Fog

[phanakapan]

Sorry for a repeat post, (last one on sunshine's coming)- but the weather forecast from the Met Office for the English channel on Sat/Sun says a threat of mist and fog patches esp in the East- I'm planning on sailing Brighton-Fecamp- do I need to reconsider? I havn't got access to my weather books- is the sort of fog you get in high pressure likely to happen at any particular time of day, near coasts or all over? or just in the shipping lanes!

<hr width=100% size=1>

 

[AndrewB]

I wouldn't worry unduely about fog patches, but if the shipping forecast mentions fog banks then don't go unless you have radar and an unflappable temperament.

Its scarey in fog in the shipping lanes out there, but I've been caught several times and at least everyone is taking extra care. Ships' radars are pretty good these days. Assume you have a decent radar reflector.

<hr width=100% size=1>

 

[BrendanS]

I haven't seen the forecast you mentioned, but sea fogs are caused by different conditions to local shore based fogs or inland fogs..

Not enough detail from what you've posted to advise, but if the forecast is a shore based one, it's not likely to apply to offshore. For mid channel fog forecasts, you need an offshore forecast or local knowledge

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide

 

[LadyInBed]

GENERAL An area of low pressure will be slow moving near Iceland on Friday
with an associated frontal system stretching across S UK towards the Azores.
The low and the frontal system will gradually decay during Saturday as high
pressure begins to build to the W of France and N Spain. High pressure will
drift N across the UK during Sunday then become slow moving over the North
Sea on Monday.
FRIDAY
NW Scotland will have an increasingly windy day with bright or sunny
intervals and showers, perhaps becoming prolonged later. Elsewhere in
Scotland, together with Northern Ireland, N England and N Wales: a bright,
breezy day; a few showers but often dry in sheltered E parts where the best
of any sunny intervals will be. For S Wales, parts of SW England and the N
and W Midlands; overnight rain only slowly fading away later in the day.
Cloud amounts staying rather large, but becoming brighter. Rain edging
slowly to SE areas. Temperatures close to average.
SATURDAY
Scotland and Northern Ireland and N England
Wind - SW fresh to strong gradually easing SW light to moderate although
remaining fresh at times in W Scotland.
Weather - Occasional showers affecting W coasts of N England, Scotland and
Northern Ireland but otherwise dry and bright with some good sunny spells.
Visibility - Mainly good but occasionally moderate in showers.
Elsewhere
Wind - SW light to moderate becoming variable mainly W to SW light.
Weather - Cloudy with showery outbreaks of rain at first along with some
mist patches. Rain slowly clearing with dry and bright conditions
developing, although with a continuing risk of mist especially in the E.
Visibility - Although mainly good or moderate it will occasionally be poor
in mist patches.
SUNDAY
All areas
Wind - Will gradually become S or SE light or moderate.
Weather - Generally fair and bright with some good sunny spells although
with a few mist patches and a risk of fog at times along Channel and North
Sea coasts.
Visibility - Will be mainly good or moderate but will be poor at times in
any mist or fog.


<hr width=100% size=1>

 

[BrendanS]

Looking at it overall, there's a chance of patches. For a novice crossing, look at the forecast again tomorrow.

Unlikely the forecast is going to be that accurate at this range. A few knts increase in the wind forecast, and no mist or fog.

If there is a real chance of mid channel fog, and no radar, then think again.

Then again, they may have it correct, and we are in for a heat wave next week!



<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide

 

[Deleted member 478]

If in doubt, head for the pub. It's much safer in there.

<hr width=100% size=1><A target="_blank" HREF=http://www.stingo.co.uk>http://www.stingo.co.uk</A> <font color=blue>- still showing at a computer near you</font color=blue>

 

[ParaHandy]

Re: a pub with tv ....

... might be a good idea (for the english) with all that rain and no play at lords ... hah! escaped a whitewash again

<hr width=100% size=1>

 

[Deleted member 478]

Re: a pub with tv ....

Strange game...

It seems to me that it is all about being in or out;
You go in until you are out and when you are out you go in.
If you are out you are not in and visa versa.
When 10 out of 11 of your team are out, your whole team is out.
When none of your team are out, only two of your team are in.

