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The Fohn Effect


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Posted
  • Location: Ireland, probably South Tipperary
  • Weather Preferences: Cold, Snow, Windstorms and Thunderstorms
  • Location: Ireland, probably South Tipperary

    I've had my La Crosse weather station since Christmas and the temperature has always been close to the relatively local met stations. At the moment though, it's reading 15.3C when the highest anywhere else the Ireland is 12C.

    I live in a something of a valley though with foot of the Knockmealdowns to the South West and Galtees a similar distance to the north, and with the wind being South Westerly at the moment, could the high temperatures be something of a local Fohn effect?

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    Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

    without seeinf a contour map I could not say really but it sounds a bit like it may be. What is the check station you use showing wind direction and your own?

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    Posted
  • Location: Ireland, probably South Tipperary
  • Weather Preferences: Cold, Snow, Windstorms and Thunderstorms
  • Location: Ireland, probably South Tipperary

    Thanks for the reply.

    I can't seem to find a free topographic map but the mountains to the south west are just over 2000ft and just a few miles from my house. I have an anemometer on the shed roof and would use Cork or Shannon as the nearest station.

    Temperature is down to 12.7C so more inline with the rest of the country.

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral

    The things you should be looking for in a foehn effect is a significant drop in humidity, (ie low dewpoints), a significant rise in temperature (relative to other places), and visually a partly clear or clear sky with a cloud bank to your southwest over the mountain ranges.

    Here today the maximum has been 13.1C the joint highest however the dewpoint was 8C so it cannot have been a foehn effect despite the broken cloud and sunshine at times. We do get some impressive foehn effects though here.. January 2000 is a great example here. A band of light (mainly orographically heavy rain) was tracking in from the southwest however here the skies cleared and the sun came out and temperature rose to 16C, whereas in Manchester it was 7C and raining. Come the night the cloud quickly filled in the temperatures quickly dropped back to average.

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    Posted
  • Location: Ireland, probably South Tipperary
  • Weather Preferences: Cold, Snow, Windstorms and Thunderstorms
  • Location: Ireland, probably South Tipperary
    The things you should be looking for in a foehn effect is a significant drop in humidity, (ie low dewpoints), a significant rise in temperature (relative to other places), and visually a partly clear or clear sky with a cloud bank to your southwest over the mountain ranges.

    Just checked back over the data for earlier and during the time the temperature was higher, humidity had dropped from 94% to around 78% before going back up to around 90% when temps levelled off again. Dunno if that counts as a significant drop though. From what I can remember, skies were partly clear though I wasn't checking much.

    The highest the temperature got up to was 15.4C.

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    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL

    Some rough calculations from your values give this:

    (Temp/Dewpoint)

    15.4/11.5

    Then

    12.7/11.1

    Very little change in the dewpoint there (essentially negligible).

    It doesn't really scream out "foehn effect" to be honest. However, clearly the dewpoint depression does increase for some reason. If you could correlate that to small changes in wind direction it may tell you something.

    Just at first glance it suggests to me that perhaps the wind dropped off a little, and if the skies were clear this allowed some additional heating. That caused the temperature to rise by a few degrees, and then it retreated when the wind picked up.

    In summary, something caused your air to warm up but not notably dry out. So it doesn't quite correlate to the foehn effect, at least not to my eyes. That doesn't mean that there wasn't some foehn drying going on somewhere, there's the chance that it was occurring aloft and you got some of that warmer stuff briefly mixed down to surface level. But really if you're looking for classic foehn stuff you would want humidities in the region of 5-50%.

    Keep a close eye on what happens next time you have mild southwesterlies ahead of an approaching cold front.

    Although the traditional explanation of the foehn effect requires precipitation on the windward side of the barrier, this is not actually necessary. If you have a very stable southwesterly, which is saturated in lower levels but dry higher up, then you may end up having that drier air forced over the barrier, and then descending on the otherside. As it's so dry, it will heat up quite considerably and you will have a warm, dry wind hitting you.

