Fog - Visibility Question

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Paul C
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Fog - Visibility Question

Post by Paul C » Sat Jan 17, 2015 9:11 pm

Darren raised a point regards the recording a day or Fog when visibility was poor as a result of falling snow

I thought it was a decent question

Darrens Q - " May seem a daft Q - but if VIS goes down to 3 at OBS in a very hvy snow shwr (15 mins), do you also record that as a 'Day of Fog'? "

Carls thoughts " No. Code for a heavy snow shower is C86, fog would be in the range of C40+ the higher code
always wins!"

I agreed with Carl, but got me thinking

So I sent this Question to the Met Office "Question has arisen yesterday. As I was taking my 0900hr Obs and heavy snow shower rolled in. I recorded weather code 86. Visibility Code 4.

If I had selected Visibility Code 3 - would this have classed as a day with Fog ( even though it was just a passing shower that had reduced visibility for a brief period)

Many thanks and I hope the question makes sense"

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Re: Fog - Visibility Question

Post by Paul C » Sat Jan 17, 2015 9:15 pm

Darren also sent an email to Philip Eden

his reply was "Yes, codes 1, 2 and 3 due to snow or smoke (unlikely these days) are counted as fog as well. There's a 9-group which you use (off the top of my head it's 94477)."

My reply from the Met Office was

Good question. Regardless of visibility code 4 or code 3 you did not have a day of fog - the reduction in visibilty was caused by precipitation rather than a suspension of tiny water droplets reducing the visibility to less than 1000 m in all directions being at least 2 metres above the ground (< 2 metres =shallow fog and does not count as 'day of fog') and the relative humidity being 100% or very nearly so - official description.
p.s. Dense fog is a reduction in visibilty to <200m.
Mist and haze are visibilties >1000m but < 10,000m with relative humidity values of 95% or more for mist and <95% for haze.

However they are now having a think

"As I said, the debate goes on. One of my colleagues and the literature is saying viz <1000m due to any of several meteorological phenemonen whilst the actual definition does say 'Day of Fog'. Mmmm, sorry for jumping the gun earlier but Ill have to ring Edinburgh on Monday for a final decision and Ill get back to you then."

So we now have Met Office Edinburgh on the case :D

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Re: Fog - Visibility Question

Post by darrog » Sun Jan 18, 2015 10:58 am

I do like a good Q.

credit to the Met'O chap for owning up, just a shame that his first answer was a little too smart.

I know what camp I'm in - if on the tin it says fish, then inside you are going to find .... fish
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Paul C
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Re: Fog - Visibility Question

Post by Paul C » Mon Jan 19, 2015 10:32 am

OK - Official RESULT

"Hi Paul,
Ive checked up and apparently if the visibility is reduced to less than a 1000 metres, whatever the meteorological cause, then it gets classed as a Day of Fog. Your example therefore should get classed as a Day of Fog."

From The Met Office

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Re: Fog - Visibility Question

Post by darrog » Mon Jan 19, 2015 11:54 am

I would be lying if I said I wasn't laughing.
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Re: Fog - Visibility Question

Post by Carl M » Mon Jan 19, 2015 12:32 pm

Both then!
In the Register for Climat. Obs. you would put C86 (moderate-heavy snow shower) in the present weather section
as that is the higher code, there is no entry for fog so put a note in the diary for that.
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Re: Fog - Visibility Question

Post by darrog » Mon Jan 19, 2015 6:23 pm

Below is the answer supplied by HO (Exeter)

I can confirm that you do also include this as a day of fog. It is a strange one but if the visibility goes below 1000 meters for any reason at 0900 then this counts as a day of fog.

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Re: Fog - Visibility Question

Post by Paul C » Thu Jan 22, 2015 5:49 pm

Darren

I think we may be getting mixed up with weather codes and visibility

Carl is right, code 86 outstrips any fog code and the higher the code always winds for the "weather at the time" I personally didnt think to much about the fog/visibility side of things but it makes sense. However I bet we would never include it as a day of fog if it happened at any other time apart from at 0900hrs. I can think of numerous times when a heavy shower has come along, reduced visiblity for 10mins and never recorded a day of fog - just a thought.

From the list I have

dense fog 1- code x
dense fog 2 - code E
Thick fog 1 - code 1 or 2
Thick Fog 2- code 3
Fog - code 3
moderate fog - 3 or 4

Why you would have different codes for the same visibility is another question.

Clearly open to debate and each observer will have her/her favourite !

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Re: Fog - Visibility Question

Post by darrog » Sun Mar 12, 2017 2:17 pm

Mr. Crabtree - just saw your reply re my Vis code for yesterday - did you open the can of worms deliberately?

It is not my fault that a murky and drizzly day brings the code down to 3 and hence MOD FOG - even if it isn't technically foggy.

We can not have 'technical fog' and by which we only go down to code 4 and no further unless it is pure, genuine fog.

I don't agree that a misty day can only be recorded as mist if the humidity is >=95% and if less than 95 it goes as haze, when it clearly isn't haze because its mist - albeit WOW tells you what to do but if you put in code 10 and humidity of 90% it still lets you submit it.

can of worms back to you.
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Re: Fog - Visibility Question

Post by embayweather » Mon Mar 13, 2017 9:52 am

I had always thought that humidity played a part here in the definition of fog. Was the humidity high when the snow was falling? With mist of course you have to have 95% humidity to call it mist when the vis is reduced to that level.

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