The fohn effect

[Potent Gust]

Simple question could somebody explain to me how the fohn effect works please?

[The PIT]

Simple question could somebody explain to me how the fohn effect works please?

Will this help???

http://www.bbc.co.uk/weather/features/unde...hn_effect.shtml

[Potent Gust]

Will this help???

http://www.bbc.co.uk/weather/features/unde...hn_effect.shtml

Yes that is great thanks!

[chionomaniac]

The Fohn effect is an example of adiabatic heating. I didn't have a clue what that meant until I came across this link: http://en.wikipedia.org/wiki/Adiabatic_process

c

[johnholmes]

I did an item on it some time ago I'll try and dig it out it may help

[Franko83]

it's a terrible dry effectlast year, in January, there were temperatures of 29° in NW of Italy!

[SP1986]

There's no need to add to what will/already has been suggested, but living at the lee of the Welsh mountains, fohn effects are fairly common in southwesterly winds here, its an effect I am particularly interested in, as I have know differences of 5-6C between here and Manchester during fohn effects.

Last winter was a marked fohn effect here with 17C being recorded by myself (personal measurement) in January.

Although fohn effects are more associated with winter months around here (due to lower temperatures), August 7th 2003 was particularly strong as a fohn (SEly wind), in Valley, Anglesey sored to 33C at 11am!

[J07]

I think it's a splendid feeling in winter, feels quite surreal. In summer though, less fun...."the hot summer wind that drops torrents on the west coast, then rips across the plains, sucking the soil dry, snapping tempers and distoring television reception".

The effect on farming can be quite negative. In winter, when there is not much grass, any feed that is put out for stock can be blown away entirely. And then in spring, lambing season, if there is a succession of fohn winds it will be too dry for grass to grow and you end up with serious problems at a very important time of the year. Then there's the fire risk in summer, and the possibility of any exposed top soil being blown out to sea.

[SP1986]

I think it's a splendid feeling in winter, feels quite surreal. In summer though, less fun...."the hot summer wind that drops torrents on the west coast, then rips across the plains, sucking the soil dry, snapping tempers and distoring television reception".

The effect on farming can be quite negative. In winter, when there is not much grass, any feed that is put out for stock can be blown away entirely. And then in spring, lambing season, if there is a succession of fohn winds it will be too dry for grass to grow and you end up with serious problems at a very important time of the year. Then there's the fire risk in summer, and the possibility of any exposed top soil being blown out to sea.

Gnerally true of most horticulture too, a plant/flower may produce a new shoot due to a fohn effect, which is then cut down when the colder temperatures return, and at the risk of losing more tender pplant species.

[Devon-Nelly]

Its sounds stupid but, we get days during the summer, when the rest of the country basks in a heatwave, its usually cooler here around 24/5c, because we get sea breeze which can get quite strong. But then the following day theres no breath of wind at all from the sea and it in fact comes from inland (land breeze) and its a warm breeze and the temperature rockets up to 28/9c.

I discovered this in 2005 during the two very pleasant weeks that month, cant remember dates, but on the monday i think it was, it was hot and sunny, with a breeze coming from inland and it reached 28c, (recorded 30c in my old garden but it was a sun trap), then ther following day we had a sea breeze and it only reached 25c, then the following day the land breeze returned and we reached 28c again, then the following day a strong sea breeze kicked in and it only reached 24c, and high pressure was sat over the uk so there wasnt really a general wind direction.

Sorry for blabbing abit but is this a fohn effect, but during the summer?

[Stratos Ferric]

Its sounds stupid but, we get days during the summer, when the rest of the country basks in a heatwave, its usually cooler here around 24/5c, because we get sea breeze which can get quite strong. But then the following day theres no breath of wind at all from the sea and it in fact comes from inland (land breeze) and its a warm breeze and the temperature rockets up to 28/9c.

I discovered this in 2005 during the two very pleasant weeks that month, cant remember dates, but on the monday i think it was, it was hot and sunny, with a breeze coming from inland and it reached 28c, (recorded 30c in my old garden but it was a sun trap), then ther following day we had a sea breeze and it only reached 25c, then the following day the land breeze returned and we reached 28c again, then the following day a strong sea breeze kicked in and it only reached 24c, and high pressure was sat over the uk so there wasnt really a general wind direction.

Sorry for blabbing abit but is this a fohn effect, but during the summer?

