Fog tutorial

[johnholmes]

hi

Fog tutorial

As we start a spell of anticyclonic settled conditions it might be interesting to see what the picture is on day 1, Monday 18 December 2006.

Providing current weather charts and computer predictions is fairly easy. However, I had hoped to put up some of the methods/diagrams that I used when I first started forecasting over 30 years ago. Sadly my very limited copying and picture enhancing features are pretty basic so its causing some problems.

I may have to be content with the current and forecast charts.

In the first one, around 1700, I think, the XC weather chart shows where the poor visibility is late afternoon/early evening. It's the areas shown as dark. Visibilities generally into fog limits(definition of fog is visibility BELOW 1000m).

The satellite visual UK shows that skies are largely clear over the areas where fog is/has formed.

and the enlarged chart for the UK, again the Visual

The next diagram is a skew-t, it's the very bottom we are interested in. trying to explain how it is used by forecasters, or indeed a computer is programmed to do these things in the place of a forecaster is very difficult. But I'll have a go based on my own experience

So what of the overnight period and into tomorrow?

John

now the more difficult part, trying to explain how a forecaster this evening, with or without the computer would be trying to predict fog/not fog for his/her area.

the chart below and attached comments may help indicate what I'm trying to get across.

j

[johnholmes]

this is a VERY basic chart to try and show how a forecaster, using a t-phi or skew-t for that afternoon would go about trying to predict for his/her RAF/civil station, will it or will it not become foggy this evening/overnight!

What it assumes is that he/she has already decided what the minimum temperature will be and has a pretty good idea of what the dewpoint will do overnight as well. These can be deduced either by more diagrams or more likely these days from the computer products. See our own Net Wx extra charts of Minimum temperatures and predicted dewpoints at various times for 0-384 hours ahead. These are taken from the basic GFS data.

John

the basic diagram for trying to forecast fog. This is just one, there are 4 of these, each with different temperature and dewpoint profiles. This will do to try and show how it can be done.

To explain the letters

A= what the maximum air temperature was around 1500 local time, whilst B, on the base line is what the dewpoint was at the same time.

The first dotted line on the far right is through the inflexion point on the dry bulb temperature graph and is made parallel to the DALR line in red(Dry Adiabatic).

The second dotted line is from the dewpoint at the same time as the maximum temperature constructed upwards and parallel to the moisture content line(blue) HMR.

Next the line parallel to the surface pressure line is made from where the line just mentioned reaches the the first dotted line from A.

Our next line is from where the line just mentioned crosses the dewpoint line at C. We then produce the line to D, which is parallel to the HMR line(blue)

The point at which this line cuts the surface pressure line is called the FOG point.

Not easy to take on board but please try and assimilate it.

I will do my best to try and answer anyone on this thread if you get stuck.

good luck

John

there are other variations but for the time being I think they are best left out of this. All of them depend on the temperature and dewpoint changes in the bottom 1-3,000ft. In some cases the dewpoint increases with height(within the limits just given) and at other times it decreases. Obviously this has an impact on how to construct the diagram to give a predicted fog temperature.

Edited December 19, 2006 by johnholmes

[andy_leics22]

Thanks for this John It certainly is quite a complex thing to understand for little rookies like me but you have gave a very good explanation on it.

[Stratos Ferric]

John, a very useful thread. I for one have never been able to get my head around the lapse charts. Looking at the example you gave higher up, how are the two projections from Td and Ta created? I'd always thought they were drawn from radiosonde data and reflected actual observed temperatures? If not, how could they create those big shifts in the dewpoint line?

Also, am I right that cloud would ordinarily be expected in the zone where the dewpoint line is effectively above the SALR line?

[johnholmes]

hi

yes the cloud base would in fact, in this instance be at B.

The Td and Ta are taken from the radio sonde ascent,

like this one

mind you any cloud on this ascent above from Albemarle this lunch time would be a bit where the small inversion is, nothing by convetion which is what the example I showed on the diagram gives!

hope that helps

John

Edited December 18, 2006 by johnholmes

[Nick F]

I'll be interested to know how to forecast fog using a tephi/skew t John, only ever use the tephi to forecast instability using the lapse rate aloft or looking for inversions. As most radiation fog occurs below 950mb near the surface I would find it more difficult using the conventional radiosonde ascents in Skew-t format such as you show for 12z Abermarle as there's little 'detail' below 950mb in the area we look at for fog.