    The issue with this is that 600m is typically not high enough to break into drier air, it's likely to still be saturated at that level. So then you end up with the moist air coming over the barrier, which is the traditional explanation. It would hopefully precipitate onto the windward side, and dry out somewhat throughout that process, and then descend on your side and heat up a fair bit.

    A good aid would be to study tephigrams taken on the windward side of that barrier (assuming there are any) and from that you can figure out some properties of the air coming in. If it's unstable, you'll never get a foehn effect.

    Another possible trouble is that of how long is your barrier? Even in stable airflows, if the range is not very long, you may just end up with the air damming up on the windward side, and then rushing around the edges to eventually meet you in some form. This doesn't allow for any rising or drying out of the air, and so no warming.

    I've attached two pictures I took of the sorts of cloudbanks that Stephen was talking about. They are amazing to watch. From a distance, they appear stationary. But if you get close enough you can see the movement, you essentially see the drying happen before your eyes. The capping cloud is the remainder of the moisture that condenses out at the very peak of the ascent. Then the air is dry and able to be compressed and warmed considerably on its leeward descent.

    post-7526-1236483048_thumb.jpg

    post-7526-1236483081_thumb.jpg

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    Posted
  • Location: Ireland, probably South Tipperary
  • Weather Preferences: Cold, Snow, Windstorms and Thunderstorms
  • Location: Ireland, probably South Tipperary

    I sort of knew it wasn't a classic fohn, I just thought it might have been a slightly lesser version. There isn't any weather stations that display much public info within about 80km of my house so that can be something of a hinderence. My weather station is set to take just hourly readings so I can't check for subtle wind direction changes.

    The mountains to the north are a similar distance away, over 50km long and are over 1000m high so I'm probably more likely to get a better effect from them.

    They're some nice photos there alright, will definitely have a look out for that kinda thing if I can.

    Is it still possible to get a fohn in the summer? I'm in college 5 days a week so am only able to have a look out during the weekends. Next summer though I'm hoping to do my dissertation on the effect of the mountains on the weather where I live

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral

    Yes it's very very possible to get a foehn in the summer. A great example was 2003, Anglesey not noted for it's hot summer got a southerly of Snowdonia (circa 1000m) and it had reached 34C by 9am in the morning, whilst places around it where at 25 or 26C absolutely amazing. In the afternoon however RAF Valley (the place in Anglesey) was back at 21C with a notable sea breeze, whereas my location had warmed to 33C on the account of the 'normal' heat.

    I often get foehns in winter and Spring here. I can remember many, January 2006 I reached over 15C 7 times, due to partial or full foehn effects. In summer they are less common, but always there because of Wales steep up ones side mountains and slowly declining inclines on the other side.

    Actually as I speak there is a clear sky all around, temperature has risen to 8C, and the cloud seems to be staying over Wales. Perfect oppurtunity for a foehn effect, however the dewpoints and humidity are too high - they do have the ability to filter down here, but probably wont. It's bone dry here compared to surrounding areas, but thats more a topographical barrier issue than a foehn issue. That said, everytime there is a southwesterly blowing with low dewpoints behind the front, I get excited.

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    Posted
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]
  • Location: Canmore, Canada [4296ft] & North Kent [350ft]

    I live for the fohn effect here in winter (or what we call a chinook here in canada). I have seen the thermometer on my deck go from minus 10oC to plus 10oC in the dead of winter within hours. Its such a nice feeling. Living on the eastern foothills of the rocky mountains, with a predominatly westerly wind flow, we receive them fairly often

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral
    I live for the fohn effect here in winter (or what we call a chinook here in canada). I have seen the thermometer on my deck go from minus 10oC to plus 10oC in the dead of winter within hours. Its such a nice feeling. Living on the eastern foothills of the rocky mountains, with a predominatly westerly wind flow, we receive them fairly often

    I have indeed seen temperature observations where the whole of Montana, USA is virtually -20C and certain areas get +12C it's really something. Actually right now in Montana there is a major foehn effect.. Fort Huachuca is recording 19C whilst Helena is -7C. Impressive! The rockies certainly are the place to be for foehn effects!

