What is the UV index and how to read it
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The UV index (or ultraviolet index) is a number that indicates the intensity of the sun's ultraviolet radiation at a given time and place — and, above all, the risk of your skin burning. It runs from 0 (no risk) to 11 or more (extreme). The higher the number, the faster your skin is damaged. Learning to read it is the simplest way to decide when and how to protect yourself.
What the UV index is
It's an international scale standardised by the World Health Organization (WHO) and the World Meteorological Organization (WMO). It translates into a single number the amount of UV radiation reaching the surface — the same radiation responsible for sunburn, premature skin ageing and much of the risk of skin cancer. Because the scale is universal, a UV index of 8 means the same thing anywhere in the world.
The UV index scale
| UV index | Risk level | What to do |
|---|---|---|
| 0–2 | Low | You can be outdoors safely; minimal protection needed. |
| 3–5 | Moderate | Seek shade around midday; use sunscreen, a hat and sunglasses. |
| 6–7 | High | Reduce exposure; sunscreen, shade, hat and sunglasses are essential. |
| 8–10 | Very high | Avoid the sun during the central hours; step up all protection. |
| 11+ | Extreme | Avoid direct exposure; maximum protection on every front. |
How to read the number in practice
The most useful rule is simple: from a UV index of 3, you should protect yourself. Between 3 and 5, your usual care during the sunniest hours is enough. From 6, protection is no longer optional. And at 8 or more, unprotected skin can burn in just a few minutes, so it's best to avoid direct sun altogether during the central hours.
A trick to gauge it without checking anything: look at your shadow. If it's shorter than you are, the sun is high and UV is strong; if it's longer, the radiation is lower.
What makes the UV index rise or fall
- Time of day: it peaks at solar noon — see what time the sun is strongest at the beach.
- Season: it's higher in summer, when the sun is more directly overhead.
- Latitude: it rises as you get closer to the equator.
- Altitude: it increases with height — about 10% more per 1000 metres.
- Clouds: they let much of the UV through; an overcast sky is deceptive and still burns.
- Ozone layer: it filters part of the radiation; where it's thinner, the index is higher.
- Ground reflection: light sand and water reflect part of the radiation (and snow reflects a lot), increasing your total exposure.
Common mistakes when reading the UV index
- Confusing heat with UV: the index doesn't depend on temperature. A cool day can have very high UV.
- Thinking cloudy days are safe: up to about 80% of UV passes through clouds.
- Thinking wind reduces UV: it cools the skin, but the radiation remains.
- Thinking only fair skin needs protection: all skin is damaged by high UV radiation.
Where to check the UV index
You'll find the UV index in your national weather service's daily forecast and in most weather apps, often hour by hour. If you're in Portugal, see also our guide to the UV index in Portugal.
How to protect yourself at each level
No single measure is enough: effective protection combines several layers. Shade for the most exposed area — the face and head — plus sunscreen, clothing, a hat and UV-filter sunglasses for the rest of the body. The higher the index, the more layers you need and the more important it is to stay out of the sun during the central hours.
A personal shade like the Sunsha creates a shaded zone dedicated to one person's face and head, with UPF50+ fabric. It doesn't cover the whole body or replace sunscreen and clothing — use it together with them for more complete protection.
Frequently asked questions
From what UV index should I protect myself?
From 3. That's when the radiation starts to pose a risk to unprotected skin.
Is the UV index the same all over the world?
Yes. It's a scale standardised by the WHO, so a UV index of 8 means the same in any country.
Can I get sunburnt when the UV index is low?
The risk is lower, but prolonged exposure — or highly reflective surfaces like snow — can cause burns even at low values.
Is the UV index higher at the beach?
The baseline radiation is the same, but light sand and water reflect part of the UV, increasing your total exposure.
Do cloudy skies reduce the UV index?
Only partly. Much of the UV radiation passes through clouds, so you can still burn on an overcast day.
The Sunsha is handmade in Portugal and protects the face and head from direct sun. Always use sunscreen and suitable clothing to protect the rest of the body.