My basic grasp of radiation fog formation, the variety we have atm, is that you need cooling of stagnant moist air to it's dew point and that cooling needs to go on long enough for the depth of saturated air to be sufficent to form cloud resting on the ground - this relying a sky clear of cloud, a very light wind and of course damp air. A light wind rather than no wind can help stir the cold air to displace it upwards and aid condensation of air.

Of course a general regional/area forecast for fog is more preferable than a localised forecast, particualrly when there's areas of cloud floating around and it's not clear where breaks may occur.

[Stratos Ferric]

I'll be interested to know how to forecast fog using a tephi/skew t John, only ever use the tephi to forecast instability using the lapse rate aloft or looking for inversions. As most radiation fog occurs below 950mb near the surface I would find it more difficult using the conventional radiosonde ascents in Skew-t format such as you show for 12z Abermarle as there's little 'detail' below 950mb in the area we look at for fog.

My basic grasp of radiation fog formation, the variety we have atm, is that you need cooling of stagnant moist air to it's dew point and that cooling needs to go on long enough for the depth of saturated air to be sufficent to form cloud resting on the ground - this relying a sky clear of cloud, a very light wind and of course damp air. A light wind rather than no wind can help stir the cold air to displace it upwards and aid condensation of air.

Of course a general regional/area forecast for fog is more preferable than a localised forecast, particualrly when there's areas of cloud floating around and it's not clear where breaks may occur.

I do agree that these set ups are actually very interesting from a forecasting perspective, and it will be fascinating to see how the next few days pan out. In Stratosdale we often get excellent valley inversions; following the snow last March I think I may have posted one or two piccies. Obviously with snow on the ground the set up is well-nigh perfect under slow moving air, and I vividly recall an occasion in 1979 standing in a field at my parents, up to my thighs in snow, my knees in the fog and my head above it. On that occasion the gradient across a metre or so was 3C as measured by my old mercury thermometer, and still one of the most remarkable weather evenings I have experienced.

In November 1978 we had a not dissimilar set-up to the forthcoming one, and on that occasion freezing for did become widespread, though I seem to recall that that time the system came off the continent - drier air - and the ground was less saturated.

[Nick F]

In November 1978 we had a not dissimilar set-up to the forthcoming one, and on that occasion freezing for did become widespread, though I seem to recall that that time the system came off the continent - drier air - and the ground was less saturated.

I recall fog of a slightly different type, advection fog I think, at the end of snowy spells back in the 80s - where mild air slowly encroached from the SW and as it came in contact with the slowly thawing snowcover it created quite thick fog - due to the moister and relatively milder air moving over the snow being cooled to it's dew point.

Don't recall such an event since then though, at least in the SE.

[Stratos Ferric]

Tonight's minor mystery re location of frost is clarified. Contrary to my expectations there is sone higher cloud showing up, which, having checked the latest "Bracknells" is actually only to be expected...

[johnholmes]

I'll be interested to know how to forecast fog using a tephi/skew t John, only ever use the tephi to forecast instability using the lapse rate aloft or looking for inversions. As most radiation fog occurs below 950mb near the surface I would find it more difficult using the conventional radiosonde ascents in Skew-t format such as you show for 12z Abermarle as there's little 'detail' below 950mb in the area we look at for fog.

My basic grasp of radiation fog formation, the variety we have atm, is that you need cooling of stagnant moist air to it's dew point and that cooling needs to go on long enough for the depth of saturated air to be sufficent to form cloud resting on the ground - this relying a sky clear of cloud, a very light wind and of course damp air. A light wind rather than no wind can help stir the cold air to displace it upwards and aid condensation of air.

Of course a general regional/area forecast for fog is more preferable than a localised forecast, particualrly when there's areas of cloud floating around and it's not clear where breaks may occur.

your comments are about right Nick

I recall fog of a slightly different type, advection fog I think, at the end of snowy spells back in the 80s - where mild air slowly encroached from the SW and as it came in contact with the slowly thawing snowcover it created quite thick fog - due to the moister and relatively milder air moving over the snow being cooled to it's dew point.

Don't recall such an event since then though, at least in the SE.

yes it is advection fog and quite different in forecasting to the radiation fog I have been posting about.