I am sure we could go on and on here



<hr width=100% size=1><A target="_blank" HREF=http://www.stingo.co.uk>http://www.stingo.co.uk</A> <font color=blue>- still showing at a computer near you</font color=blue>

 

[ParaHandy]

Re: when you\'re in you\'re out ...

no surprise you're winning ... you've read the rule book ..

and you know you're out when an old git sticks one finger up. he could put 2 fingers up but that would, apparently, be rude ....

<hr width=100% size=1>

 

[billmacfarlane]

I've crossed the Channel many times and met fog when it hasn't been forecast. So when it's mentioned in a forecast, I've got a simple rule - I don't go !!! Despite the fact that I'm off on holiday in about 2 hours time, I will not cross the Channel with a good chance of fog, and I've got radar. I'll go along the coast away from major shipping channels but not cross channel. I might even see you in Brighton.

<hr width=100% size=1>

 

[qsiv]

Today in Jersey we've had air temperatures within 0.2 degrees of the dewpoint - that has to mean that likelihood of radiation fog is high - implying visibility will be well below 1 Km. Unless there are particular reasons for going to sea under these circumstances, I'd stay put, or avoid heavy traffic areas.

The charts show the humidity decreasing rapidly from Saturday morning, so it is quite possible that the risk will be short lived.

Longer term certainly shows an area of very slack pressure over the Western Approaches, which would certainly ring warning bells to me - if any warm moist air flows back from Europe to the sea the scope for advection fog is very high, and is often seen (thats usually the fog forming on coasts and spreading out to sea)

<hr width=100% size=1>

 

[Gunfleet]

It's because cold water from the Atlantic meets warm water from the Dover straights just abt in a line between Le Havre and Brighton. So when the air conditons are right you get a fog, esp in summer. Having been caught by one of those (non-forecast) fogs when I was between the two shipping lanes and suffered a night from hell as a result (we could hear them, we could feel their wash, that's it) I would advise as strongly as I can don't go if there's any fog forecast for the channel at all. So I would check the forecast very carefully just before you go. Have a nice trip. John

<hr width=100% size=1>

 

[peterb]

Are you sure you've got it the right way round?

Radiation fog is caused by the ground radiating its heat to space on a calm, clear night, so that the air near the ground falls below its dewpoint and mist forms. The sea doesn't cool down in the same way, so radiation fog can't form at sea; it can, however, drift out to sea a short way from the land.

Advection fog is when warm, moist air is blown over a cold sea. It's the usual reason for fog in the Channel Islands with a SW wind.

If the forecast is for radiation fog, then usually it's safe to set out. Out to sea it's unlikely to be foggy, and anyway the fog is likely to burn off as the sun warms things up. Advection fog can be much more dangerous.

<hr width=100% size=1>

 

[qsiv]

I understood radiation fog to be cased when the air temperature cools to a point below the dew point. Advection was when warm moist air (from any source) moves over a colder mass (be it land or sea) and is cooled as a result of the change in location of the air mass.

But from what you say mt understanding is flawed!

<hr width=100% size=1>

 

[BrendanS]

This is from my course notes from Chichester Maritime Yachtmaster.

It's copyrighted, so may disappear in the early hours of the morning, but I will ask them if I have permission to post here. The actual course comes with graphics which show how it all works.

Brendan
-----------------------------------------------------------------------------------------------------------

Advection fog is formed when warm moist air passes over a cold surface. For sea fog to form, the critical factors are the temperature of the sea-surface, and the dewpoint of the air.

If warm moist air moves in over a cold sea, with a temperature below that of the dew point of the air, any water vapour will condense into fog.

Where the sea temperature is well below the dewpoint of the air mass widespread fog will form, but where the difference is small only banks of fog will form.

During early spring when the sea temperature around the coast is at it's lowest, advection fog forms in the tropical maritime air mass that approaches the UK from the south west. Areas that are particularly likely to be affected are those with strong tidal streams and rocky bottoms that cause cold water to well up to the surface.

As the summer approaches and coastal waters warm up, the colder water and therefore the sea fog are found further out to sea.

During the winter when the polar continental air mass approaches the UK from Russia one would not expect to have advection fog as the air mass is cold and dry. However, the North Sea is cold on its outer fringes but relatively warm in its centre, so the air mass warms up slightly as it passes over this central region and absorbs moisture. When the air mass finally hits the cold waters on the east coast of the UK it cools and sea fog forms.

