Uv Index Peaks In The Uk & Cloud Types Affecting It?

[conor123]

I was wondering how high the UV index can actually get in the UK and what causes it to be say 6.3 (today), 5.9 (tommorow) etc, counting the sky is clear.

The peak time is usually the 21st June apparently but ive noticed that next Sundays UV index is forecast a 5.7 whilst Thursday

is forecast 7.0 (providing clear sky) which is quite confusing?

Also ive noticed that on a day like today with scattered clouds the suns strength seems to dramatically increase shortly before going

into Cumulus clouds and Cirrus seems to make it stronger to.

Does the sun reflecting off surfaces like windows/cars etc magnify its strength aswell?

Thanks.

Edited June 13, 2009 by conor123

[J07]

I was wondering how high the UV index can actually get in the UK and what causes it to be say 6.3 (today), 5.9 (tommorow) etc, counting the sky is clear.

The peak time is usually the 21st June apparently but ive noticed that next Sundays UV index is forecast a 5.7 whilst Thursday

is forecast 7.0 (providing clear sky) which is quite confusing?

Also ive noticed that on a day like today with scattered clouds the suns strength seems to dramatically increase shortly before going

into Cumulus clouds and Cirrus seems to make it stronger to.

Does the sun reflecting off surfaces like windows/cars etc magnify its strength aswell?

Thanks.

AFAIK, a few things contribute (at least): Ozone depth, latitude, time of the year (ie distance from the sun), pollution and cloud.

I really know little about how ozone depth varies, but obviously the layer is very shallow near the Ozone hole and when parts of it break off. Elsewhere, I think typically there is less ozone in the tropical atmosphere than in temperate regions. I don't know what governs the day-to-day movement of different depths of ozone.

Where do you get your UV forecasts? It sounds like it's pulled straight out of a computer model, which is probably trying to juggle a few different variables at once. See the two attached files for an example of the type of thing we get down here. It's also trying to take into account the cloud, but I don't know how sophisticated is the cloud model. Clearly it recognises different depths - with the UVI being 8 underneath one area of cloud but 5 under another area.

Yours are clear sky forecasts, I wonder if it's just translating forecast ozone depth directly into a surface UVI?

But I found that UV forecasts can be just as wrong as temperature forecasts, at least down here anyway.

I suppose you would get your highest UV within 2 weeks of the Solstice. Worth noting that the sun angle changes *very little* around the solstice, so being bang on June 21st is probably not much different to being in the first week of July when it comes to UV.

Highest UVI ever recorded in the UK was 8, somewhere in the South...don't have it to any greater accuracy than that. UVI would be limited by the relatively polluted atmosphere over the British Isles, the high latitude and low sun angle, and the fact that the Earth is furthest from the Sun around July 4th - hence the seasons are "backwards".

I don't have any sources at hand, but I think the highest sea level UVI was 17 in Carnarvon, Western Australia but mountainous tropical regions in the Southern Hemisphere (eg Andes, PNG) get UVI up to 25.

The "high cloud magnifies UV" is a mystery to me, I've often heard it but really don't know.

[conor123]

AFAIK, a few things contribute (at least): Ozone depth, latitude, time of the year (ie distance from the sun), pollution and cloud.

I really know little about how ozone depth varies, but obviously the layer is very shallow near the Ozone hole and when parts of it break off. Elsewhere, I think typically there is less ozone in the tropical atmosphere than in temperate regions. I don't know what governs the day-to-day movement of different depths of ozone.

Where do you get your UV forecasts? It sounds like it's pulled straight out of a computer model, which is probably trying to juggle a few different variables at once. See the two attached files for an example of the type of thing we get down here. It's also trying to take into account the cloud, but I don't know how sophisticated is the cloud model. Clearly it recognises different depths - with the UVI being 8 underneath one area of cloud but 5 under another area.

Yours are clear sky forecasts, I wonder if it's just translating forecast ozone depth directly into a surface UVI?

But I found that UV forecasts can be just as wrong as temperature forecasts, at least down here anyway.

I suppose you would get your highest UV within 2 weeks of the Solstice. Worth noting that the sun angle changes *very little* around the solstice, so being bang on June 21st is probably not much different to being in the first week of July when it comes to UV.

Highest UVI ever recorded in the UK was 8, somewhere in the South...don't have it to any greater accuracy than that. UVI would be limited by the relatively polluted atmosphere over the British Isles, the high latitude and low sun angle, and the fact that the Earth is furthest from the Sun around July 4th - hence the seasons are "backwards".

I don't have any sources at hand, but I think the highest sea level UVI was 17 in Carnarvon, Western Australia but mountainous tropical regions in the Southern Hemisphere (eg Andes, PNG) get UVI up to 25.

The "high cloud magnifies UV" is a mystery to me, I've often heard it but really don't know.

Thanks for the reply :unsure:

I always use this website which estimates the UV index/ozone etc by longitude, latitude though ive found it to be unreliable recently.

http://www.temis.nl/uvradiation/nrt/uvinde....6&lat=52.8

Indeed i think the highest the UV has ever been here was about 7.7 with the Ozone column around 308DU.

I never knew that such high UV could be possible, a UVI of 25 must surely burn the skin in seconds as ive developed

quite a dark tan recently from days in the sun with a UV index of just 6 most days.