[MAF]

I recall fog of a slightly different type, advection fog I think, at the end of snowy spells back in the 80s - where mild air slowly encroached from the SW and as it came in contact with the slowly thawing snowcover it created quite thick fog - due to the moister and relatively milder air moving over the snow being cooled to it's dew point.

Don't recall such an event since then though, at least in the SE.

I seem to remember that type of fog too. but as you say, never since repeated. in fact todays fog is by far the worst i have seen for, possibly, a few years. and even so, the 10 miles from home to work here in central london has dissolved almost completely

[johnholmes]

Advection fog can occur after a cold spell as milder air runs up over the cold land.

another advection type fog is the sea fog you see roll inland around our coast. The moist air rolls inland and brings the sea fog with it, obviously covering hills as well near to the coast.

Hill fog is something different in that its simply relatively low cloud sitting on the hill tops. In the valleys there will be no fog, and sometimes visibility wll be quite good in the valley.

I will slowly give more information about radiation fog. As I've already said its a complex subject and just about the most difficult to forecast. Its also, with my very limited copying and printing, even more difficult to show easily on the web.

The point Nick makes about the ske-t diagrams being very small is quite valid. One has to copy them and paste into(in my case) ms paint) and then enlarge.

Obviously in the Met O(they use t-phis) they have them larger and thus easier to see. In fact they used to have a special enlarged version up to about 850mb which was plotted additionally if the forecaster felt there was a need for it.

John

Edited December 19, 2006 by johnholmes

[MAF]

you need a PA John, perhaps Lady P would oblige.

[johnholmes]

you need a PA John, perhaps Lady P would oblige.

lovely thought! However, I have always made it a rule not to get involved with married ladies. My face is bad enough without an irate husband making it worse!

j

I'll be interested to know how to forecast fog using a tephi/skew t John, only ever use the tephi to forecast instability using the lapse rate aloft or looking for inversions. As most radiation fog occurs below 950mb near the surface I would find it more difficult using the conventional radiosonde ascents in Skew-t format such as you show for 12z Abermarle as there's little 'detail' below 950mb in the area we look at for fog.

My basic grasp of radiation fog formation, the variety we have atm, is that you need cooling of stagnant moist air to it's dew point and that cooling needs to go on long enough for the depth of saturated air to be sufficent to form cloud resting on the ground - this relying a sky clear of cloud, a very light wind and of course damp air. A light wind rather than no wind can help stir the cold air to displace it upwards and aid condensation of air.

Of course a general regional/area forecast for fog is more preferable than a localised forecast, particualrly when there's areas of cloud floating around and it's not clear where breaks may occur.

not really true Nick. Remember that a local forecaster has all the major tools at his/her disposal and a much better local knowledge than someone scores or perhaps hundreds of miles away. The Met O have followed that line, due to economies being placed on them by government, and many local airports and authorities would welcome a more local input. Hence the expansion by ex met staff into various local authorities using internet.

John

[johnholmes]

this chart from CX Weather shows where the fog is at 0820z with the darker the colour for the worst visibility.

this fits in with the 00z skew-t ascents where Albemarle(ne England) shows fog highly unlikely as its too dry in the very bottom layers.

Watnall and Herstmaceux both show sufficient moisture and the correct temperature-dewpoint profile below 900mb to give fog, Watnall especially.

On the Watnall ascent it will take about 6-7C for the fog to lift and clear.

John

Edited December 19, 2006 by johnholmes

[MAF]

thanks John. shows quite well that London is clear (ish) compared to outer london.

[Nick F]

Watnall and Herstmaceux both show sufficient moisture and the correct temperature-dewpoint profile below 900mb to give fog, Watnall especially.

On the Watnall ascent it will take about 6-7C for the fog to lift and clear.

John

Interestingly John, the fog seemed to have cleared Nottingham (Watnall) around lunchtime looking at the Satellite picture for around noon:

NOAA-18 1205-1220 19/12/06:

http://saturn.unibe.ch/rsbern/noaa/dw/real...c-corrected.jpg

- the skew-t upto 700mb from the 12z Watnall ascent suggested that the air was 'drying out' above the ground - with the dew point curve (on the left) and temperture curve (on the right) becoming more spaced apart on ascent - this is in line with clearer air on the satellite image above over the N Midlands:

Here's the 12z Nottingham (Watnall) ascent with skew-t to 700mb:

... conversely, the 12z Larkhill ascent suggested saturated air from the ground upto 1000mb (303 metres) as shown by the dew point and temperature line ascending close together - in otherwords the air upto 303metres had reached it's dew point and had condensed into fog. The Satellite image above indeed shows fog in the area of Larkhill where the ascent took place in the West Country.

