The Fohn Effect

[Yeti]

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

:lol:

I think I had foehn today. At one point the temp reached 12.3 when W of the Pennines it was 6-8; 64% humidity and much warmer than pretty much anywhere else in the UK - also sunny, with a huge, abrupt bank of cloud to the W. Sounds like it could have been to me, I was amazed at how high the temp got!

[SP1986]

:winky:

I think I had foehn today. At one point the temp reached 12.3 when W of the Pennines it was 6-8; 64% humidity and much warmer than pretty much anywhere else in the UK - also sunny, with a huge, abrupt bank of cloud to the W. Sounds like it could have been to me, I was amazed at how high the temp got!

Perhaps, we got 10C here but it was just my location its usually warmer than surrounding areas. Your best bet is with SWly high pressure (and mine too)

[J10]

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

Well I can honestly say I didn't know that.

[Yeti]

Perhaps, we got 10C here but it was just my location its usually warmer than surrounding areas. Your best bet is with SWly high pressure (and mine too)

Yes you are near the coast whereas I'm inland and at 185m so you would expect it to be a good 1c colder than the Vale of York - however it was a lot warmer! The Pennines are to my immediate W. Although I don't think a SW'ly is the best direction - the hills simply aren't high enough despite the airmass being better. A straight W'ly or NW'ly drags the air across 600-900m hills rather than 400-600.

[J07]

Here is a little something on the health impacts of the foehn:

Interesting that some people claim they have symptoms ahead of a foehn actually kicking in. I'm not really sure if this sort of thing is psychosomatic

Across Canterbury the symptoms were very similar, but the big difference was that some people said that they got the symptoms when the nor'wester blew, but others said that they could forecast when the nor'wester was going to arrive. Those who reported the effects when the nor'wester blew were inland, and those who could forecast its arrival were on the coast.

The scientists' original theory was that the static electricity in the dry air created positive ions that changed people's seratonin levels because it had been shown in mouse studies in Europe that positive ions increased their seratonin levels. The inland people were in the area of the positive ions, but the coastal people were in a sea breeze. It took six to eight hours for the nor'wester to blow away the sea breeze.

"The seabreeze question took me back to my understanding of the ionosphere where the positive ions are creating an electric field between the ionosphere and the earth," Neil recalled. "The positive ions in the nor'wester are creating a stronger local electric field and those norwest ions travel over the sea breeze creating the electric field down to earth, so both of the populations were receiving the changed electric field from the ions.

"I looked up the literature on melatonin and seratonin and found that electromagnetic fields in many studies of electrical workers show reduced melatonin. That suggested that the nor'westers were altering the seratonin/melatonin ratio through the altered electric fields. When those electric fields flow through your body they create a current which creates magnetic fields so they become electromagnetic fields."

As luck would have it, there's a foehn over the south of the country right now. You can see the places ahead of the cold front (scrawled in by yours truly...), with the likes of 22/05.

On the otherside of the divide (west coast), places have about 14/11. So there is 8C of warming in that wind, and dewpoints reduced by 6C. The humidity on the west coast is 84%, on the east coast it is 32%.

Behind the front, clearly is a very different air mass, and also very clear that despite a westerly wind at Dunedin there is definitely no foehn effect.

This is not a classic foehn set up, because the entire synoptic flow is southwesterly. Therefore, the air that is being foehn warmed is starting off really quite chilly anyway. Clearly though it is stable enough and saturated enough to be capable of being foehned.

A better example is to have a warm sourced synoptic flow, which gives much more impressive effects.

If you're on the lookout for classic tell tale signs of foehn winds, then lenticular clouds are useful. They suggest significant disruption to the flow of air at mid-levels, and have a very dramatic and beautiful shape at times:

(But seeing these clouds does not necessarily imply that there will be a foehn wind blowing in your location).