The UV index is a numbered scale that tells you how strong the sun’s ultraviolet radiation is at a given place and time. It runs from 0 (nighttime or very weak sunlight) up into the mid-teens under extreme conditions, with higher numbers meaning faster potential damage to your skin and eyes. Most weather apps and forecasts include it alongside temperature and humidity, but the number itself often goes unexplained.
What the Numbers Mean
The scale breaks into three practical tiers based on how quickly unprotected skin can be harmed:
- 0 to 2 (Low): Minimal risk. You can be outside comfortably without sun protection.
- 3 to 7 (Moderate to High): Skin damage becomes a real possibility. Seek shade during late morning through mid-afternoon, apply broad-spectrum SPF 15 or higher sunscreen, and wear a hat and sunglasses.
- 8 and above (Very High to Extreme): Unprotected skin can burn quickly. Avoid prolonged time outdoors during midday hours. Sunscreen, shade, protective clothing, and sunglasses are all essential.
A quick trick at high levels: if your shadow is shorter than your height, the UV radiation overhead is intense enough to warrant extra caution. The World Health Organization recommends starting sun protection at a UV index of 3, which is lower than many people assume. On a clear summer day in most of the continental U.S., the index easily reaches 8 to 10 by midday.
How the UV Index Is Calculated
The number you see in your weather forecast isn’t a raw measurement of sunlight. It’s a weighted calculation designed to reflect how human skin actually reacts to different wavelengths of ultraviolet light. Shorter UV wavelengths cause more damage per photon, so they’re given a heavier weight in the formula. This weighting system, developed using a model of how skin reddens and burns, ensures the final number tracks closely with real-world sunburn risk rather than just total UV energy.
The calculation starts by measuring or modeling UV strength at every wavelength between 290 and 400 nanometers, then multiplying each by its biological weight. Those weighted values are added together to get a single number representing total skin impact. That total is then adjusted for local elevation and cloud cover before being divided by 25 and rounded to the nearest whole number. The result is the UV index.
Why the Same City Gets Different Readings
Several factors push the UV index up or down throughout the day, the year, and across different locations.
Time of day is the biggest variable. UV peaks between roughly 10 a.m. and 4 p.m. when the sun is highest and its rays travel through less atmosphere. Early morning and late afternoon readings drop sharply.
Season and latitude matter because the sun’s angle changes. Closer to the equator, UV is stronger year-round. At higher latitudes, winter UV drops dramatically, but summer readings can still reach the “very high” range.
Altitude increases UV intensity by 10% to 12% for every 1,000 meters you climb. A hike at 3,000 meters exposes you to roughly 30% to 36% more UV than at sea level, even if the temperature feels cooler.
Cloud cover absorbs some UV radiation but not as much as most people expect. Thin or scattered clouds let a large portion through, and you can still burn on overcast days. Thick cloud cover does reduce the index noticeably, which is why the calculation includes a cloud adjustment.
Surface reflection adds UV exposure from below. Fresh snow reflects 50% to 88% of UV radiation back at you, which is why skiers burn so easily. Dry beach sand reflects 15% to 18%. Water, concrete, and grass reflect smaller amounts but still contribute, especially in open areas with no shade overhead.
What the Ozone Layer Filters Out
The atmosphere does a lot of heavy lifting before UV radiation ever reaches you. The ozone layer blocks the most dangerous wavelengths entirely: UVC radiation, the highest-energy type, never reaches Earth’s surface. Only about 10% of UVB radiation (the primary cause of sunburn) makes it through. UVA radiation, which penetrates deeper into skin and contributes to aging and long-term damage, passes through the ozone layer largely unimpeded. The UV index primarily reflects UVB because of its outsized role in burning, but broad-spectrum sunscreens are designed to block both UVA and UVB.
UV Exposure and Vitamin D
Your skin produces vitamin D when UVB light hits it, and this is the main argument people raise against aggressive sun protection. Under favorable conditions, 10 to 15 minutes of sun on your arms and legs a few times a week generates nearly all the vitamin D most people need. That’s a modest amount of exposure, well short of what causes visible reddening or burning. The tradeoff doesn’t require skipping sunscreen during long outdoor stretches. It means that brief, incidental sun exposure during daily life often covers your vitamin D needs, particularly in spring and summer when the UV index is above 3.
In winter at higher latitudes, the UV index can stay so low that skin produces very little vitamin D regardless of time spent outside. This is one reason vitamin D supplements are commonly recommended for people living far from the equator during colder months.
Reading the UV Index in Practice
Most weather apps display the UV index as a peak forecast for the day, meaning the highest value expected around solar noon. The actual index is lower in the morning and evening. If your app shows a UV index of 7, that’s the midday peak. By 8 a.m. or 5 p.m., it may be 2 or 3.
Values above 11 are labeled “extreme” and occur regularly in tropical and equatorial regions, at high altitudes, and in areas with thinner ozone coverage. Parts of Australia, the Andes, and equatorial Africa routinely see readings of 12 to 14. In those conditions, unprotected fair skin can begin to redden in under 10 minutes.
The index doesn’t account for your individual skin type. People with darker skin have more natural protection against UV-induced burning, but the index still reflects the objective strength of the radiation reaching the ground. Everyone’s risk increases as the number climbs, though the threshold for visible damage varies from person to person.