Almost Trivia - When am I in cloud?
Joined: Nov 2003
Posts: 115
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From: Townsville Australia
I would like to eat crow
Hello all,
I posted the reply stating that Fog is cloud that touches the ground. That's what my Meteorology instructor told me and he worked for the Bureau of Meteorology here in Australia... so naturally I took his word literally. And the definition of low level cloud is cloud that occurs from ground level to 2500m, then mid-level and high level cloud.
But I would like to add the following, which may make my original post inaccurate and could open a can of worms... but here goes.
I said that Fog was stratiform cloud that touches the ground. I would like to correct my statement and say that cloud, according the Bureau, and aviation meteorology books, is formed when a lifting mechanism causes the air to become saturated. The normal lifting mechanisms are orographic, frontal, convective or mechanical.
Fog, on the other hand, can be caused by conduction, radiation or advection.
So let me correct myself and say that cloud is the formation of visible moisture and the product of a lifting mechanism and fog is the formation of visible moisture and the product of cooling without there necessarily being a lifting mechanism.
Fog, by definition, has visibility less than 1000m and mist has visiblity greater than 1000m.
So that's the way I interpret it and what I guess is one of the reasons why fog is treated separately to cloud in forecasts and reports.
I welcome any input if I'm incorrect.
I posted the reply stating that Fog is cloud that touches the ground. That's what my Meteorology instructor told me and he worked for the Bureau of Meteorology here in Australia... so naturally I took his word literally. And the definition of low level cloud is cloud that occurs from ground level to 2500m, then mid-level and high level cloud.
But I would like to add the following, which may make my original post inaccurate and could open a can of worms... but here goes.
I said that Fog was stratiform cloud that touches the ground. I would like to correct my statement and say that cloud, according the Bureau, and aviation meteorology books, is formed when a lifting mechanism causes the air to become saturated. The normal lifting mechanisms are orographic, frontal, convective or mechanical.
Fog, on the other hand, can be caused by conduction, radiation or advection.
So let me correct myself and say that cloud is the formation of visible moisture and the product of a lifting mechanism and fog is the formation of visible moisture and the product of cooling without there necessarily being a lifting mechanism.
Fog, by definition, has visibility less than 1000m and mist has visiblity greater than 1000m.
So that's the way I interpret it and what I guess is one of the reasons why fog is treated separately to cloud in forecasts and reports.
I welcome any input if I'm incorrect.
Thread Starter
Joined: Jun 2005
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From: North of Eq
ConwayB
Wow! For me at least that is good stuff.
Crab@
Interesting comments too.
But you say “…the mechanism for forming cloud is the same as that for fog…”. It would seem that might not be absolutely accurate (see ConwayB's last post).
You also say “When are you clear of cloud? - when you are not in it!” Care to say what you think “it” is in terms of visibility?
Wow! For me at least that is good stuff.
Crab@
Interesting comments too.
But you say “…the mechanism for forming cloud is the same as that for fog…”. It would seem that might not be absolutely accurate (see ConwayB's last post).
You also say “When are you clear of cloud? - when you are not in it!” Care to say what you think “it” is in terms of visibility?
Joined: Nov 2003
Posts: 115
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From: Townsville Australia
Hidden Agenda,
I think what crab was saying was that the mechanism for forming fog (ie cooling of air until it is completely saturated and moisture becomes visible) is the same as that for forming cloud (ie cooling of air [by a lifting mechanism which cools it because of the reduction in pressure and the change of temperature with altitude] until it is completely saturated and moisture becomes visible).
The same basic requirements need to be met:
cooling;
moisture;
nucleii to condense onto.
The point of mine that I was clarifying is that the result is the same but the manner in which the cooling occurs is slightly different.
Sorry for the confusion.
CB
I think what crab was saying was that the mechanism for forming fog (ie cooling of air until it is completely saturated and moisture becomes visible) is the same as that for forming cloud (ie cooling of air [by a lifting mechanism which cools it because of the reduction in pressure and the change of temperature with altitude] until it is completely saturated and moisture becomes visible).
The same basic requirements need to be met:
cooling;
moisture;
nucleii to condense onto.
The point of mine that I was clarifying is that the result is the same but the manner in which the cooling occurs is slightly different.
Sorry for the confusion.
CB
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
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From: EGDC
Tees - I think we have established that to all intents and purposes, fog is cloud on the ground (my aviation met book from the Met Office agrees). However since the cloudbase is a vertical measurement from a cloudbase recorder and indicates where the thickness of the cloud (highest relative humidity probably) is sufficient to reflect the light or laser and the horizontal visibility (RVR in this case) is measured using a transmissometer over a fixed range (usually 200m) they are not stricly comparable - one is a reflected light measurement and the other a measure of the transparency of the air using a one way light transmission.
