The UV Index (short for "Ultraviolet Ray Index") is a next-day forecast of the amount of skin-damaging UV radiation that's expected to reach Earth's surface when the sun is highest in the sky (solar noon). It was created to help people make informed decisions about the amount of time that they spend in the sun.
How Do You Read It?
The UV Index uses a numerical scale to rate the strength of the sun's UV exposure level. The higher the UV Index level, the greater the strength of the sun's UV rays—and the faster you can burn!
| Index Scale |
UV Exposure Level |
| 0—2 |
Very low |
| 3—4 |
Low |
| 5—6 |
Medium |
| 7—9 |
High |
| 10+ |
Very high |
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How Should You Use This Index?
The UV Index is issued daily to advise you on the strength of the sun's UV rays in your region. Make a habit of checking the index so you'll know how much sun protection you'll need each day.
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How Accurate Is the Index?
The UV Index is based on monitoring the sun's position, cloud movements, altitude, ozone data, and other factors. Each year the National Weather Service performs a validation of the UV Index forecasts by incorporating the help from several government agencies and private companies, hospitals, and colleges, that provide observations of surface UV radiation. From these observations, statistical corrections are made to ensure accuracy of the index.
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Ozone & Local Effects
While it is well known that the Earth's ozone decreases the amount of UV rays you receive, the exact impact of ozone depletion is not yet fully understood. However, some local factors such as smog, or the type of reflective surface you're near, can also determine the amount of exposure you receive. For example, water, sand, snow, and concrete can all reflect ultraviolet rays, increasing your exposure.
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