diamond geezer

 Sunday, March 26, 2017

As we edge into British Summer Time, is your sunscreen ready?

London's just entered the half of the year when the sun's rays are strong enough to cause skin damage. Indeed this is the very week when the UV Index ticks up from definitely 'Low' to potentially 'Moderate'. To help demonstrate this, here's a graph showing the UV Index in sunny London yesterday.

The UV Index appears in many weather forecasts, and acts as a guide to ultraviolet exposure during the sunniest parts of the day. For "the average person" it goes something like this...

UV IndexExposureAction
0-2LowYou can safely enjoy being outside.
3-5ModerateStay in the shade near midday, wear protective clothing, apply sunscreen.
6-7HighReduce time in the sun, wear protective clothing, apply sunscreen.
8-10Very HighMinimise time in the sun, wear protective clothing, apply sunscreen.
11+ExtremeAvoid time in the sun, wear protective clothing, apply sunscreen.

n.b. If you have lighter or darker skin colouring than average, click here for a reassessment of your risk. I'm fortunate enough to tan rather than burn, so for me the danger kicks in at 5 rather than 3.

The UV Index is a global scale, so scores of 11 and above tend to occur in the tropics. 10 is the maximum midsummer value in Toronto, where the UV Index was devised. In the UK we rarely go higher than 7, with 8 possible in late June in Cornwall and the Channel Islands.

I've long wondered how the UV Index is calculated, so have done some digging, and I can confirm that it's very complicated. Specifically a computer model is required to relate a) the strength of solar ultraviolet radiation, b) the concentration of ozone in the stratosphere, c) the amount of cloud in the atmosphere and d) the elevation of the ground. For the latter, UV intensity increases by about 6% for every 1000m above sea level. As for clouds, clear skies allow virtually 100% of UV to pass through, scattered clouds transmit 89%, broken clouds transmit 73% and overcast skies transmit 31%. This is why the UV forecast varies so much.

But it is possible to simplify things by ignoring actual weather conditions and focusing on what would happen in one place if there were clear skies. This splendidly basic website (circa 2003) allows me to calculate the maximum UV Index at any point on the globe at any time on any date. I've chosen London, obviously (other locations further north or south in the UK would be little different).

Assuming clear skies in London, what's the UV maximum at different times of year?

My first table shows the maximum UV Index in London at noon on the 21st day of each month. That's solar noon, the point when the sun is highest in the sky, which is around noon in the winter months and around 1pm in the summer. Theoretically, this is the highest the UV Index can reach. I've calculated the numbers to one decimal place.

Maximum UV Index in London at noon on the 21st of the month
UV Index0.

If you look at the green entries you'll see that the maximum UV Index is 'Low' for half of the year, from the start of autumn to the end of winter. Specifically the UV Index in London is always below 3.0 from 22nd September to 23rd March, which is why you haven't needed your sunscreen for the last six months. But last Thursday was the first day with a theoretical maximum in the yellow zone, and by May we'll be up into the oranges. Specifically the UV Index in London can be 6 or more from 13th May to 31st July. Notice that in this theoretical model London never quite reaches 7, although in reality certain atmospheric conditions can tip the index this high.

'Moderate' UV levels kick in when the sun is more than 40ยบ above the horizon, which happened for the first time this year at noon last week. Because the UV Index varies with the height of the sun in the sky, the maximum varies considerably according to the time of day. At dawn the UV Index is always 0 because the sun is on the horizon, and then on a cloudless day it rises through the morning and falls away in the afternoon.

My next table shows the maximum possible UV Index in London at various times of day on the 21st day of each month. I've only considered the spring and the summer - the rest of the year is all green. To keep things simple I'm using whole numbers. Note that all the times in March are GMT, and from April onwards they're BST.

Maximum UV Index in London on the 21st of the month
Time21 Mar21 Apr21 May21 Jun21 Jul21 Aug21 Sep
12 noon2466642

The first column is for last Tuesday, and shows how the UV Index rises to 2 between 10am and 2pm, but never quite reaches 3. The second column is for 21st April, which shows a 'Moderate' risk kicking in by 11am and remaining until 3pm. In May, June and July, a 'High' risk exists between noon and 2pm, with a 'Moderate' risk between 10am and 4pm. All of this assumes a clear sky, with lower values in case of cloud. But this helps to explain why you burn in the summer (either side of the solstice) and not in March or September.

This next table I'm calling my You Might Burn table. It shows the calendar dates when a certain UV Index is possible for a prolonged length of time.

UV Index3456
noon-2pm28 Mar-18 Sep12 Apr-1 Sep29 Apr-13 Aug22 May-18 Jul
11am-3pm9 Apr-5 Sep27 Apr-14 Aug21 May-6 Jul-
10am-4pm2 May-6 Aug---

For example, I'm particularly interested in a UV Index of 5, because that's when my skin becomes susceptible to burning. I need to watch out between noon and 2pm from the end of April to the middle of August, and between 11am and 3pm from the middle of May to the middle of July. But if your skin burns when there's a UV index of 3, you might need to slap on sunscreen between noon and 2pm starting this week, between 11am and 3pm starting in a fortnight's time, and between 10am and 4pm from the beginning of May.

If that was a bit confusing, try this. My final table shows the potential danger periods on the 21st day of the month over the summer. The times are a little more approximate here. Again the data is for London.

UV Index3456
21 Apr10.30am-3.30pm11.30am-2.30pm--
21 May9.45am-4.15pm10.30am-3.30pm11am - 3pm12 noon - 2pm
21 Jun9.30am-4.30pm10am - 4pm10.45am-3.15pm11.30am-2.30pm
21 Jul9.45am-4.15pm10.30am-3.30pm11am - 3pm12 noon - 2pm
21 Aug10.30am-3.30pm11.30am-2.30pm--

On 21st April a UV Index of 3 is possible from 10.30am onwards, and 4 from 11.30am to 2.30pm. By the time we reach 21st May, the UV Index could be 5 from 11am onwards, and 6 from noon to 2pm. The peak risk is on the day of the summer solstice, with the UV Index above 3 for almost the entire working day. That's assuming it's perfectly sunny, of course. A more typical British summer's day wouldn't be quite so intense.

If you're still with me, I hope that all this research and number crunching has been interesting. It's been reassuring to discover that there is genuinely no need for sunscreen in London between late September and late March. It's been a salutary reminder that it'll soon be time to get the sunscreen out again, or face the consequences. And it's helped me to understand why I got the worst sunburn of my life in San Francisco even though it was only April (it never gets to 7.5 in London, but it does over there). If I could offer you only one tip for the future, sunscreen would be it.

» UV Index forecast (Met Office)
» UV Index - Wikipedia
» How the UV Index is calculated
» UV Index calculator (simple)
» Daily UV graphs for a dozen UK locations (Defra)
» Graph showing maximum UV Index for New York City throughout the year
» 2-page UV radiation explanatory leaflet (USA)

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