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Author Topic: FOG......I have a question about fog.  (Read 8094 times)

Offline Firegirl

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FOG......I have a question about fog.
« on: May 17, 2006, 02:46:13 AM »

Ok Guys........  I know what fog is.  Fog occurs when the air near the ground is saturated and condensation occurs on tiny particles suspended in the air. 
My Question is when does fog become a cloud?  I know fog is a cloud, so to speak, but when does fog become a cloud?  Is it when you can see under the fog with a certain length of visibility?  Is it when it's a 100' off the ground that fog technically becomes a cloud?  Say if you are on a mountain side hiking around at about 4000' in elevation and you hike into a fog bank.  It's still considered ground fog to you but to others at lower elevations on the mountain isn't it considered as "just a cloud" to them? 

Any thoughts on this?
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Offline Gulfstream Driver

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Re: FOG......I have a question about fog.
« Reply #1 on: May 17, 2006, 02:52:11 AM »
I think it's no longer fog when you get a hard ceiling on ATIS or AWOS, etc.  If it gives a vertical visibility or "undefinite ceiling," that's fog.

Does everyone remember the different types of fog?
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Offline Mike

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Re: FOG......I have a question about fog.
« Reply #2 on: May 17, 2006, 03:45:32 AM »
Ok, but when does ATIS or AWOS start seeing a ceiling??
That's what my question would be... :P
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Offline Roland

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Re: FOG......I have a question about fog.
« Reply #3 on: May 17, 2006, 08:17:23 AM »
Right, here comes my (german) definition of the difference between fog and cloud:

Fog is if the cloud lies on the ground ( ;D). Cloud is if there is some space between ground and the cloud.

Both are made of the same material, but the definition depends, the way as I understand it, where this collection of condensation is to be found. On ground: fog. In the air: cloud. Simpel, isn't it?
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Offline Sleek-Jet

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Re: FOG......I have a question about fog.
« Reply #4 on: May 17, 2006, 03:53:14 PM »
Quote from: Mike on May 17, 2006, 03:45:32 AM
Ok, but when does ATIS or AWOS start seeing a ceiling??
That's what my question would be... :P

I like Roland's explanation... :D

I think that anything under  a 100' ceiling would be considered ground fog... I can't remember what the answer was when I studied for my weather observer test...  ???
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Offline Roland

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Re: FOG......I have a question about fog.
« Reply #5 on: May 17, 2006, 04:00:49 PM »
Quote from: Gulfstream Driver on May 17, 2006, 02:52:11 AM

Does everyone remember the different types of fog?

Different types of fog?  ;)

Well, we rotorheads have this definition: fog you can see at least to the next light pole = flyable weather conditions, fog you can’t see your shoelaces = no flying  ;D

In Port Harcourt, Nigeria, we had yellow fog (dust from the Sahara). Flying Amsterdam – Lagos over the Sahara one can see red clouds (fog off the ground or airborne fog).  :)
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Offline Mike

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Re: FOG......I have a question about fog.
« Reply #6 on: May 17, 2006, 06:53:19 PM »
Quote from: Sleek-Jet on May 17, 2006, 03:53:14 PM
Quote from: Mike on May 17, 2006, 03:45:32 AM
Ok, but when does ATIS or AWOS start seeing a ceiling??
That's what my question would be... :P

I like Roland's explanation... :D

I think that anything under  a 100' ceiling would be considered ground fog... I can't remember what the answer was when I studied for my weather observer test...  ???

In my whole time teaching nobody ever asked a similar question like firegirl and the more I think about it, the less I am seeing my way through it.

Is it a 100' ceiling? The question is, does the ATIS only do 100' increments and is that why it is a 100' ceiling? Or is there some really smart explanation somewhere in a really smart book written by really smart people who are probably pedestrians (and we don't mingle with thos people  ;))

For aviation it's probably 100', what do you guys think?
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Offline Roland

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Re: FOG......I have a question about fog.
« Reply #7 on: May 17, 2006, 06:57:32 PM »
Well, honestly I thought I gave a smart answer ... ::)
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Offline Sleek-Jet

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Re: FOG......I have a question about fog.
« Reply #8 on: May 17, 2006, 07:05:20 PM »
The reason I say 100ft, is that would be the lowest "ceiling" able to reported (I think, I've never seen a 50' ceiling reported), anything lower would be indefinate.