If you could measure horizontal and vertical visibility in flight you would always get variations as cloud is not a homogenous mass and has variations in relative humidity giving variations in visibility.
If you could measure horizontal and vertical visibility in flight you would always get variations as cloud is not a homogenous mass and has variations in relative humidity giving variations in visibility.
Joined: Aug 2003
Posts: 3,833
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From: Sale, Australia
Scenerio. I live on a mountain peak X,000 feet tall. I wake in the morning and peer out the window and can't see my hand in front of my face. Is it foggy or cloudy? I would say it depends on your point of view. To me its foggy, to an observer at the mountains base it is cloud cover of Y octas with a mountain peak creating cumulo granitus.
Cheers,
Brian
Cheers,
Brian
Joined: Nov 2003
Posts: 115
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From: Townsville Australia
Brian,
that's a very good point, and one that I was going to use to illustrate my own.
Then, when I delved a little deeper, I found that cloud is defined as the product of the cooling of air specifically due to some sort of lifting mechanism. (I didn't differentiate between this subtle difference between fog which forms because of some other cooling method such as radiation, conduction or advection).
So in your scenario, I would suggest that orographic lifting was the most likely cause of your 'fog/cloud'.
That being the case, the mountain is in cloud... and, as you so rightly pointed out, cumulus granitus if you don't maintain your LSALT.
Anyway, I had to eat crow from my previous posts. The BOM website has a glossary of meteorological terms where I discovered I was wrong in my definition of fog as being cloud that touches the ground. But now we could be accused of getting into semantics.
Say g'day to Pete T for me. He used to monster me on my check rides and would ask me questions like these on my instrument rating tests. (How's his golf swing, these days?)
Happy flying.
Conway
that's a very good point, and one that I was going to use to illustrate my own.
Then, when I delved a little deeper, I found that cloud is defined as the product of the cooling of air specifically due to some sort of lifting mechanism. (I didn't differentiate between this subtle difference between fog which forms because of some other cooling method such as radiation, conduction or advection).
So in your scenario, I would suggest that orographic lifting was the most likely cause of your 'fog/cloud'.
That being the case, the mountain is in cloud... and, as you so rightly pointed out, cumulus granitus if you don't maintain your LSALT.
Anyway, I had to eat crow from my previous posts. The BOM website has a glossary of meteorological terms where I discovered I was wrong in my definition of fog as being cloud that touches the ground. But now we could be accused of getting into semantics.
Say g'day to Pete T for me. He used to monster me on my check rides and would ask me questions like these on my instrument rating tests. (How's his golf swing, these days?)
Happy flying.
Conway
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
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From: EGDC
Brian, the Met Office answer to your question would be 'Hill Fog' which they forecast regularly in UK, usually with a visibility in hundreds of metres.
If you live in the SW of UK, the moist Tropical Maritime airmass often produces very low cloud along with 30 -40 kt winds - to an observer at sea there is a low cloud base - an observer on a 200' cliff can see less than 1000m because he is in cloud but it looks just like fog.
There is no difference between fog and cloud, other than what triggers the condensation process - it all depends on where you are viewing the cloud/fog from.
If you live in the SW of UK, the moist Tropical Maritime airmass often produces very low cloud along with 30 -40 kt winds - to an observer at sea there is a low cloud base - an observer on a 200' cliff can see less than 1000m because he is in cloud but it looks just like fog.
There is no difference between fog and cloud, other than what triggers the condensation process - it all depends on where you are viewing the cloud/fog from.
Joined: Aug 2003
Posts: 3,833
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From: Sale, Australia
Since my intrest was piqued I got the old Air Ministry Meteorological “Handbook of Aviation Meteorology” out (an old, old publication) which has the following to say,
“A fog composed of water droplets (or possibly of ice crystals) may be described as a cloud on the surface. Over high ground fog may indeed be merely one or other of the usual cloud types, requiring adiabatic ascent for its foundation; the hills may protrude into a sheet of cloud or may themselves be the cause of local orographic cloud.”
It then goes on to describe Radiation Fog, Advection Fog, Steaming Fog and Frontal Fog. It says fog is used to describe vis less than 1100 yards, mist a vis between 1100 yards and 6.25 miles caused by condensed water drops.
Blue Skies (no fog – maybe a mental one though),
Brian
“A fog composed of water droplets (or possibly of ice crystals) may be described as a cloud on the surface. Over high ground fog may indeed be merely one or other of the usual cloud types, requiring adiabatic ascent for its foundation; the hills may protrude into a sheet of cloud or may themselves be the cause of local orographic cloud.”
It then goes on to describe Radiation Fog, Advection Fog, Steaming Fog and Frontal Fog. It says fog is used to describe vis less than 1100 yards, mist a vis between 1100 yards and 6.25 miles caused by condensed water drops.
Blue Skies (no fog – maybe a mental one though),
Brian
