On Tuesday, 28 May 2019 19:22:38 UTC+1, Nick Gardner wrote:
On 27/05/2019 23:22, Nicholas Randall wrote:
UV levels are not related to temperature so they would not be lower because of a colder air mass and they would not be higher because of a warmer air mass. They relate to the position of the sun in the sky. At this time of year, it is at its highest point at 1 pm and this is when UV levels can be high.

Thanks for this. If only it was that simple (as the Met Office seems to
think so).

As Graham has already stated our findings with regards to UV, the only
thing I can add is that it is thought that there is ozone depletion
above tropical maritime air masses. The mechanisms for this, I believe,
are poorly understood but someone (on this newsgroup) said that they
were aware of this when they were studying atmospheric sciences back in
the 1960s. It seems that this knowledge has been lost over the years.

Here in the SW we get some of the highest UV levels in northern Europe.
And, given a tropical air mass, throw in a bit of mist and a few clouds
and from mid-May to early August the UV can reach as high as the extreme
value of 10. On days like that I often do a scan of UV levels across the
rest of Europe and places as far south as Gibraltar and Cyprus can be
'wallowing' in levels around 7.

So stating that the UV levels are directly linked to elevation of the
sun is overtly simplistic and could be misleading.

--
Nick Gardner
Otter Valley, Devon
20 m amsl
http://www.ottervalleyweather.me.uk

I am aware that there are other factors that affect UV levels but I only stated one. Others are latitude, altitude and ozone. Lower ozone values lead to higher UV levels and higher ozone values lead to lower UV levels. With latitude, the lower the location is, the higher the UV levels are. With altitude, at the top of a mountain UV levels are higher because the air is thinner and cleaner. Another factor that affect UV levels is reflection which I have seen Graham mention today. Most surfaces reflect UV levels but some do more than others. Water does reflect UV and that has been mentioned here. I do not think high humidity or mist cause high UV levels. When Graham mentions sea mist, I think it would be the water causing them to be high through reflection.

Nicholas
Meir Heath, Stoke-On-Trent 250 metres above sea level.