Now I'll have to do some research...  ;D
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Offline Mike

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Re: FOG......I have a question about fog.
« Reply #9 on: May 18, 2006, 03:21:33 AM »
Quote from: Roland on May 17, 2006, 08:17:23 AM
Right, here comes my (german) definition of the difference between fog and cloud:

Fog is if the cloud lies on the ground ( ;D). Cloud is if there is some space between ground and the cloud.

Both are made of the same material, but the definition depends, the way as I understand it, where this collection of condensation is to be found. On ground: fog. In the air: cloud. Simpel, isn't it?


It is a smart answer Roland.
BUT!!!
You said SOME space between the ground and the cloud.
How much is it?

See what I am getting at??  ;)
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Offline Gulfstream Driver

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Re: FOG......I have a question about fog.
« Reply #10 on: May 18, 2006, 04:57:27 AM »
I'm pretty sure the lowest ceiling you can have is 100'.  I've never seen anything else.  I looked through my Aviaiton Weather Handbook by Terry Lankford.  It's an excellent book that has everything you need to know about weather except the answer to this question.  :-)

The different types of fog I was referring to were radiation, advection, upslope, rain-induced (frontal), and steam.
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Offline Roland

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Re: FOG......I have a question about fog.
« Reply #11 on: May 18, 2006, 05:39:14 AM »
OK, Mike, I see your point. Well, here comes a rotorhead definition again: some space is that amount of space between ground and cloud which allows you to get the landing skids off the ground to skim across but also to have the head (rotor head for beginners, pilots head for experienced) out of the cloud. ;D

I’m such a smarta§§ … ::)
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Offline Roland

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Re: FOG......I have a question about fog.
« Reply #12 on: May 18, 2006, 01:21:43 PM »
I gave in „Air America“ and got this. (Fog or clouds?)
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Offline C310RCaptian

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Re: FOG......I have a question about fog.
« Reply #13 on: May 18, 2006, 03:20:51 PM »
The Aim Might have the answer!!!!  I found this when talking about a metar coding. read with caution. Im no Mr. Spock and my logic can be flawed.

Obstructions to visibility. There are eight types of obscuration phenomena in the METAR code (obscurations are any phenomena in the atmosphere, other than precipitation, that reduce horizontal visibility):
FG fog (vsby less than 5/8 mile)
HZ haze
FU smoke
PY spray
BR mist (vsby 5/8 - 6 miles)
SA sand
DU dust
VA volcanic ash
NOTE-
Fog (FG) is observed or forecast only when the visibility is less than five-eighths of mile, otherwise mist (BR) is observed or forecast.

Also found this!

For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration. Also there is no provision for reporting thin layers in the METAR code. When clouds are thin, that layer shall be reported as if it were opaque.

Now here is Vertical Visibility (Whiteman AFB loves to use VV)

Vertical Visibility (indefinite ceiling height). The height into an indefinite ceiling is preceded by "VV" and followed by three digits indicating the vertical visibility in hundreds of feet. This layer indicates total obscuration.
EXAMPLE-
1/8 SM FG VV006 - visibility one eighth, fog, indefinite ceiling six hundred.
 
So this is my conclusion. Fog has no ceiling but can have a Vertical visibility value. (kind of like an RVR value only vertical) Fog is when the ground visibility is being obstructed below 5\8ths of a mile. Therefore if you have good visibility on the ground then it is not fog but mist if it is over 5\8 of a mile. When good visibility prevails on the ground (5/8 mile +) and there is Vertical Visibility value this changes it from fog to a cloud. Keep in mind that this is based on relatively flat terrain and VV does not mean that is where the bases start. Just where visibility gets bad again. So you could have the bases start at 2 feet with 6/8 mile visibility under the 2 feet and 4/8 mile visibility above 2 feet and a VV of 600 feet. It is now deemed a cloud( if youwere laying under it getting that 6\8 mile visibility, standing up its still fog) On a mountain I guess its all relative to where you are. In it and the right conditions are met, its fog. Out of it, it’s a cloud. ;D  Make any sense at all??? I hope it did…. :-[
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Offline Firegirl

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Re: FOG......I have a question about fog.
« Reply #14 on: May 18, 2006, 05:10:56 PM »
An absolutely fabulous answer Captain!! 

I also like Roland's smart#*@ answers too.
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