See today's UV index for your location – hour by hour. When is UV at its highest and when should you apply sunscreen?
The UV index is an international scale describing the strength of the sun's ultraviolet radiation at ground level. It starts at 0 and is effectively open-ended. At mid-latitudes it rarely exceeds 8, while in the tropics and high in the mountains it can pass 11. The higher the number, the less time it takes to get sunburnt.
The UV index depends mainly on how high the sun is in the sky. That is why it peaks around solar noon and is greatest in summer near the longest day of the year. Clouds, the ozone layer, altitude and reflection from snow or water also affect the value. Note that UV does not follow the temperature — you can get sunburnt on a cool, clear spring day.
The WHO recommends sun protection whenever the UV index is 3 or higher: use sunscreen of at least SPF 30, seek shade around midday, and wear sunglasses and a hat. About two-thirds of the day's total UV falls between 10:00 and 16:00. The radiation is weakest around sunrise and sunset, as the chart above shows.
The protection window above shows the hours when the UV index is 3 or higher today — the period when sunscreen and shade actually matter. The dashed outline in the chart shows how high UV would reach under a completely clear sky: once the clouds break, the real radiation moves closer to that value.
See today's UV index hour by hour for a city such as London, New York, Los Angeles, Sydney or Singapore.
The UV index is deliberately built to be acted on rather than admired, and the bands are what carry the meaning:
The scale is roughly linear in intensity, so a UV index of 8 is about twice as strong as 4 — meaning it takes about half the time to do the same damage.
UV is driven overwhelmingly by solar elevation, because that determines how much atmosphere the radiation has to pass through. A sun 60° above the horizon shines through roughly the thickness of one atmosphere; a sun 15° up shines through nearly four times as much, and the extra ozone and air in that path absorbs the shorter wavelengths preferentially.
The practical consequence for northern latitudes is stark. In London the midday sun reaches about 62° in June but only around 15° in December — which is why the UV index there peaks around 7 or 8 in high summer and sits at 0 or 1 for months on end in winter. You genuinely cannot get sunburnt in a British December, no matter how bright and clear the day is.
The same principle explains the daily curve on the chart above: UV peaks tightly around solar noon, not around the hottest part of the afternoon. Roughly two-thirds of a day's total UV arrives in the four hours centred on solar noon — and remember that solar noon is often well after 13:00 on the clock once daylight saving and your position within the timezone are accounted for.
The standard advice holds up well, and it is worth being precise about the parts people get wrong. SPF 30 blocks about 97% of UVB and SPF 50 about 98% — the gap is much smaller than the numbers suggest, and both are far more sensitive to how much you apply than to the rating. Most people use around a quarter to a half of the amount the tested figure assumes, which effectively drops an SPF 30 to something closer to SPF 10.
Reapplication matters more than the initial coat: every two hours, and immediately after swimming or towelling, regardless of what the bottle claims about water resistance. And "broad spectrum" is the label to look for, since SPF alone describes only UVB protection while UVA is what drives most long-term skin ageing and contributes to skin cancer risk.
Shade, clothing and a hat outperform any sunscreen. Note also that UV passes through cloud and through glass differently: ordinary window glass blocks most UVB but transmits a good deal of UVA, which is why drivers in some countries show more sun damage on the side of the face nearest the window.
See today's UV index hour by hour for a city such as London, New York, Los Angeles, Sydney or Singapore. For how the sun's height varies through the year where you are, see the year chart, and for where these figures come from, how we calculate the times.
Protection is recommended from 3 upwards. From 6 the risk is high and unprotected fair skin can burn in well under half an hour; from 8 it is very high, and from 11 extreme. The scale is roughly linear, so a UV index of 8 does the same damage in half the time as a 4.
Yes. Thick overcast cuts UV substantially, but light or broken cloud may reduce it very little, and scattered cloud can briefly push UV above the clear-sky value through reflection off cloud edges. UV also has nothing to do with how warm it feels — a cool, clear spring day can burn you.
Around solar noon, when the sun is highest. Roughly two-thirds of a day’s total UV arrives in the four hours centred on solar noon — which is often well after 13:00 on the clock once daylight saving and your position within the timezone are accounted for.