than the minimum reportable value of 001, or 100'. Perhaps it was just possible to discern the base of the cloud. More usually in visibilities of less than 600 metres, the report would be VV////, or 'sky obscured' because it's not possible to make out the bottom of the cloud. Perhaps the horizontal viz was around 900 metres, borderline for being mist, which strictly starts at values in excess of 1,000 metres.

Have a look in the UK-AIP GEN 3.5 pages 30-39. It's free to view or download at www.ais.org.uk.

Hope the wx improves for you soon! Saturday pm might be better, or Sunday am, providing it's not too windy.

Must be great flying regularly at Duxford, enjoy!

Cheers,
The Odd One

[AerBabe]

Thanks for clearing that up for me (no pun intended). I must have known all this when I did my PPL. :O

Must be great flying regularly at Duxford, enjoy! Yes, it must. The training I was referring to, though, was my FISO validation! ;)

[Keygrip]

AB - Fog? Wassat?

I---I

[AerBabe]

Fog? Wassat?
It's like when the cloud cover is OVC, but at 0 feet. :}

I---I ;)

[Keygrip]

Oh, yeah - forgot that stuff.

I'll say "Hi" to Mickey for you.

I---I