Ultraviolet radiation is invisible and cannot be felt: in cool or hazy weather, skin can burn without any warning. The UV index is the international scale that turns the intensity of the radiation reaching the ground into a simple number, from 0 to 11 and above.
Why this indicator matters
Its value depends on the height of the sun, and so on the time of day and the season, on altitude, which adds roughly ten per cent per thousand metres, and on the ozone layer and clouds, which filter part of the radiation. In Europe, the index peaks between 12:00 and 15:00 in summer, and rarely exceeds 2 in winter.
From an index of 3, the World Health Organization recommends protecting yourself: shade in the middle of the day, covering clothes, a hat, sunglasses and sunscreen. Sunburn in childhood is an established risk factor for melanoma in adulthood.
How to read it
The box gives the peak index of the next twenty-four hours, the time of the peak and the time range in which the index stays at 3 or above, when it is best to seek shade. It also shows the approximate time to sunburn for fair skin.
That time is 200 divided by the index, in minutes: at index 5, unprotected fair skin can redden in forty minutes; at index 10, in twenty. Sunscreen does not lengthen this time indefinitely; it reduces cumulative exposure and is no substitute for shade.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- The indicator speaks up as soon as the maximum reaches 3; below that, protection is not recommended.
- From 3 to less than 6: moderate index; from 6 to less than 8: high index; from 8: very high.
- Time to sunburn for fair skin (skin type II, no protection): 200 ÷ index, in minutes.
- The “in the shade” range is the continuous run of hours in which the index stays at 3 or above.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13: UV index of 3 around 13:00: stay in the shade around 13:00. Fair skin burns in about 66 minutes without protection; wear sunscreen and a hat if you are outdoors.
Its limits
The figure of 200 divided by the index is a simple rule, correct to within a factor of two for skin type II: very fair skin burns faster, darker skin more slowly. The model's index allows for the forecast clouds, but not for reflection off snow, water or sand, which can add up to eighty per cent.
Frequently asked questions
Can you get sunburn on a cloudy day?
Yes: thin cloud lets through up to eighty per cent of UV, and some sunny spells even boost it. Only a very heavy overcast sharply reduces the index.
Does sunscreen let me stay in the sun longer?
SPF 30 filters about 97 % of UVB, provided you apply a thick layer and reapply it. It limits the damage but does not make prolonged exposure in the middle of the day acceptable.
Why is UV stronger in the mountains?
Because the air is thinner: at 2,000 metres, the index is about twenty per cent higher than on the plain, and snow adds reflection.