Ultraviolet radiation reaches the ground mainly as UVA and UVB; UVA penetrates deep into the skin and drives premature ageing, while UVB burns the surface and is the main trigger for sunburn and, with repeated overexposure, skin cancer. Both also damage the eyes over time, contributing to cataracts. The UV Index, developed by the World Health Organization and used worldwide, condenses the strength of this radiation at ground level into a single number, typically from 0 to 11 or more, so you do not need to know the underlying physics to judge your risk on a given day. Crucially, UV Index has very little to do with how hot or cold it feels: a clear, crisp spring day at altitude can carry a higher index than a hazy, warm summer afternoon.
What drives the UV Index up
The sun's angle is the single biggest factor: UV is strongest within a couple of hours of solar noon and during the months when the sun sits highest in the sky, and it is naturally more intense closer to the equator year-round. Altitude matters too, because thinner air absorbs less radiation, adding roughly 10% more UV for every 1,000 metres of elevation, which is part of why sunburn catches people off guard in the mountains. The ozone layer is the atmosphere's main UV filter, and its thickness varies by location, season and from year to year, with thinner ozone allowing more radiation through. Clouds usually cut UV exposure, but thin or scattered cloud can scatter radiation without blocking it, occasionally pushing levels briefly higher than under a fully clear sky. Finally, the ground itself matters: fresh snow reflects up to 80% of UV back upward, sand around 15%, and water enough to meaningfully add to the dose you receive from above, which is why sunburn is so common at the beach and on ski slopes alike.
How to protect yourself
As a rough guide, an index of 0-2 needs no special precaution, 3-5 calls for shade around midday and a hat, 6-7 means sunscreen and protective clothing are genuinely necessary, and 8 and above, classed High to Extreme, means unprotected skin can burn in well under an hour and active protection is essential. The most effective habits are simple: stay in the shade during the two to three hours around solar noon when you can, wear a wide-brimmed hat and UV-blocking sunglasses, cover up with tightly woven clothing, and apply a broad-spectrum sunscreen of at least SPF 30, reapplying roughly every two hours and immediately after swimming or heavy sweating, since water and towels remove most of it. Remember that cool air, overcast haze and even a light breeze give no real protection on their own, so the index itself, not how the day feels, should guide your decisions, especially for children, fair-skinned individuals and anyone near snow, sand or water. The UV Index layer on ngmeteo.com's forecast maps lets you check expected levels for any location and day before heading outdoors.