Why does sea fog not move in despite onshore wind?

[IO540]

This seems to be quite common.

Is there really no wind where the fog is, say 300-1000ft AGL, despite 10kt onshore at surface?

Or does the fog form where temp=DP (well obviously yes it does) but somehow the two separate once over land, due to radiation from the land?

[BackPacker]

My guess would be the latter.

I once cycled into a coastal area where sea fog was present and the temperature near the coast (<200m from the coastline) was significantly lower than further away.

[stickandrudderman]

The land mass warms the air. Warm air can contain more moisture before reaching DP.

[Molesworth 1]

sea fog over the sea of sea fog over the land?

[ShyTorque]

Relative humidity changes as the fog gets blown inland over a warmer surface; a single degree temperature change will put the air above it's dew point and the fog disperses. The fog is still forming at the origin, which makes it appear to be stationary.

It's the opposite of advection fog, which may occur when warm(er) "sea" air moves over an increasingly cold land surface, causing the air to go below dew point temperature.

[UL730]

I remember being examined at the General Belgrano where knowledge of advection fog was one of the topics explored. This was about 19 years ago but a little of the met theory stuck and I think it goes something like this.

Sea fog or advection fog is a movement fog caused when warm moist air flows over a cold surface.

If you have an air mass moving towards a shore - say 10kts at +10*C with a dewpoint of +8*C over a sea at +6*C and a shoreline of +6*C – then you a likely to encounter advection fog or what we call up norf in Durham –“haar”.

As the parcel of air moves further inland - encounters land at over + 8*C the condensation dissipates as the ambient temperature is above the dewpoint of the parcel of air. The distance between land at +6*C and +8*C can be as little as a few hundred yards but I have seen over several miles – which begins to impinge on aerodromes like EGNT and EGNV.

If the onshore wind rises much above 15kts then the fog clears to low ST.

Slightly different to steaming fog where air from a very cold land in moving over a warmer sea but this is very unusual in my local latitude- 55N but I have seen this off Kirkenes (70N) in Norway.

[pistongone]

There was a great example of this yesterday in the Calais area. Hot and sunny untill 15 miles from french coast then thick fog and a massive temp drop. a few miles inland and the temp rose markedly(not sure of the exact amount). By the time we got to St Omer it was really warm, appx 28'. Also found a delightfull restaurant called L'en Vol on the field with a flying owner and a lovely terrace to enjoy the day.
Have a look at www.lenvol-restaurant.fr to see whats on, booking is adviseable at the weekend! Made a nice change from the usual:ok: I also saw a Jodel fitted with a Peugeot Diesel, lifted straight out of a 600 series i think he said. It was a four pot turbo, appx 130hp but weighs in at 200kg inc all bits and bobs. Gave 100kn at 17ltr/hr, whats the rules on doing this sort of thing here?
PS sorry for thread creep chaps

[kevmusic]

I once witnessed a view of Snowdon in incredible vis, from Anglesey; a tail of cloud was visible at its summit, which stayed there for ages. I can only assume the moisture in the wind condensed out at the summit, then as the air warmed going down the lee side, the cloud couldn't be supported.

[sycamore]

Kev, that would be the `FOHN effect`(umlaut over O,pronounced `fern) where the air on the down-slope is warmed at the Dry Adiabatic lapse rate>3*C/1000, as opposed to the Saturated LR on the upslope; so the cloudbase is higher,and warmer downwind.. I think....

[Whopity]

A bit like a lenticular cloud or wave cloud which remains stationary in a strong wind.

[IO540]

I find the stationary clouds over hills easy to understand, because the fall and rise in the air temp happens purely as a result of the air flowing up the hill and then down the hill.

What I find more suprising is how fog can dissipate so markedly after passing over just say 100-200m of the land, in onshore wind of say 10-20kt.

In just some tens of seconds, the air must be warming up by a few degrees, and it can only be from radiation from the ground. It cannot be conduction.

[jabberwok]

I took this shot a couple of years ago. The sea fog walled up against the coastline for several hours but as the sun set and the land cooled down it rolled right across the airfield (in about two minutes).

The fog was approx 250ft high.

http://www.homepages.mcb.net/bones/WebPost/Fog2.jpg

[vee-tail-1]

Happened to me at about 17.00 local, overhead Carmarthen on route to Haverfordwest, one autumn evening.
Within 5 mins Cardiff, Swansea, Aberporth, and Haverfordwest were fog bound.
Only just made it by sneeking in to Pembray at 50ft AGL. ILAFFT :uhoh:

[Mike Cross]

Bleedin simple really. The fog actually does move inland, but not necessarily very far.

Sea does not heat up as quickly as land, as anyone who's gone swimming in it knows.

Land warms up, air in contact with land warms up, forms bubbles of warm air that rise up from the surface as thermals. Rising air is replaced by cool air moving in from the sea, what we all know as a sea breeze. See breeze brings the fog with it. Cool moist air meets warm land and starts to warm up, as it warms it rises above dew point and the fog turns to water vapour. The land has cooled down as a result of heat transfer to the cool incoming air and has also been shielded from the sun by the fog so land near the coast gets cooler and the fog front gradually moves further inland as the day progresses.

[ShyTorque]

A bit like fog on a conveyor belt........ ;)

[Tmbstory]

It does depend on which part of the world you are flying in.

In the past I have operated in the Japan area and at Sapporo International Airport the fog would come in off the sea and in all respects, it was fog, but the wind speed would be between 10 and 20 knots.

Tmb

[Hyperborean]

Try the Northern N Sea, sea fog and 40kts is not unheard of. The significant thing to remember is that sea fog is not radiation fog, which requires only very slight air movement to provide the mixing. Sea fog is formed by, essentially warm (relative), moist air being cooled below its dew point as it moves over the cold sea; hence its formation when the wind would be too strong for radiation fog. The classic case is, perhaps, early summer with a marked SE wind. Hot dry air from the continent starts its sea track towards UK from say the Dutch coast. It absorbs moisture from the sea and cools slightly in the process. By the time it reaches the vicinity of the Scottish coat the dew point falls below the surface temperature of the sea and you get Haar from Fife up to Peterhead and beyond.