Exceptionally Low Rh?

[skifreak]

The Summit Weather Station on CairnGorm Mountain is reporting a Relative Humidity of just 9% as off 10pm and it's been falling steadily as an inversion sets up. It's been down into the mid 20s% at the mid station as well.

What is the record low for RH in the UK? How exceptional is a single digit value here?

[knocker]

The Summit Weather Station on CairnGorm Mountain is reporting a Relative Humidity of just 9% as off 10pm and it's been falling steadily as an inversion sets up. It's been down into the mid 20s% at the mid station as well.

What is the record low for RH in the UK? How exceptional is a single digit value here?

Without checking any stats 9% is very exceptional if it's correct but I'm not sure that the the Summit Weather Station on CairnGorm Mountain should be taken as a comparison with he rest of the UK. Above an inversion?

[skifreak]

Yes Summit is above an inversion, Summit was at 7c at 10pm and at Loch Morlich at the foot of the mountain at 0.6c.

[summer blizzard]

Considering that we are surrounded by sea i would say that is exceptional.

[Evening thunder]

No idea when but I remember several years ago a BBC forecast saying something on the lines of "(some station) in north west Scotland recorded a relative humidity of just 3% earlier today, and said the process of how it happened, I think they said when the air rises over the mountains and the moisture condenses out then the dry air warms up as it descends the other side.

Cant remember the wording very well but I'm sure the figure was 3%, not sure how to check though.

[Terminal Moraine]

No idea when but I remember several years ago a BBC forecast saying something on the lines of "(some station) in north west Scotland recorded a relative humidity of just 3% earlier today, and said the process of how it happened, I think they said when the air rises over the mountains and the moisture condenses out then the dry air warms up as it descends the other side.

Cant remember the wording very well but I'm sure the figure was 3%, not sure how to check though.

If I remember correctly I think that was at Altnaharra but I can't remember the year. R.H values below 10% are pretty rare in Britain no matter what the location.

[Methuselah]

March and April 2003 had similarly arid conditions up in the NW of Scotty. The air was so dry that heath fires were breaking out...

[knocker]

Yes Summit is above an inversion, Summit was at 7c at 10pm and at Loch Morlich at the foot of the mountain at 0.6c.

Not a very good comparison but an attempt to illustrate my thinking. Looking at Cambone's midday ascent you can see the humidity at 880mb is 3% so you can get very low humidities at low heights often just above an inversion. This could possble occur at the summit of Cairngorm.

Edited March 23, 2012 by weather ship

[knocker]

No idea when but I remember several years ago a BBC forecast saying something on the lines of "(some station) in north west Scotland recorded a relative humidity of just 3% earlier today, and said the process of how it happened, I think they said when the air rises over the mountains and the moisture condenses out then the dry air warms up as it descends the other side.

Cant remember the wording very well but I'm sure the figure was 3%, not sure how to check though.

This is the principle of the foehn wind. Orographic uplift causes the air to expand and be adiabatically cooled at the dry adiabatic lapse rate until it becomes saturated at which point latent heat is released when WV begins to condense which slows the rate of cooling to the saturated adiabatic lapse rate.

Condensation turns to precipitation on the windward side of the mountain so the drier air descends and compresses down the dry adiabatic lapse rate on the leeward side which leads to warming and arid conditions and considerable increase in temperature. Probably the best known example being the Chinook. There are well known examples in the UK but I’ve no idea how low the humidity can get here. This of course wouldn’t be the case at the summit of Cairngorm

Edited March 23, 2012 by weather ship

[knocker]

If I remember correctly I think that was at Altnaharra but I can't remember the year. R.H values below 10% are pretty rare in Britain no matter what the location.

I think you are correct TM.

Altnaharra is not the Sahara

A Highland village has not set a new UK record for low humidity despite weather reading equipment indicating it had, according to the Met Office.

The reading showed that Altnaharra in north west Scotland was drier than the hottest desert on Tuesday.

However, the Met Office has said that equipment, which showed humidity at 1.6%, was incorrect.

Met Office meteorologist Andy Yeatman said the humidity was still exceptionally low, at about 8% it was lower than most deserts.

http://news.bbc.co.u...and/2777173.stm

Edited March 23, 2012 by weather ship

[Scorcher]

I seem to remember widespread RH values below 20% in July 2006, but I can't recall many days below 10% in this country.

[reef]

The lowest humidity Ive ever recorded was actually last April, which was 30%.

Anything below 40% is pretty unusual here so close to the North Sea.

[johnholmes]

I recall some time in the 60's a fairly widespread very low RH, I'll try and dig something out, I was at Manchester Aurport and 'think' the RH got to 10-12% but there were lower values reported?

[Interitus]

I posted in January this year when Cairngorm had a RH below 10% and similarly for Aonach Mor so though unusual it isn't unheard of for these high altitude stations with the right conditions of subsiding air -

http://forum.netweather.tv/topic/72246-monday-16th-january-2012-weather-general-chat/page__view__findpost__p__2214445

[AderynCoch]

I recall some time in the 60's a fairly widespread very low RH, I'll try and dig something out, I was at Manchester Aurport and 'think' the RH got to 10-12% but there were lower values reported?

Cape Wrath recorded 6% humidity on 2nd March 1963. I'm not sure if that was an isolated case or not.

http://www.personal.dundee.ac.uk/~taharley/1963_weather.htm