Misty Warm Sector = High UV

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Misty Warm Sector = High UV

Postby Graham Easterling » 2012 Jul 17, 16:22

Never fails. UV hit 8.9 on Scilly during some afternoon sunshine when the mist broke. Once the cold front goes through, the UV will be much lower.

Quite surprising that clear Polar maritime air gives lower UV than warm sectors, but it's very noticeable. Hence the saying down here that mist burns you. It certainly can, I've seen St Martins record > 6 in sea mist, the highest readings as mist just clears, or just before it forms.

I know Martin once suggested that the high readings were due to the upper atmospheric conditions in warm sector ridges, whilst others have suggested the affect of the high humidity at the lower levels. Any theories welcome.

The highest UV I've seen at St. Martins is 10.2, and at Camborne 9.8, both in conditions very similar to this, only late June rather than mid July. I think Nick Gardner might also have a useful input here.
Last edited by Graham Easterling on 2012 Jul 18, 18:26, edited 1 time in total.
Graham

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Re: Misty Warm Sector = High UV

Postby Martin Rowley » 2012 Jul 17, 20:48

Graham Easterling wrote:Never fails. UV hit 8.9 on Scilly during some afternoon sunshine when the mist broke. http://www.weatherlink.com/user/taperki ... &headers=1 . Once the cold front goes through, the UV will be much lower.

Quite surprising that clear Polar maritime air gives lower UV than warm sectors, but it's very noticeable. Hence the saying down here that mist burns you. It certainly can, I've seen St Martins record > 6 in sea mist, the highest readings as mist just clears, or just before it forms.

I know Martin once suggested that the high readings were due to the upper atmospheric conditions in warm sector ridges, whilst others have suggested the affect of the high humidity at the lower levels. Any theories welcome.


... I don't want to set myself up as some sort of expert on this ... my 'knowledge' such as it is was gleaned from many years ago by studying the works of one of my meteorological heroes, GMB Dobson (who died in 1976): he spent the largest part of his career studying ozone in the stratosphere. His name has been linked with Alan Brewer (died 2007), born in Canada but raised in England and who worked for the Meteorological Office & later at Oxford University (as did Dobson), also researching stratospheric motion [ hence the Brewer-Dobson circulation of the diagram below ].
Dobson found by measurement (he designed the Dobson spectrograph, a network of which he arranged to be set up across the globe) that total column ozone varied in a non-intuitive manner: it was *most intense* at high latitudes and less intense/lower concentration in the equatorial zone. As the intensity of solar radiation is greatest across the tropical regions, it might be expected that the stratosphere would carry the greatest concentration there - but Brewer showed that the stratospheric circulations led to the distributions found.
Here is a link to the Wikipedia chart based on the NIMBUS observations in the 1980s. http://upload.wikimedia.org/wikipedia/c ... lation.jpg
You can see that air with an origin poleward of 60 degN/S. will have considerably more total column ozone (the greater part being within the stratosphere) than air with an origin in the sub-tropics.
Dobson codified these variations (depending upon changes in synoptic pattern) as follows:-
*High Ozone:-
- Cyclonic wind circulation at tropopause level
- High temperature in the stratosphere
- Low temperature in the troposphere
- Low level of tropopause
- Low absolute pressure
[ i.e., conditions associated with low-level cyclonic types]
*Low Ozone:-
-Anticyclonic wind circulation at the tropopause
-Low temperature in the stratosphere
-High temperature in the troposphere
-High level of tropopause
-High absolute pressure
[i.e., conditions associated with high-pressure types]
So, now we've got this thing the BBC have suddenly found out about - the jet stream - that's moved north, we've got sub-tropical origin air aloft - with the characteristics of the second part of the table above edging north.
Low total ozone means less 'shielding' in the UV bands that cause sunburn.

Martin.
Martin Rowley
West Moors, East Dorset [ altitude: 17 m/56' ]
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Re: Misty Warm Sector = High UV

Postby Nick Gardner » 2012 Jul 18, 18:12

Graham Easterling wrote:Never fails. UV hit 8.9 on Scilly during some afternoon sunshine when the mist broke. http://www.weatherlink.com/user/taperki ... &headers=1 . Once the cold front goes through, the UV will be much lower.


Same here yesterday Graham, UV reached 8.9. Equal highest this year with the 2nd June.

As you say, always high in a warm sector.
Near Newton Poppleford, Sidmouth - Devon
50:41N 3:17 W; 20 metres AMSL; Station Grade B-BAAA37
http://www.ottervalley.co.uk
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Location: Rural riverside valley location near the village of Newton Poppleford and 2 km from the sea.


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