Science & history · Light and energy
UV is not the same as brightness
A hazy summer day can feel dim and still carry a high UV Index; a bright winter afternoon can carry a low one.
What the index is
The UV Index is a forecast of the expected risk of overexposure to the Sun's ultraviolet radiation, on a scale from 1 to 11 and above.
What changes it
The EPA lists ozone, season, weather, the time of day and latitude, and notes that UV intensity rises about 6% per kilometre of elevation. Clouds cut it, but not to zero: the EPA's figure for overcast skies is about 31% getting through. None of that is visible-light brightness, which is why this site never turns a sky calculation into a UV claim.
Sources
- US Environmental Protection Agency Learn About the UV Index (opens in a new tab) for UV Index scale and what changes it (elevation ~6% per km, clouds, ozone, solar noon) · checked 2 October 2026