The UV index, explained

The UV index is a number from 0 upward that says how strong the sun’s ultraviolet rays are at midday. This guide explains the scale. It is general information, not medical advice.

The scale below comes from the United States Environmental Protection Agency and was checked on September 19, 2026. See the EPA UV index scale.

The scale

• 1 to 2, low. The EPA says no protection is needed and you can safely stay outside using minimal sun protection.
• 3 to 7, moderate to high. The EPA suggests seeking shade in the middle of the day, wearing protective clothing, a wide-brimmed hat, and sunglasses, and applying a broad-spectrum sunscreen of SPF 15 or higher.
• 8 and above, very high to extreme. The EPA advises extra precautions, especially from late morning through mid afternoon, and says to seek shade and wear protective clothing and sunscreen.

The shadow rule

The EPA describes a simple check. If your shadow is taller than you are, in the early morning or late afternoon, UV exposure is lower. If your shadow is shorter than you are, around midday, UV exposure is higher and it is time for shade and protection.

When the number matters

The UV index is highest around midday, in summer, at high altitude, and near water or snow, which reflect sunlight. Clouds reduce it but do not remove it. The index describes the sun, not the temperature, so a cool, bright day can have a high index.

What this means for Bold Weather

Bold Weather shows the UV index on the details screen with a sentence that says whether it matters today, such as “UV index 8. Very high. Seek shade in the middle of the day.” Explore the hourly and 10-day forecasts or learn about the TestFlight beta.