Nope, just the effect of differential heating in the case of an onshore breeze (most of the UK will get onshore winds in the right conditions in summer). A Fohn only really kicks in in the lee of large mountains, and then when the air is fairly moist: the critical point is that the moisture laden air cools at a lower rate than the dry air warms when descending in the lee.

[SP1986]

Devon-Nelly, this is a simple diagram of a foehn effect situation, or a classic textbook definition.

[Potent Gust]

Thanks for all your comments folks. I think I understand it now. Very interesting stuff!

[Somerset Squall]

Yes, interesting stuff. The links were very useful. The Fohn effect is certainly a weird aspect of weather, which I never really understood- but I think I understand the basics now. It would still be good if you could dig out the work you did on it John.

[SP1986]

The lee of the welsh mountains (Hawarden, SW Wirral), is getting a fohn today, already 14C and fairly gusty - (the 'symptoms') Theres no knowing what temperatures it could reach if it keeps up - anything from 15 to 17C keep an eye on the temperatures, itll give you an idea of the heating capabilites of the Fohn.

[johnholmes]

hi

I promised to try and give an explanation that was not complicated so I hope this will do.

Fohn winds or Fohn effect

The name comes from a German term given to a wind local in the Alps, which is most noticeable in late winter and spring. Its seen also to the lee of our own mountain chains, especially in eastern Scotland, to the lee of the Grampians. Quite large rises in temperature can occur, 3-5C is often seen when compared to values on the western side of the Grampians and can exceed 10C. One of the most notable was in January 2003 when Arboyne recorded 18.3C, which is the joint highest for the UK in January. Similar events can occur in North Wales.

The most spectacular is in the Canadian Rockies where rises of 20C or more can occur in a very short time. Here and in the Alps this wind is responsible for a very rapid decrease in snow depth, often, as it’s a very dry wind, with no apparent water run off.

It is also blamed for people having sudden headaches and mood swings.

If you go on holiday in mountain regions, even now, but more so when wood was often the sole building material, fire was a great hazard and there are very full documented accounts of these in the Alps. This is why, in winter, fire hydrant point are the first to be cleared of snow even before roads and paths are cleared.

On a more technical level let us take an example of a moist west or south west wind blowing over the Scottish mountains. It has a temperature of 10C. As it rises it is forced to cool, at the DALR (Dry Adiabatic Lapse Rate) =approx 3C per 1,000ft until it becomes saturated, it then rises at the SALR (Saturated ALR) =approx 1.5C per 1,000ft, and it will rain or drizzle from it. As the air continues to rise so the rain and drizzle will eventually decrease the amount of water vapour present. Or put another way it releases latent heat. Once it reaches the mountain top, it may have lost all its available water vapour so it soon becomes cloud free as it starts its descent on the leeward side. Once the cloud has cleared then the air descends at the DALR (see above) and thus the air warms until it reaches the valley floor.

See the simple diagram below. In this, we assume the air starts at a temperature of 10C. It cools at the DALR to A, at which point it has cooled by 3C=7C. It then rises at the SALR to the mountain top at 3,000ft where its temperature is now 4C at B. It starts to run down the lee side and having lost all its moisture it falls at the DALR. Thus by the time it reaches C its temperature is 13C. (its fallen 3000ft at 3C per 1,000ft=DALR).

So even with relatively low mountains a noticeable rise occurs along with the air feeling very dry, and obviously its sunny as opposed to cloudy and damp on the windward side.

diagram

hope it helps

pm me or ask on this thread if anyone needs more explaining

[chionomaniac]

Thank you for that clear explanation, John' which has helped me to understand adiabatic lapse rates a little bit better.

c

[johnholmes]

glad it helped

perhaps you might like to dip into here, all kinds of things to do with upper air.

Don't be afraid to ask questions

http://www.netweather.tv/forum/index.php?showtopic=16002

[Devon-Nelly]

Thanks Stratos Ferric and Stephen Prudence, for the explanations, and diagrams, i understand more now.

[Franko83]

it' s a very good example to stau and fohn effect

http://www.youtube.com/watch?v=OjEs2ohSqAE

see it, it' wonderfull!

[Franko83]

it' s a very good example to stau and fohn effect

http://www.youtube.com/watch?v=OjEs2ohSqAE

see it, it' wonderfull!

[johnholmes]

its good Franko but can you get a larger image please, difficult to read your numbers etc.

thanks

[Stoxs]

WOW

very interesting stuff folks thanks

it is simliar to how an air conditioning unit works but in reverse,maybe who ever created it got the idea from this ?

thats about al i can comment on

thanks again for the as usual top discriptions