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    Posted
  • Location: City of Gales, New Zealand, 150m ASL
  • Location: City of Gales, New Zealand, 150m ASL
    I sort of knew it wasn't a classic fohn, I just thought it might have been a slightly lesser version. There isn't any weather stations that display much public info within about 80km of my house so that can be something of a hinderence. My weather station is set to take just hourly readings so I can't check for subtle wind direction changes.

    The mountains to the north are a similar distance away, over 50km long and are over 1000m high so I'm probably more likely to get a better effect from them.

    They're some nice photos there alright, will definitely have a look out for that kinda thing if I can.

    Is it still possible to get a fohn in the summer? I'm in college 5 days a week so am only able to have a look out during the weekends. Next summer though I'm hoping to do my dissertation on the effect of the mountains on the weather where I live

    From the little I know about Ireland, the only mountains in the country over 1000 metres are away to your southwest.

    It would be harder to get a foehn wind from the north. Again it would need to be stable, so northwesterlies behind a cold front would usually not be suitable. A stable northeasterly flow could do the trick. You can get foehn warmed southeasterlies over here, so presumably you could get the equivalent, but the air would have to be very damp on the windward side of the range. But again the local geography might scupper you. Does the range extend away to your northeast?

    I have indeed seen temperature observations where the whole of Montana, USA is virtually -20C and certain areas get +12C it's really something. Actually right now in Montana there is a major foehn effect.. Fort Huachuca is recording 19C whilst Helena is -7C. Impressive! The rockies certainly are the place to be for foehn effects!

    I think that greatest temperature changes in the world have been recorded in lee of the Rockies. Some amazingly huge jumps:

    Loma, Montana boasts as having the most extreme recorded temperature change in a 24-hour period. On January 15, 1972, the temperature rose from −54 °F (-48 °C) to 49 °F (9 °C), a 103 °F (57 °C) change in temperature; a dramatic example of the regional Chinook wind in action.

    The Black Hills of South Dakota are home to the world's fastest recorded rise in temperature. On January 22, 1943, at about 7:30am MST, the temperature in Spearfish, South Dakota was -4 °F (-20 °C). The chinook kicked in, and two minutes later the temperature was +45 °F (7 °C). The 49 degree (27 °C) rise in two minutes set a world record that is still on the books. By 9:00am, the temperature had risen to 54 °F (12 °C). Suddenly, the chinook died down and the temperature tumbled back to -4 °F. The 58 degree drop took only 27 minutes.

    Yes it's very very possible to get a foehn in the summer. A great example was 2003, Anglesey not noted for it's hot summer got a southerly of Snowdonia (circa 1000m) and it had reached 34C by 9am in the morning, whilst places around it where at 25 or 26C absolutely amazing. In the afternoon however RAF Valley (the place in Anglesey) was back at 21C with a notable sea breeze, whereas my location had warmed to 33C on the account of the 'normal' heat.

    When was that? I can "only" (!) find a 33C for RAF Valley on the 5th August.

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)

    The best places to see foehn in the UK, in terms of effects, are Aberdeenshire and the Helm Wind off Cross Fell in the Pennines, where a classic "Helm Bar" forms:

    helm%20bar-s.jpg

    This also explains why Aberdeen has the highest maximum ever recorded in January, of over 18c. I occasionally get foehn here although I don't remember it ever happening in summer; however I can remember a straight westerly occasionally giving 15c+ temps with below 10 to the W of the Pennines. Unfortunately though, 700m just isn't quite high enough for it to happen regularly and lapse rates etc. need to be pretty steep.

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral
    When was that? I can "only" (!) find a 33C for RAF Valley on the 5th August.

    Yes you're right, it was 33C in Anglesey that day and 34C here - I always get it mixed up :lol:

    Still impressive though.

