UV Index
Also called: ultraviolet index, UVI
The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.
Why It Matters
The UV index tells you how quickly the sun can damage unprotected skin, which the temperature does not. It depends mainly on the height of the sun in the sky, the amount of ozone in the stratosphere, and cloud cover.
UV Index Exposure Categories
| UV index | Category | Protection |
|---|---|---|
| 0 to 2 | Low | No protection needed: minimal sun protection is enough |
| 3 to 5 | Moderate | Protection needed: seek shade from late morning through mid-afternoon, and use SPF 15 or higher sunscreen, protective clothing, a hat, and sunglasses |
| 6 to 7 | High | Protection needed, as for Moderate |
| 8 to 10 | Very High | Extra protection needed: be careful outside from late morning through mid-afternoon, seek shade, and cover up |
| 11 or higher | Extreme | Extra protection needed, as for Very High |
Source: U.S. Environmental Protection Agency: UV Index Scale
How to Read It
- The forecast value is for solar noon. UV is weaker earlier and later in the day.
- There is no upper limit. The index can reach 15 or 16 in the tropics at high elevations under clear skies.
- A quick check from the EPA for very high readings: if your shadow is shorter than you are, seek shade, cover up, and use sunscreen.
- A cool day can have a high UV index. It follows the sun's height, ozone, and cloud, not the air temperature.
Frequently Asked Questions
At what UV index do you need sunscreen?
At 3 and above. From 3 to 7, protection is needed: shade around midday, SPF 15 or higher sunscreen, clothing, a hat, and sunglasses. At 8 and above, extra protection is needed.
What is the highest UV index?
The scale has no upper limit. Anything 11 or higher is classed as Extreme, and values of 15 or 16 can occur in the tropics at high elevations under clear skies.
What time of day is the UV index highest?
At solar noon, when the sun is highest in the sky. The published UV index is the forecast for that time.
Related Terms
Heat Index
The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.
AQI
The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
Wet-Bulb Temperature
Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is always at or below the air temperature, and the drier the air, the larger the gap between the two. Forecasters use it to judge heat stress and how much falling rain or snow can cool the air.
UV Index: Guides and Explainers
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In the WeatherAI App
WeatherAI shows the UV index where the selected forecast provider supplies it, and the Apple Watch app includes a UV Index complication.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions.