Grey skies can create a false sense of safety. The temperature drops, glare disappears and sun protection may seem unnecessary. Yet clouds do not block all ultraviolet radiation, and enough UV can still reach the ground to harm skin and eyes. The useful guide is not how warm or bright it feels, but the forecast UV index for your location and time.
What the UV index measures
The ultraviolet index (UVI) expresses the intensity of solar UV radiation at the surface. It is open-ended: the higher the number, the greater the potential for harm and the less time damage may take. The World Health Organization advises checking the UVI and using protection when it reaches 3 or above.
Forecasts commonly show the daily maximum, usually reached around solar noon. AEMET's official UV forecast uses the established categories: low (0–2), moderate (3–5), high (6–7), very high (8–10) and extreme (11+). The level changes during the day, so the maximum is a planning value rather than a constant reading.
Why clouds do not remove the risk
Clouds alter UV radiation, but the effect depends on their thickness, type and coverage. Dense overcast generally reduces UV substantially, whereas thin or broken cloud can transmit a considerable amount. With moving gaps, exposure may also change quickly. Looking at the sky is therefore no substitute for checking the index.
Visible light and heat are poor guides as well. Wind, altitude and cloud can make conditions feel cool while UV exposure continues. Sunburn often becomes noticeable only later, after the damage has already occurred.
Factors that influence exposure
- Time: UV is usually strongest around solar noon.
- Altitude: higher ground has less atmosphere above it to absorb radiation.
- Latitude and season: both affect the Sun's elevation and the path through the atmosphere.
- Ozone: atmospheric ozone affects how much UV reaches the surface.
- Reflection: snow, water and light surfaces can add indirect exposure.
At the beach, on a boat or in the mountains, combine the weather forecast with the UVI. A cloud shadow may remove glare while reflected radiation from the surroundings remains relevant.
How to act on a cloudy-day UVI
Check the daily maximum and, when available, the hourly curve. At UVI 0–2, everyday risk is generally low. From UVI 3, seek shade around midday, wear covering clothing, a hat and UV-protective sunglasses, and apply sunscreen to exposed skin according to its instructions. At UVI 8 and above, make a particular effort to limit time outdoors around midday.
These measures work together. Sunscreen should not be used to extend exposure deliberately. Children, fair-skinned people, those taking photosensitising medicines and anyone outdoors for long periods may need extra precautions; seek professional medical advice for individual concerns.
A quick check before leaving
- Check the official UV index for your exact area, not just the cloud icon.
- Note when the maximum is expected.
- Consider snow, water, altitude and the availability of shade.
- Carry protection even if the morning begins overcast.
- Refresh the forecast during a long activity.
The practical conclusion is simple: clouds may reduce ultraviolet radiation, but they do not guarantee a low level. A cool, cloudy day can still require protection whenever the UV index says it does.