The UV index is a standardized scale that measures the strength of ultraviolet radiation reaching the Earth’s surface at a given place and time. It starts at zero and has no fixed upper limit, though values above 11 are rare outside of high-altitude equatorial regions. The higher the number, the faster unprotected skin can burn.
How the Scale Is Organized
The UV index was developed jointly by the World Health Organization, the United Nations Environment Programme, the World Meteorological Organization, and the International Commission on Non-Ionizing Radiation Protection. It’s used worldwide, which means a UV index of 7 in Sydney means the same thing as a UV index of 7 in Miami.
The scale is broken into five exposure categories:
- 1 to 2 (Low): Minimal risk for most people. You can be outside comfortably without special precautions.
- 3 to 5 (Moderate): Fair-skinned people can burn in under 30 minutes of direct midday sun. Shade, sunscreen, and a hat become worthwhile.
- 6 to 7 (High): Unprotected skin burns more quickly. The EPA recommends seeking shade from late morning through mid-afternoon and wearing broad-spectrum SPF 15 or higher sunscreen, protective clothing, a wide-brimmed hat, and sunglasses.
- 8 to 10 (Very High): Burn times shorten significantly. Limiting midday outdoor time and covering exposed skin become important.
- 11+ (Extreme): The most intense UV exposure possible at the surface. Even brief unprotected time outdoors can cause damage.
At index values of 8 or above, the EPA advises extra caution, especially between late morning and mid-afternoon. A useful quick check: if your shadow is shorter than your height, UV levels are elevated. If your shadow is taller than you, exposure is lower.
What the Number Actually Measures
The UV index isn’t a raw measurement of sunlight intensity. It’s weighted toward the specific wavelengths of ultraviolet light that cause sunburn in human skin. Scientists call this the “erythemal action spectrum,” but in practical terms it means the index reflects how damaging the UV radiation is to your skin, not just how much of it exists. Shorter UV wavelengths (UVB) cause more burning per unit of energy than longer ones (UVA), so they count more heavily in the calculation.
The result is a dimensionless number. Each single-point increase on the index represents a meaningful jump in burn risk. Going from UV index 4 to UV index 8 doesn’t just double the intensity; it roughly doubles how quickly unprotected skin will redden.
What Pushes the UV Index Up or Down
Several environmental factors shift the UV index at any given location, sometimes in surprising ways.
Time of Day and Season
UV radiation peaks when the sun is highest in the sky, typically between 10 a.m. and 4 p.m. local time. In summer months at mid-latitudes, the index regularly reaches 8 to 10. In winter, the same location might see values of 1 to 3.
Altitude
UV intensity increases with elevation. At lower altitudes, air molecules and dust scatter more UV radiation before it reaches the ground. Higher up, that filtering layer is thinner, so more UV arrives at the surface. A rough rule of thumb: UV exposure increases about 6 to 8 percent for every 1,000 meters (roughly 3,300 feet) of elevation gain. This is why sunburn risk is notably higher when skiing or hiking in the mountains, even when temperatures feel cool.
Clouds
Thick, heavy cloud cover does reduce UV transmission, but not as much as people assume. Thin or broken clouds can let 80 percent or more of UV radiation through. In some cases, clouds actually increase surface UV levels: towering cumulus clouds can reflect UV radiation off their sides, boosting exposure at ground level beyond what a clear sky would deliver. Overcast days are not sunburn-proof.
Surface Reflection
Snow reflects up to 80 percent of UV radiation back at you, effectively doubling your exposure. Sand reflects about 15 percent, and water reflects around 10 percent. Grass and soil reflect very little. This reflected UV hits you from below, reaching areas like the underside of your chin and nose that direct sunlight often misses.
Latitude and Ozone
Locations closer to the equator receive more direct sunlight year-round and consistently see higher UV index values. The thickness of the ozone layer also matters: ozone absorbs a large share of incoming UV radiation, so regions where the ozone layer is thinner (including parts of the Southern Hemisphere) tend to have higher UV readings for their latitude.
How High the Index Can Go
In most populated areas, the UV index tops out between 10 and 12 on the hottest summer days. Tropical locations at high altitude are the exception. Parts of the Andes in South America, particularly in Bolivia and Peru, have recorded UV index values above 20. These are among the highest surface measurements on Earth, driven by a combination of equatorial sun angle, extreme altitude (above 4,000 meters), thin ozone, and reflective terrain.
By contrast, polar regions stay much lower. At the South Pole, typical summer UV index values range between 2 and 3.5, with a recorded maximum of just 4. Barrow, Alaska, tops out around 5, and Summit, Greenland, has reached 6.7. Even at peak summer, these locations never approach the levels seen in the tropics.
How to Use the UV Index Day to Day
Most weather apps and forecasts include the UV index as a standard part of their daily report. The number shown is usually the peak value expected for that day, which almost always falls around solar noon (roughly 1 p.m. during daylight saving time).
At index values of 1 or 2, most people can spend extended time outside without any sun protection. From 3 to 7, sunscreen, shade during midday hours, and a hat make a real difference, especially if you have lighter skin. At 8 and above, limiting direct sun exposure during peak hours is the most effective strategy. Sunscreen helps but works best as a supplement to shade and clothing, not a replacement.
Skin type matters. People with very fair skin that burns easily can start to redden at a UV index of 3 in under 20 minutes of unprotected exposure. People with darker skin have more natural protection from melanin but are not immune to UV damage, particularly to the eyes. UV radiation contributes to cataracts and other eye conditions regardless of skin tone, which is why sunglasses with UV protection are recommended for everyone when the index climbs above 3.