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)

    Wow, it seems like I may have a foehn effect today! Current temp 11.1 and rising (nearest observations five to eight); humidity 68% and DP 5.4 with these two dropping fast! There is also a huge, sudden bank of cloud to my W although this could be anything. Could it be!?

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    Posted
  • Location: Western Isles
  • Location: Western Isles

    I was reading about this and that temps can rise 20 to 30 degrees in a matter of hours,

    dose anybody know the fastest change in temps during a fohn event?

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
    I was reading about this and that temps can rise 20 to 30 degrees in a matter of hours,

    dose anybody know the fastest change in temps during a fohn event?

    Read J07's earlier post! :doh:

    Loma, Montana boasts as having the most extreme recorded temperature change in a 24-hour period. On January 15, 1972, the temperature rose from −54 °F (-48 °C) to 49 °F (9 °C), a 103 °F (57 °C) change in temperature; a dramatic example of the regional Chinook wind in action.

    The Black Hills of South Dakota are home to the world's fastest recorded rise in temperature. On January 22, 1943, at about 7:30am MST, the temperature in Spearfish, South Dakota was -4 °F (-20 °C). The chinook kicked in, and two minutes later the temperature was +45 °F (7 °C). The 49 degree (27 °C) rise in two minutes set a world record that is still on the books. By 9:00am, the temperature had risen to 54 °F (12 °C). Suddenly, the chinook died down and the temperature tumbled back to -4 °F. The 58 degree drop took only 27 minutes.

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    Posted
  • Location: Aberdeen 33m asl
  • Location: Aberdeen 33m asl

    Yes, the fohn effect is very significant for Aberdeen - I remember playing golf in a polo shirt in January about maybe 10 years ago and also this effect explains why Aberdeen can reach summer maximums in excess of 79°F.

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)

    Why is it that in southern Germany/Austria, the foehn wind is notorious for giving people headaches, drowsiness etc.? It's hated over there.

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)

    Thanks OON - I've always wanted to see that. I guess you've seen that a fair few times then?

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    Yes it's very very possible to get a foehn in the summer. A great example was 2003, Anglesey not noted for it's hot summer got a southerly of Snowdonia (circa 1000m) and it had reached 34C by 9am in the morning, whilst places around it where at 25 or 26C absolutely amazing. In the afternoon however RAF Valley (the place in Anglesey) was back at 21C with a notable sea breeze, whereas my location had warmed to 33C on the account of the 'normal' heat.

    Indeed Hawarden often gets the warmest conditions in Wales due to the foehn effect.

    Over the coinitent the foehn effect is associated with very dry air, so I imagine this can effect people with breathing type problems, of course it can also cause rapid snow melt over the Alps, so this also make it unpopular for some.

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    Posted
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
  • Location: Cambridge (term time) and Bonn, Germany 170m (holidays)
    Indeed Hawarden often gets the warmest conditions in Wales due to the foehn effect.

    Over the coinitent the foehn effect is associated with very dry air, so I imagine this can effect people with breathing type problems, of course it can also cause rapid snow melt over the Alps, so this also make it unpopular for some.

    Apparently suicide rates rise by 10% when Foehn blows. Known for migraines and psychosis!

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    Posted
  • Location: Heswall, Wirral
  • Weather Preferences: Summer: warm, humid, thundery. Winter: mild, stormy, some snow.
  • Location: Heswall, Wirral

    :lol: I'll bear that one in mind next time the SW'ly blows :winky:

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    Posted
  • Location: Western Isles
  • Location: Western Isles
    Apparently suicide rates rise by 10% when Foehn blows. Known for migraines and psychosis!
    These winds are often associated with illnesses ranging from migraines to psychosis. A study by the Ludwig-Maximilians-Universität München found that suicide and accidents increased by 10 percent during föhn winds in Central Europe. The causation of Föhnkrankheit (English: Föhn-sickness) is yet unproven. Labelling for preparations of aspirin combined with caffeine, codeine and the like will sometimes include Föhnkrankheit amongst the indications.

    http://en.wikipedia.org/wiki/Foehn

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