Here's the 12z Larkhill ascent with skew-t to 700mb:

Also notice on the right hand-side of the Larkhill skew-t, the wind barbs suggesting little or now wind upto around 950mb, to disperse the fog.

[andy_leics22]

John I saw the other day you mentioned being in disbelief at the fact the BBC could forecast fog for like 5 days in a row... Because you said its very hard to forecast when a place is going to have foggy conditions. So far though the BBC has been correct, well for me anyway :lol: and its forecasting more fog up until monday! I wouldn't bet against that being right.

[johnholmes]

fairly easy to say fog will occur in a wide area. But not so when you try to pin it down to a specific point at a specific time.

For instance here has had fog/no fog both yesterday and today in the space of several minutes. Last night no fog or low cloud but predicted. This morning no fog and blue sky then low Stratus for about 1.5 hours, then clear until about 1 hour ago.

Its this spatial and time change which causes all the problems being headlined for the airports. That and the fact that apart from Aberdeen, no airport on the UK MAINLAND has any Met O or indeed any other forecaster on duty there. I may be wrong in as much that Manchester the forecaster in underneath the flight path(about 2.5 miles from touch down), assuming the approach is made to the runways 24 Left or Right. at the weather centre. This is due to close.

John

Edited December 20, 2006 by johnholmes

[Nick F]

The noon Nottingham (Watnall) sounding today was as good as any ascent to demonstrate fog in the lowest layer of the atmosphere. If we look at the skew-t upto 700mb from the 12z ascent we can see the dew point and temperature lines are 'stuck' together - showing the saturated air and fog upto around 200m or just below 1000mb where the lines part and the air 'dries' above this:

... also, notice how the temperature increases from below freezing at the bottom (ground) to around 4C at 321m above or 1000mb before dropping upto 956m or 925mb then risng again to 5C above this at around 850mb - showing there were two inversions.

[johnholmes]

yep I've copied it as its as good an example as one is likely to see.

jh

[johnholmes]

before I lose where I've put it here is as good an example of a fog set up as will be seen.

and the enlarged section to show more clearly the bottom part with the fog set up.

John

and the 12z for the following day from the same place

unfortunately I was not on line in the afternoon so the only visibility is for earlier and then that evening.

and the evening chart

Edited December 21, 2006 by johnholmes

[The PIT]

John I saw the other day you mentioned being in disbelief at the fact the BBC could forecast fog for like 5 days in a row... Because you said its very hard to forecast when a place is going to have foggy conditions. So far though the BBC has been correct, well for me anyway :lol: and its forecasting more fog up until monday! I wouldn't bet against that being right.

On the other nside of the coin they've forecast it here quite a few days and have been wrong bar one day. Even then many wouldn't have seen it as it lifted by 6:15.

Edited December 21, 2006 by The PIT

[Stratos Ferric]

John I saw the other day you mentioned being in disbelief at the fact the BBC could forecast fog for like 5 days in a row... Because you said its very hard to forecast when a place is going to have foggy conditions. So far though the BBC has been correct, well for me anyway :lol: and its forecasting more fog up until monday! I wouldn't bet against that being right.

Yeah, but if they've been forecasting fog for pretty much everywhere then in these conditions they're going to be right some of the time. My guess, as PIT's response infers, is that there'll be a good few false positives in there too.

[johnholmes]

Ill say it again and probably again another time, forecasting fog, be it on a country wide scale or worse on a local scale is very very difficult.

It is much easier to do a forecast, for instance, England and Wales, ' continuing dry but rather cold with fog and frost in places night and morning. the fog possibly persisting all day in a few places'

Yes?

Now try and tell someone when it will be foggy and when it will clear at, for the sake of argument, where Pit lives.

first, what height is his station, is he prone to ana or katabatic winds, is it a built up area or open countryside, etc etc. Then start to consider all the variables for forecasting the onset and clearance of fog.

believe me I would always prefer someone to ask will it snow, where and when than will it be foggy, where and when.

John

Edited December 21, 2006 by johnholmes