The UK has for a long time been described has having a maximum UV index of 8, at one location in Cornwall, rarely. This was challenged at the MetO in recent years until they adjusted their forecast UV range from 1-7 to 1-8. A factor of 8 is now more commonly forecast eg on the BBC website in June & July.

This interesting link gives a UV reading in Reading in June 2000 (this was, admittedly, a notable dry air high UV event) of about 9.6. Reading is some distance inland away from the S coast where higher UV indices are often recorded.There are other events I am sure if anyone has the time to scroll through the site.

On that June day I was walking north in the Wallop Brook valley and had my shirt off for about 15 minutes before my wife told me to put it back on. I have a white skin that tans easily. I burned. I just thought the UV index was a worthwhile topic- some of us look out for it cranking up to 2 in March each year with some excitement!

https://uk-air.defra.gov.uk/data/uv-index-graphs

Forgot to add- notice on some days the curve is constant, but these are not the days with the highest UV- the spikier days, suggesting cumulus or other interruptions, reach the highest values; cloud cover or air type intensifying the UV?

As the proud owner of my own little green power station - my 15 panel  solar array - I notice how the solar output ticks up markedly under any sun at the very start of March, compared to a week or two beforehand.  Exciting times, I relish this time of year, the best of the astronomical winter just about to come, and the promise of increasing daylength and some sparkling sunshine just around the corner. 

Good website that. 

March 7th is the day when the sun starts to turn my skin a healthier looking colour. Tanning opportunity continues until about 2nd October down here for me. By mid January I start to look decidedly pale !

Between 7-14th March the sun really does ramp up.

Not long now...

I agree- at least, in our sort of latitude; I do remember a slight bronzing (you'd have to want to see it, if you know what I mean) out on Purbeck in Feb 87, under long periods of sun, but my earliest gentle tanning recently was first week of March, re-roofing the shed. The shadow of the house down the lawn shortens quickly through March too.

http://www.bbc.co.uk/weather/2633858

First UV2 forecast for the Itchen Valley today. That'll open the crocuses.

Just seen yesterday's data- it did just reach 2 at around 1200 but sadly I was locked away in a dark box.

UV max at Reading 1.7 on Saturday and 1.6 yesterday despite the cloud.

Reading max UV 2.1 yesterday; highest of the year so far.

UV 1.6 max at Reading yesterday.

Reading UV yesterday 1.4 max.

Reading UV max 1.8 yesterday. Already 1.3 at 9am this morning.

Above 2 is the threshold I look for, because it's only above that number that exposure to UV enables the body to start generating vitamin D.

Peaked at 1.9 today. Thought it could have passed two easily.

A few extra minutes of solar heating in my famed west-facing hobby room this afternoon.  That's a few less minutes saved on the central heating. Of course, that only applies when the sun is actually out, like it has been off and on today.

Only managed 1.3 today under extensive cloud- but that in itself is interesting, as Boxing Day was a beautiful sunny morning and it only reached 1.3 in the sunshine- so UV levels under the cloud now match those in bright sunshine near the solstice.

Reached 1.8 at 11a.m. before the weather turned for the day. Another week and we'll be seeing 2.5, I'm sure. Interesting that the Beeb/ MetO seem to be quite frequently forecasting 2s for the south earlier in the year than usual.

1.4 today.

Glad you could join the thread Lumi!

It will be interesting to compare the Santa Monica/ various UK locations as the year progresses. I'd like to draw your attention to the day in June 2000 when phenomenally dry air gave a level in Reading of > 9. Some UK spots may have approached 10 that day. The previous forecast range of 1-7 for the UK (which, after my contestations with the metO was upped to 😎 were clearly related to general data, rather than extreme. I bet Santa M gets a 14 on the most extreme!

8 is a regular June/ July forecast in S. UK now. 6 from May to August is common. 

Generally days with lower upper air humidity (like today!) are the days with deep blue skies and puffy cumulus clouds, so the UV reading between clouds will be higher.

Hazy summer days with higher humidity and a bit of general high cirrus tend to have fewer cumulus clouds.

This is a good link showing current and forecast upper air humidity. Dark browns (low humidity) are usually associated with deep blue skies (assuming there is no lower level cloud obscuring it of course!) 

http://www.instantweathermaps.com/GFS-php/showmap-euroupper.php?run=2017022806&var=RH&lev=300mb&hour=000

1.7 today.