The UV index indicates the level of ultraviolet radiation at the earth's surface and does not coincide with either temperature or the amount of visible light.
This guide explains what the UV index means, how the scale works, where to check it and what the number does not tell those who play sport.
What the UV index is: the WHO definition
The UV index is a number that tells you how much ultraviolet radiation is reaching the ground in a given place at a given time. The higher the number, the more intense the radiation, and the less time it takes for skin and eyes to be affected. It does not indicate how hot it is or how much visible light there is: it indicates the level of UV radiation.
It is not a value set by whoever publishes the forecasts. It is a single scale, defined by the World Health Organization together with other international organisations, and this means that an index of 7 measured in Bolzano means the same thing as an index of 7 measured in Australia. The number comes from a measurement of the intensity of the radiation, converted into a scale that is simple to read.
Two things are worth knowing. The first: the index is built mainly around UVB rays, the ones that cause sunburn, while UVA rays have little effect on the final value. It is a practical choice, because sunburn is the most immediate and measurable effect, but it means that the number does not describe the whole exposure: the difference between UVA and UVB rays explains why this matters. The second: the value reported in bulletins is almost always the daily maximum, that of the four hours around solar noon.
The 0-11+ scale: what each level means
The scale starts at zero and has no formal upper limit: by convention, all values equal to or above 11 are communicated as "11+". Each range corresponds to a category of exposure, with behavioural guidance defined by the WHO.
|
UV Index |
Category |
What it means for those training outdoors |
|
0-2 |
Low |
In normal conditions, for short exposures the risk is limited. In the presence of reflective surfaces such as snow, sand or water, actual exposure may increase. |
|
3-5 |
Moderate |
The threshold from which the WHO recommends protective measures: cream, sunglasses, hat, shade during the central hours. |
|
6-7 |
High |
Protection is necessary. Where possible, prolonged activities can be scheduled outside the hours of greatest UV intensity. |
|
8-10 |
Very high |
Avoid exposure during the central hours where possible and adopt reinforced protective measures. |
|
11+ |
Extreme |
At these levels it is particularly important to adopt all the main protective measures. |
The operational threshold to remember is 3. From this value onwards, the WHO recommends adopting protective measures.
What the number does not tell you
The UV index provides a standardised indication of the level of UV radiation, but it does not necessarily describe each person's actual exposure. Reflective surfaces, shade, orientation and local conditions can alter exposure.
Reflective surfaces
UV radiation is reflected by the ground to very different degrees depending on the material. According to the WHO, fresh snow can reflect up to 80% of UV radiation, water around 25% and dry sand around 15%. On snow, sand or other reflective surfaces, part of the radiation can reach the skin by reflection as well, including areas that normally receive less direct exposure.
Altitude
As altitude increases, the thickness of atmosphere crossed by the radiation is reduced and absorption falls. The WHO indicates an increase in UV levels of around 4% every 300 metres of altitude. A ski touring outing at 2,500 metres on fresh snow combines the two effects: altitude and reflection.
Duration
The index describes instantaneous intensity, not the accumulated dose. Overall exposure depends both on the intensity of the radiation and on the duration of the activity. For this reason, even a moderate index can become relevant during prolonged exposure.
Clouds
Light cloud cover may only partly reduce exposure to UV rays. Actual cloudiness can also change over the course of a long training session, and the perception of reduced brightness does not necessarily correspond to a proportional reduction in exposure.
Where to check the UV index
- SunSmart Global UV app. Free, developed by the WHO together with WMO, UNEP and ILO. It gives a five-day UV forecast for any searchable location and flags the time slots when protection is needed. It is the most consistent option if you train in different countries, because it uses the same scale everywhere.
- Your national weather service. Most European meteorological agencies publish a UV index forecast alongside the standard weather data, usually for the current day and the next two.
- General weather apps. Convenient, but the reference altitude of the point does not always match the actual altitude where you train: in the mountains the value shown may be underestimated.
How to use the UV index to plan your training
The UV index serves two decisions: when to train and with what level of protection. For the first you cannot always choose — a race has a fixed time, as does a group session. For the second, however, you can:
- Check the maximum forecast value and the time at which it occurs, not just the daily average.
- Where possible, schedule the longest sessions outside the central hours of the day, when UV intensity is greatest.
- Increase the level of protection: apply protection around 20 minutes before exposure and reapply every two hours, or more frequently in case of heavy sweating, water or rubbing.
- Pay particular attention at altitude, on snow, on sand or near water: reflective surfaces can increase actual UV exposure.
- Carry a suitable hat and sunglasses: they are part of protection, especially in conditions of strong exposure or reflection, such as at high altitude and on snow.
In terms of protection, for outdoor training it is important that the formula suits the conditions in which the sporting activity takes place: SUNO AIR is developed for high-sweat sports, while SUNO FACE is the specific formula for protecting the face during sport. For the choice of protection factor, the dedicated article explains which SPF to choose according to phototype and discipline.
The most widespread myths about the UV index
"If it isn't hot, UV is low"
Temperature and UV index are independent quantities. An April day at 2,000 metres, with cold air and snow on the ground, can present higher UV index values than a muggy day on the plain. Ultraviolet radiation is not perceived as heat.
"In winter the UV index doesn't matter"
In many lowland areas winter values are generally lower, but the situation can change with latitude, altitude and atmospheric conditions. In the mountains, the combination of altitude and snow reflection can significantly increase exposure, especially on clear days in late winter and early spring.
"In the shade the UV index is zero"
Shade reduces exposure but does not eliminate it. Radiation scattered by the sky and reflected by surrounding surfaces continues to arrive: under a tree or a parasol the reduction is partial.
"A high UV index means I will burn"
The index describes the intensity of the radiation, not the outcome for the individual person. The same value can produce different effects depending on individual characteristics, including phototype and sensitivity to UV. The absence of erythema does not mean the absence of exposure to UV rays: the effects of exposure do not necessarily show themselves through immediate sunburn.
FAQ
From what UV index value is protection needed?
From 3. The WHO indicates this threshold as the point from which to adopt protective measures: sunscreen, sunglasses, hat and seeking shade during the central hours of the day.
Does the UV index take altitude into account?
UV index forecasts take into account the environmental factors that influence radiation, including altitude; however, in a mountain environment the forecast value may not perfectly represent the conditions at the point where you are.
Does the UV index change during the day?
Yes, markedly. The maximum occurs in the four hours centred on solar noon. In the early morning and late afternoon the rays cross a greater thickness of atmosphere and the intensity falls.
Does the UV index tell me how long I can stay in the sun?
No. It indicates the intensity of the radiation, not a safe duration of exposure. Reaction time also depends on phototype, and no value should be read as authorisation for prolonged exposure without protection.
What to do with it, in practice
Checking the UV index takes a few seconds and affects two concrete decisions: the time of your outing and the level of protection with which you tackle it. There are three reference points to keep in mind: the threshold of 3, the central hours of the day when UV intensity is greatest, and the role of altitude and reflective surfaces, which can increase actual exposure.
The next step concerns execution. Quantity, timing and correct reapplication during activity are fundamental aspects for maintaining adequate protection, and this is the subject of how to apply and reapply sunscreen during a race.