All fog that forms more than 2 miles off the coast is advection fog. Sea fog is slow to disperse and requires a change to a colder, drier air mass or a wind of F6 or above. The sun has no effect on sea fog.


Radiation fog

Radiation fog is land fog. To form it requires a cold clear night, illustration F, (Image sequence - hover over the olive words), a saturated air mass, and a very light breeze.

Heat radiates away from the earth and the ground becomes very cold.

Without any breeze the saturated air mass cools to its dewpoint and droplets of dew form on the grass. The light breeze simply stirs the air sufficiently to keep the water droplets suspended so that fog forms.

On most cold, clear nights there is no gradient wind at surface level but with the first rays of the rising sun, convection starts and this tumbling air causes fog to form in a relatively thin layer, particularly over damp areas and in valleys where cold air collects.

As the sun heats the land, the air above warms and the fog is absorbed. By mid-morning virtually all traces of the fog should have disappeared.

Radiation fog only ever forms over land but, like sea fog, it can spill over the coastline and move a little way out to sea. If you are heading out to sea the fog will peter out and most will disappear by late morning when the sun has heated the land.


Is is advection or radiation fog?

If you are within 2 miles of the coast the fog could be of either type but there are some clues:

What time of year is it? Radiation fog forms more readily during autumn and winter when relatively warm days saturate the air and the nights are cold and clear.


Is it early morning? Radiation fog forms in the early morning on the first zephyr of breeze.


Was the previous day warm and night clear and cold? Radiation fog forms under these conditions.


Is the sea temperature relatively warm (September/October around the UK coast)? Unless the air mass is saturated it is unlikely to be advection fog close to the coast. Advection fog tends to form further out to sea where cold currents are found or where a change of tide brings cold water to the surface.


Is the sea temperature relatively cold (February/March around the UK coast)? If the sea temperature is cold and the air mass saturated advection fog will form readily.

------------------------------------------------------------------------------------------------------------------


So, Is sea fog or land fog forecast this weekend?


<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide

 

[BrendanS]

CM have given permission for this to be posted, as it's for educational purposes.

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide

 

[cgull]

Did you go?

Was it foggy?

<hr width=100% size=1>

 

[phanakapan]

Thankyou to all for the fog info- in the end I did go- and it was a bit misty but not really thick fog. Spent the hours before I set off reading the radar manual - and found using the radar incredibly reassuring for crossing the shipping lanes.Set off from Brighton to Fecamp at midnight Friday a week ago, no wind, motored all the way. What struck me most about my first channel crossing was that once past the shipping lanes we had hours and hours without seeing another vessel-great excitement at one point when we saw a dolphin though. Another bit of equipment I really loved was the old Yeoman chart plotter that came with the boat- had never used it before- as I can easily get seasick even on the flat sea that we had, it was great to minimise the time spent down below, it's so quick to use to put a dot on the chart. Didn't end up doing a great deal of sailing-spent a couple of days in Fecamp, my partner mended the mainsheet track that was parting company with the deck, and I recarpeted the saloon sides and floor with a 10 quid offcut from the local B&Q. We had some weird German weather forecasts from the bloke in the boat next to us (F2-3 gusting 7-8!) which put us off a bit, and as we had to be back in Brighton by Sunday, we decided to make our way up to Dieppe so that if the weather got nasty we could catch the Dieppe-Newhaven ferry back, and return later to get the boat back across the channel. So we pottered up to St Valery en Caux, then to Dieppe. We left Dieppe at midnight Friday, only to turn back at the harbour entrance because the radar and chartplotter weren't working- morning revealed nothing more sinister than a blown fuse, so off we set again-good thing we had mended it as visibility was poor most of the way and the eastbound shipping lane was very busy. As it turned out, the hardest thing was spotting our well known marina entrance in the dark- especially as there were loads of fireworks being set off and the red flashing light at the marina entrance wasn't working.
Lessons learnt-
I would never go out in the chance of fog without the radar, we would have been stuffed on the way back without it;
We were relying on Marinecall for our longer range forecasts and didn't realise that you can't phone premium rate nos from a mobile abroad.



<hr width=100% size=1>